Acoustic microscope

Details Acoustic microscope Acoustic microscope

1995-11-13

1997-10-07

Brock, Michael

Measuring and testing

Surface and cutting edge testing

Roughness

G01B 528

Patent

active

056750757

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention concerns an acoustic microscope for examination of a specimen, with a nib affixed to an elastic beam and arranged in the near-surface area of a specimen surface, with a supersonic transducer coupled to the specimen, with a device for positioning the specimen relative to the nib, where the nib has at time average a consistent distance from the surface, and with a control and data capturing device.
An acoustic microscope of this kind is known from WO 89/12805. On this acoustic microscope, a nib attached to a tuning fork is arranged in the near-surface area of a specimen surface. Forming an elastic beam, the tuning fork of piezoelectric quartz can be excited to mechanical vibrations by application of an electrical alternating voltage via electrodes attached to its shanks. These mechanical vibrations couple by way of the nib, as ultrasound, in the surface and, depending on the interaction between the vibrating nib and the specimen surface, lead on account of the attenuation of the vibration to a shift of the vibration frequency and/or the amplitude of the tuning fork as against a free vibration. The specimen and nib allow positioning relative to each other by means of a moving apparatus, the nib being at time average arranged at a consistent distance from the surface. A control and data capturing device serves the coordination of the data sensed by the nib in contingence on the relative position of the nib to the specimen.
In this acoustic microscope the tuning fork with the nib attached to it serves both as supersonic transmitter and also as sensor. Therefore, an electronic substraction of substantially equal values for the frequency and amplitude of the mechanical vibration must be carried out for obtaining the measured values, which subtraction is relatively prone to error. The result, notably with short measuring times per measuring point on the surface, is an unfavorable, poor signal-to-noise ratio. Owing to the resonant stimulation of the tuning fork in sustained operation, a scan of the topography of the surface, separate of sensing for instance elastic properties, can basically not be performed in the coupling area between the nib and the surface. A further provision with this acoustic microscope is stimulating the tuning fork preferably at its resonant frequency in the range of about 32 kHz, so that, while the amplitude is high in relation to the stimulating force, coupled-in interference vibrations cause in this frequency range as well excessively high interference amplitudes, which unfavorably affect the signal-to-noise ratio.
Another acoustic microscope is known from the publication "Scanning Microdeformation Microscopy," by B. Cretin and F. Sthal in the magazine "Applied Physics Letters" 62 pp 829 through 831 (1993). In this device, the vibrations of the nib at approximately 50 kHz produce at the resonant frequency of the elastic beam microdeformations on the specimen surface which induce an acoustic wave in the specimen. The acoustic wave allows detection by amplitude and phase relative to the vibrations of the nib, with a supersonic transducer. Plotting the amplitude and/or phase of the supersonic wave allows the generation of an image of the elastic properties of the specimen surface scanned by the nib, in contingence on the position of the nib. Possible also is detecting and imaging material in-homogeneities contained beneath the surface. The local resolution of this microscope ranges at around 10 .mu.m.
With such an acoustic microscope it is possible to detect mechanically hard and soft areas of the specimen surface. But a topography of the specimen surface is possible only indirectly, by processing the supersonic signals received, and proves to be very difficult, notably with relatively complex semiconductor topographies. With modern semiconductor structures ranging in the order of a few .mu.m, this microscope cannot be used for high-resolution examinations on such specimens.
A further disadvantage of this acoustic microscope is the relatively low s

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