JPH0390856A - Ultrasonic microscope - Google Patents

Abstract

PURPOSE:To accurately restore a sent pulse wave and to improve the resolution in the depth direction of a sample by restoring a sent waveform from the transfer function of the passage of ultrasonic pulses which are made incident on the sample. CONSTITUTION:The converged ultrasonic wave is made incident on the sample 24 and the ultrasonic wave reflected by the sample 24 is converted into an electric signal. The electric signal is supplied to a gate circuit 30 to extract a reflected wave from an optional depth position of the sample 24. this reflected signal is converted 31 into a digital signal, which is processed through the Fourier transformation of an arithmetic circuit 32. Further, the transfer function of the ultrasonic pulses is calculated 32 from reflection position information of the reflected wave on the sample 24 which is extracted by a position detecting circuit 27 for the relative position between an acoustic lens 23 and the sample 24, the sample density and sound speed, the time difference between the reflected waves from the sample surface and the sample inside, the focal length of the lens 23 in the ultrasonic wave propagation medium positioned between the acoustic lens 23 and sample 24, the absorption coefficient of the ultrasonic propagation medium, and the pupil function of the lens 23 and observation image data is then generated according to the transfer function.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention involves injecting ultrasonic pulses into a sample, converting the ultrasonic waves reflected from the surface of the sample, defects inside the sample, etc. into electrical signals, and For example, it relates to an ultrasonic microscope that can examine the acoustic properties of a sample.

[Conventional technology]

As this type of ultrasonic microscope, one having the configuration shown in FIG. 5 is conventionally known. This ultrasonic microscope outputs a transmission pulse as shown in FIG. 6(a) from a transmission circuit 1, for example, a relatively short pulse wave with a pulse width of 5 to 10 nS, and applies this transmission pulse to a transducer 2. to generate ultrasonic pulses. This ultrasonic pulse is focused onto the surface or inside of the sample 4 by the acoustic lens 3. The space between the sample 4 and the acoustic lens 3 is filled with water 5. The reflected wave reflected from the sample 4 passes through the acoustic lens 3 , is converted into an electrical signal by the transducer 2 , and is input to the receiving circuit 6 . The electrical signal input to the receiving circuit 6 (hereinafter referred to as "received signal") is as shown in FIG. 6(b).
Reflected waves from the surface of the sample, reflected waves from inside the sample, and reflected waves from the back of the sample are received with a time delay. Based on the command signal sent from the control circuit 8, a gate circuit 7 connected to the output side of the receiving circuit 6 detects a necessary reflected wave from the received signal using a gate signal as shown in FIG. 6(C), for example. (The figure shows an example of extracting internally reflected waves), and the output of this gate circuit 7 is manually input to the peak detection circuit 9 to perform peak detection as shown in FIG. 6(d). Get a signal. The peak detection signal is A/D conversion circuit 1
0 and is converted into a digital signal and stored in the image memory 1].

Through the above operations, information about one point on the sample 4 is stored in the image memory 11.

In order to obtain two-dimensional image information, the stage 12 on which the sample 4 is placed is driven by the control circuit 8 in synchronization with the output of the transmission signal, and the sample 4 is scanned in plane with ultrasonic pulses.

As a result, two-dimensional image data is stored in the image memory 11. The two-dimensional image data stored in the image memory 11 is read out by the control circuit 8 and displayed as a visible image on the display device 13, so that the distribution of acoustic properties of the sample 4 can be observed.

By the way, in the above ultrasonic microscope, a pulse wave is used as a transmission signal in order to make the temporal resolution of a reception signal as high as possible. By using pulse waves, information regarding the position in the depth direction where a defect or the like exists inside the sample 4 can be detected with high accuracy.

[Problem to be solved by the invention]

However, conventional ultrasonic microscopes have a problem in that the transmitted waveform is not accurately restored, and therefore it is not possible to obtain sufficient resolution in the depth direction of the sample.

That is, by applying a gate to the received signal at a predetermined timing and a predetermined gate width, the reflected wave from a predetermined depth of the sample is extracted, and the peak value of this extracted reflected wave is detected. As shown in FIGS. 6(c) and 6(d), the magnitude of the peak detection signal does not change no matter where the peak value of the reflected wave is located within the gate signal. Therefore,
The time resolution is determined by the gate width of the gate signal. However, when the gate width of the gate signal is narrowed to increase the time resolution, the magnitude of the peak detection signal changes due to a slight delay in the received signal, as shown in Figure 7, and it is difficult to distinguish it from changes in the strength of the received signal. I can't stand it anymore.

Furthermore, since the transducer 2 has narrow band characteristics as shown in FIG. 8, the received signal spreads out significantly compared to the waveform of the transmitted pulse. Therefore, Fig. 9(a)
As shown in Figure 9(b), the reflected waves a and b from slightly different depth positions of the sample overlap, resulting in a complicated waveform, so the two reflected waves a and b are It becomes impossible to separate. In addition, since the phase of each frequency component of a reflected wave from a position deviated to some extent from the focus position of the acoustic lens 3, that is, a reflected wave in a defocused state, changes in a complicated manner, the waveform of the sample shown in FIG. 6(b) is Like the reflected wave from the back surface, the waveform is not a pulse wave, and it is difficult to clearly define the peak position.

For example, even if the transmitting signal shown in FIG. 6(a) is driven by a pulse wave having an extremely wide band and a resolution of several nanoseconds, the time resolution of the received signal will be extremely poor. Furthermore, many of the pulse transducers currently on the market have a center frequency of 30 to 00 MHz, and in most cases, a spread (tailing) of 2 to 3 waves or more occurs. Therefore, the limit to which different reflected waves can be resolved without complexly overlapping is about 20 nS, and in some cases, 1
Exceeds 00nS. In addition, the sound velocity of the sample was set to 5000 m/S.
Then, the round trip time is 100 nS and 0.25 mm,
Even at 20 nS, it becomes [1.05 mm], making it impossible to obtain sufficient resolution in the depth direction.

The present invention was made in view of these circumstances.
An object of the present invention is to provide an ultrasonic microscope in which a transmitted pulse wave can be accurately restored and the resolution in the depth direction of a sample can be improved.

[Means to solve the problem]

In order to solve the above problems, the present invention focuses an ultrasonic pulse with an acoustic lens and makes it incident on a sample, scans the sample in plane with this ultrasonic pulse, and returns the reflected wave from the sample to the acoustic lens. In an ultrasonic microscope that receives the electrical signal and converts it into an electrical signal, creates observation image data of the sample from this electrical signal, and visualizes this observation image data, an arbitrary part of the electrical signal is sampled in an arbitrary time width. means for extracting a reflected wave from an arbitrary position in the depth direction of the sample; A/D conversion means for converting the output of the extraction means into a digital signal; and Fourier converting the output of the A/D conversion means. Relative position information between the acoustic lens and the sample 5 Reflection position information on the sample of the reflected wave extracted by the extraction means, density and sound velocity of the sample, reflected wave from the sample surface and the sample Time difference of reflected waves from inside.

From the sunspot distance of the acoustic lens in the ultrasonic propagation medium located between the acoustic lens and the sample, the absorption coefficient of the ultrasonic propagation medium, the absorption coefficient of the sample, and the pupil function of the acoustic lens, the electric means for calculating a transfer function regarding a passage path of the ultrasonic pulse converted into a signal, dividing the Fourier-transformed value by this transfer function, and further performing inverse Fourier-transformation on the division result to create observation image data; The configuration includes:

[For production]

By taking the above-described measures, the present invention makes focused ultrasonic waves incident on a sample, and the ultrasonic waves reflected from the sample are converted into electrical signals. From this electric signal, a reflected wave from an arbitrary portion, that is, an arbitrary depth position of the sample, is extracted by an extraction means. This extracted reflected wave signal is A/D
After being converted into a digital signal by a converter, it is Fourier transformed. On the other hand, the reflection position information on the sample of the reflected wave extracted by the acoustic lens and the sample t-direction position information power 11 output means, the density and sound velocity of the sample, the reflected wave from the sample surface and the reflected wave from inside the sample. The electric signal is determined from the time difference of A transfer function regarding the passage path of the converted ultrasonic pulse is calculated, and the Fourier-transformed value is divided by this transfer function.

This division result is then subjected to inverse Fourier transform to create observation image data. According to the observation image data created in this way, the temporal spread is corrected by the transfer function, so the waveform of the ultrasonic pulse incident on the sample can be accurately restored, and the temporal resolution, that is, the depth direction of the sample The resolution of the image will be improved.

〔Example〕

Before explaining the present invention in detail, the concept of the present invention will be explained with reference to FIG. Considering this type of device as a transmission system with a transmitted pulse wave P as an input and a received wave W as an output as shown in Fig. The idea is to restore a high-resolution transmitted waveform from a received waveform with a low time resolution using a so-called deconvolution method.

Examples of the present invention will be described below.

FIG. 1 is a diagram showing the configuration of an ultrasound microscope according to an embodiment of the present invention. Reference numeral 21 shown in the figure is a transmitting circuit, and a transmitting pulse signal is generated from this transmitting circuit 21 and applied to the transducer 22. Transducer 22 converts the transmitted pulse signal into ultrasound pulses. This ultrasonic pulse is focused by an acoustic lens 23, and a sample 24 is placed at or near the focused position. The space between the acoustic lens 23 and the sample 24 is filled with water 25 as a Kabra liquid. The XYZ stage 26 moves the sample 24 in the direction of incidence of the ultrasonic pulse (two directions) and scans and drives it two-dimensionally within a plane (XY plane) formed by two axes x+Y orthogonal to the Z direction. .XYZ stage 26
The position in each direction is detected by the position detection circuit 27.

The reflected wave from the sample 24 passes through the acoustic lens 23 again, is converted into an electrical signal by the transducer 22, and is input to the receiving circuit 28. The waveform of the output signal of the receiving circuit 28 is displayed on the monitor 29. The waveforms displayed on the two monitors include those on the sample surface and inside the sample.

Contains reflected waves from the back surface of the sample, etc. An arbitrary portion of the received signal displayed on the monitor 29 is extracted by the gate circuit 30. The output of the gate circuit 30 is converted into a digital signal by a high-speed digitizer 31, and then sent to the arithmetic circuit 3.
2. Manpower. The arithmetic circuit 32 performs Fourier transform on the manually input digital signal, and divides this transform value by a transfer function to be described later.

This division result is inverse Fourier transformed and digital memory 3
Output to 3. Note that parameters necessary for calculating the transfer function are input from the position detection circuit 27 and the human power device 34. The control circuit 35 reads out the observed image data stored in the digital memory 33 and displays it on the display device 36 as a visible image.

Here, assuming that the receiving circuit 28 and other electrical systems have a sufficiently wide band, the transfer function H(f) can be roughly divided into two components.

One is the part caused by the transducer 22,
It has narrow band characteristics as shown in FIG.

Let this be H,,(f). Let the central axis of the acoustic lens 23 be two axes, and the distance from the lens center point in a plane perpendicular to this Z ITo be r. Also, the speed of sound in water is CW
, a certain frequency within the band of the transducer 22 as f. Then, the propagation vector k. W is kow””2π fo
/Cw, and if F is the sunspot distance of the acoustic lens 23 underwater, then the angular spectrum is k.

k　r　　　　(k　ow/　F　)　r can be defined.

The output V+0(Z) of the transducer 22 when the acoustic lens 23 approaches the sample 24 by 2 from the focused state is Vto (z)=CfP2 (k,)R(k,)(k,
/kz )cxp (j 2kz z)dk.

...(1) It can be expressed as.

Here, kz=kow''-, 'P(k,) is the pupil function of the acoustic lens, R(k,) is the reflection function of the sample, C
is a normalization constant.

When the surface of the perfect reflector is in focus, zmO, so the output of the transducer 22 is V+o(0)-
It can be expressed as ClF3 (k,) (k, /kz)dk, (2).

P(k,) is the amplitude of the pupil of the acoustic lens 23! If it is a 4-rate distribution and there is no change in phase, ■ (○) has the same value for all ultrasonic frequencies.

Therefore, if the output of the transducer 22 is measured while changing the ultrasonic frequency, the transducer 22
A characteristic curve proportional to the frequency characteristic H, d(f) of is obtained.

Note that instead of sweeping the frequency, the transmitted pulse wave P and received wave W shown in FIG. 2 are Fourier transformed in a focused state,
A similar characteristic curve can be obtained by calculating the ratio.

However, in any case, the frequency characteristics obtained include the influence of absorption by water, and are expressed as A 'Ho(f)'' Hw (f) −(
3). In addition, A is a proportionality constant, the term Hw(f) of absorption by water is Hw - eXp (-2Fawf2), and aw is an absorption coefficient of water. Therefore, the result of measuring the output of the transducer 22 is divided by Hw(f) to obtain a value that takes into account the influence of absorption by water.

Now, another component of the transfer function H(f) is (3) as an effect of the received signal being an opposite wave at a certain defocus state at a given depth on the surface of the sample or inside the sample. It is the part that should be added to the equation.

FIG. 3 shows the positional relationship between the acoustic lens 23 and the sample f-124, and shows a state in which the focal point of the acoustic lens 23 has moved relatively closer to the sample surface by a distance of -2. . Further, M is a reflector existing within the sample, and its reflecting surface is separated from the sample surface by a distance M-q. In this state, the output of the transducer 22 is expressed as −exp[2jk,=q]exp[−2jk,Z]dk by extending equation (1) for a certain frequency f0.
,... (4) becomes. Here, kZ ”” )Cos' −kt
'kos-2πfo/Cs, Cs is the sound velocity in the sample. Therefore, exp(j2kz q) represents a phase change due to the incident ultrasonic wave reciprocating to the reflector M within the sample. Further, T(k,) indicates the dependence of the transmittance on the incident angle when the ultrasonic wave is transmitted from water to the sample. T'
(k,) shows the dependence of the transmittance on the angle of incidence when ultrasonic waves are transmitted from the sample to water. 2 and q can be measured, and T and T' can be determined by knowing the density of sample 24 and the speed of sound, r WAVES IN LAYE! ? EDM
EDIAJ (Second EdItlon byL
, M Brekhovsklkh, Academlc.
Press, 1980). The pupil function P of the acoustic lens 23 can be known at the lens design stage and can be measured even after that. Also, the reflection function R′ of the internal reflection surface of the sample
can be determined by calculation if the density of the material forming the reflective surface and the speed of sound are known.

As mentioned above, the frequency f. In contrast, V+o(z) can be calculated for arbitrary z and q. Therefore, Vfo (
z)+Vf+ (z)+Vf2 (z)...
By calculating for all frequencies fo, f+, fz, . . . in the band, the portion added to H, (f) of the transfer function can be found. Letting this part, that is, the set of V (z) for all f, be HIN (f), the transfer function H (f) is obtained by combining equation (3) and H + N (f), ``H (f) = A- H, (f) ・H+N(
f)... (5) It is expressed as follows. Furthermore, considering absorption, H(f) −A−Ho(f) ・H+N (f)
・H, bCf) ... (6)
It can be found as Note that H,, (f) is H-
b(f)-eXp (-2(F-z)awf2-2Q
α5f2) -exp [-2f2((F-z) aw+qα5)
]...(7) Note that αS is the absorption coefficient of the sample. This value is known for each sample.

The transfer function H(f) is obtained in the above manner.

Next, the operation of this embodiment will be explained.

First, without looking at the waveform of the received signal displayed on the monitor 29, the operator adjusts the position of the XYZ stage 26 in the Z direction so that the black spot of the acoustic lens 23 is aligned with the sample surface. Next, the position detection circuit 27 is reset and the sample 24
The position in the Z direction is adjusted so that the part of the reflected wave from the part to be observed appears sufficiently large on the monitor 29. At this point, the control circuit 35 receives the distance Z from the position detection circuit 27.
is input.

Next, the timing and gate width of the gate signal are set so that the necessary portion of the received signal can be extracted. In addition, the density and sound velocity of the sample body 24 and the internal reflector M are input from the input device 34, the time difference τ between the surface reflected wave and the internal reflected wave by observing the waveform from the monitor, and the underwater focal length MF of the acoustic lens 23.
, the absorption coefficient α of water, the absorption coefficient αS of the sample, and the pupil function P of the acoustic lens 23 are input, and the transfer function H(f) is calculated. Note that the depth q of the reflector M is determined by τ and the sound velocity C of the sample body.
3, we can find q”Cs τ/2.

Next, a transmission pulse signal is generated in the same manner as in the ultrasonic microscope described in FIG. is input, and the necessary reflected waves are extracted by the gate signal set as above. The output of the gate circuit 30 is sampled sufficiently densely by a digitizer 31, A/D converted, and outputted to the arithmetic circuit 32 as digital waveform data. Arithmetic circuit 3
2 performs Fourier transform on the input digital waveform data, divides this transform value by the transfer function H(f), and further performs inverse Fourier transform on this division result.

By performing such arithmetic processing, for example, the result shown in Fig. 4 (
The reflected waves a and b from the reflectors at slightly different depths within the sample, as shown in a), overlap to form a complex waveform as shown in FIG. The deconvolution operation is performed by f), and the transmitted pulse wave is restored to an accurate depth position as shown in FIG.

The digital waveform data (observed image data) thus restored is stored in the digital memory 33 as is or after processing such as encoding the peak position and magnitude. Then, by driving the XYZ stage 26 to scan the sample 24 in a two-dimensional plane, observation image data inside the sample is stored in the digital memory 33 with extremely high resolution. By reading the observation image data stored in the digital memory 33 by the control circuit 35 and displaying it on the display device 36, it is possible to observe an extremely accurately reconstructed image of a predetermined portion of the sample.

According to this embodiment, the light is incident on the sample 24 based on the depth position q of the reflector M in the sample, the material of the sample 24, the focus state of the acoustic lens 23, etc., and is converted into an electrical signal by the transducer 22. The transfer function H(
f), and the reflected wave at an arbitrary depth extracted by the gate circuit 30 is restored by the deconvolution method in the arithmetic circuit 32, so that the restored transmission pulse waveform can be placed at an accurate depth position. can be restored, and the resolution in the depth direction of the sample can be significantly improved.

Note that the present invention is not limited to the above embodiment. For example, in the above embodiment, the transfer function H(f) is simply used as an inverse filter, but it may be modified into a Wiener filter in consideration of the S/N ratio.

〔Effect of the invention〕

As detailed above, according to the present invention, the transmitted waveform is restored from the transfer function of the passage path of the ultrasonic pulse incident on the sample, so the transmitted pulse wave is accurately restored,
It is possible to image an ultrasonic microscope that can improve the resolution of the sample in the depth direction.

[Brief explanation of drawings]

Figure 1 is a block diagram of an embodiment of the present invention, Figure 2 is a diagram for explaining the invention in detail, Figure 3 is a diagram showing the focus state of the acoustic lens on a sample, and Figure 4 (a). is a waveform diagram of two reflected waves reflected from slightly different reflectors, and (b) is a waveform diagram of two reflected waves overlapping each other.
Figure (c) is a waveform diagram of the restored reflected wave, Figure 5 is a configuration diagram of a conventional ultrasound microscope, Figure 6 is an operational diagram,
FIG. 7 is a waveform diagram of two reflected waves reflected from an extractor by a gate signal with a narrow gate width, and FIG. 7(b) is a diagram showing a state in which the two reflected waves overlap. 21... Transmission circuit, 22... Transducer, 2
3... Acoustic lens, 24... Sample, 27... Position detection circuit,... Receiving circuit, O... Gate circuit, 2... Arithmetic circuit, 34... Input device, 5. ...Control circuit.

Claims (1)

[Claims] Ultrasonic pulses are focused by an acoustic lens and made incident on a sample, the sample is plane-scanned by the ultrasonic pulses, and reflected waves from the sample are received again by the acoustic lens and converted into electrical signals. In an ultrasonic microscope that converts the electric signal into observation image data of the sample and visualizes the observation image data, an arbitrary part of the electric signal is sampled in an arbitrary time width to determine the depth of the sample. means for extracting a reflected wave from an arbitrary position in the horizontal direction; A/D conversion means for converting the output of this extraction means into a digital signal; and Fourier transformation of the output of this A/D conversion means, and and the relative position information of the sample, the reflection position information on the sample of the reflected wave extracted by the extraction means, the density and sound velocity of the sample, the reflected wave from the sample surface and the reflected wave from inside the sample. a time difference, a focal length of an acoustic lens in an ultrasound propagation medium located between the acoustic lens and the sample, an absorption coefficient of the ultrasound propagation medium;
From the absorption coefficient of the sample and the pupil function of the acoustic lens,
A transfer function regarding the passage path of the ultrasonic pulse converted to the electrical signal is calculated, the Fourier-transformed value is divided by this transfer function, and the division result is inversely Fourier-transformed to create observation image data. An ultrasonic microscope characterized by comprising means.