TW202403303A - Ultrasonic imaging device and method for removing noise of reflected wave capable of generating highly accurate image information of a bonding interface of an object by generating pixelized information using a waveform from which noise is removed - Google Patents

Abstract

The present invention provides an ultrasonic imaging device capable of generating highly accurate image information of a bonding interface of an object by generating pixelized information using a waveform from which noise is removed. An ultrasonic imaging device 100 irradiates an object formed by laminating a plurality of layers with an ultrasonic wave to obtain a reflected wave of a bonding interface of the object, and generates an image of the bonding interface based on signal intensity of the reflected wave. The ultrasonic imaging device 100 includes: a matching processing unit 56 for correcting a phase shift of a second reflected wave from a second irradiation point in the object with respect to a first reflected wave from a first irradiation point in the object; and a pixelized information generation processing unit (averaging processing unit) 57 for removing noise from the first reflected wave based on the first reflected wave and the second reflected wave after phase correction, and generating pixelized information of a portion of the bonding interface that returns an interface echo based on signal intensity of the interface echo indicating a waveform from the bonding interface in the first reflected wave from which the noise is removed.

Description

Ultrasonic imaging device and method for removing reflected wave noise

The present invention relates to an ultrasonic imaging device and a method for removing reflected wave noise.

The ultrasonic imaging device irradiates ultrasonic waves from a probe to an inspection object (subject), receives the reflected waves, and images the object interface. In ultrasonic inspection devices that repeatedly transmit and receive ultrasonic waves to an object to be inspected, a method of removing noise by averaging similar reflected waves has been proposed as a method of clarifying the waveform of the object.

Patent Document 1 discloses an ultrasonic inspection device, which includes: an ultrasonic transmitter that repeatedly transmits ultrasonic waves to an inspection target; and an ultrasonic receiver that repeatedly transmits ultrasonic waves from the ultrasonic transmitter to Reception of the inspection target ultrasonic wave propagated in the inspection object; a repetition interval setting unit that sets the repetition interval of the ultrasonic wave sent by the ultrasonic transmitting unit, and causes the repetition every time the ultrasonic wave transmission by the ultrasonic transmitting unit is repeated. The interval increases or decreases by a certain offset each time; and an averaging processing unit synchronizes the ultrasonic transmission start point of the ultrasonic transmitting unit to average the inspection target ultrasonic waves repeatedly received by the ultrasonic receiving unit. ; The offset amount is a value that satisfies the condition that the index indicating the absolute value of the ratio of the reverberation component superimposed on the ultrasonic wave of the inspection object before and after averaging by the averaging processing unit is the minimum or less than a predetermined value. [Prior technical literature] [Patent Document]

[Patent Document 1] Japanese Patent Application Publication No. 2013-29396

[Problem to be solved by the invention]

In the technology disclosed in Patent Document 1, the influence of jitter (fluctuation in the time axis direction) is not considered. In an ultrasonic imaging device, an object is irradiated with ultrasonic waves to obtain its reflected waves. However, the phase of these reflected waves is shifted due to jitter. When averaging is performed in a state where a phase shift occurs, the signal strength of the necessary waveform may be weakened. As a result, defects that should be discovered are ignored.

The present invention was completed in view of the above-mentioned problems, and its purpose is to provide an ultrasonic imaging device and a method for removing noise from reflected waves, using the waveform after noise removal to generate pixelated information, thereby being able to generate the height of the joint interface of the object. Accurate image information. [Technical means to solve problems]

In order to achieve the above object, the ultrasonic imaging device of the present invention irradiates ultrasonic waves to a multi-layered object to obtain the reflected wave of the joint interface of the object, and generates the joint interface based on the signal strength of the reflected wave. The image further includes a matching processing unit that performs a phase shift of the second reflected wave from the second irradiation point on the object with respect to the phase of the first reflected wave from the first irradiation point on the object. Correction; and a pixelation information generation processing unit, which removes noise from the above-mentioned first reflected wave based on the above-mentioned first reflected wave and the above-mentioned second reflected wave after phase correction, and based on the above-mentioned first reflected wave after removing noise The signal strength of the interface echo from the waveform of the above-mentioned joint interface is represented, and pixelated information is generated that returns the portion of the above-mentioned interface echo in the above-mentioned joint interface. The above-mentioned matching processing unit is based on the above-mentioned interface echo of the above-mentioned first reflected wave. When the signal strength is greater than or equal to a threshold value indicating a predetermined value, the phase of the peak portion of the interface echo of the second reflected wave is made to coincide with the phase of the peak portion of the interface echo of the first reflected wave. When the signal intensity of the interface echo of the first reflected wave does not exceed the threshold, the phase of the peak of the surface echo of the second reflected wave is made equal to that of the first reflected wave. The process of making the peaks of the above-mentioned surface echoes consistent in phase. Other aspects of the present invention will be described in the embodiments described below. [Effects of the invention]

According to the present invention, the waveform after removing noise is used to generate pixelated information, thereby making it possible to generate high-precision image information of the joint interface of a multi-layered object.

Embodiments for carrying out the present invention will be described in detail with appropriate reference to the drawings. FIG. 1 is a diagram showing the structure of the ultrasonic imaging device 100 according to this embodiment. Figure 2 is a diagram showing interface echo and noise as reflected waves. The ultrasonic imaging device 100 is used to detect defects in the bonding interface of an object formed by laminating multiple layers of semiconductors or the like.

The ultrasonic imaging device 100 is configured to include a control unit 50 , a scanning unit 70 (mechanical unit), and an ultrasonic probe 20 . The ultrasonic imaging device 100 irradiates ultrasonic waves through a probe to irradiation points set at predetermined intervals within the inspection range of the object to obtain the reflected waves, and extracts from the reflected waves a waveform representing the joint interface to be inspected. Interface echo is a process that converts the signal intensity of the interface echo into a positive integer value (0~255) to generate pixelated information for all illumination points or specific illumination points, and based on the pixels of the generated illumination point The information is used to generate an image of the joint interface to find defects.

However, in the reflected wave, as shown in the waveform data 31 in Fig. 2, in addition to the surface echo and the interface echo, noise such as electrical noise is sometimes superimposed. This noise affects the pixelated information generated by the interface echo, so it is necessary to remove the noise as much as possible.

Returning to FIG. 1 , the ultrasonic probe 20 includes an encoder 21 for detecting the scanning position of the ultrasonic probe 20 and a piezoelectric element 22 for converting electrical signals and ultrasonic signals to each other. The piezoelectric element 22 is a single focus type ultrasonic sensor.

The control unit 50 includes a scan control unit 51 that controls the scanning position of the ultrasonic probe 20 , a transmission and reception control unit 52 that controls the transmission and reception of ultrasonic waves, and an image generation unit 53 that is based on pixelation generated by the averaging processing unit 57 information to generate an ultrasonic image; the matching processing unit 56 corrects the second reflected wave from the second irradiation point in the subject 15 (object) relative to the second reflected wave from the first irradiation point in the subject 15 1. Shift of the phase of the reflected wave; the averaging processing unit 57 (pixelated information generation processing unit) removes noise from the first reflected wave based on the first reflected wave and the phase-corrected second reflected wave, and removes noise based on the noise removal. The signal strength of the interface echo representing the waveform from the joint interface in the first reflected wave after the signal is generated to generate pixelated information of the location where the interface echo in the joint interface is returned; the storage unit 58; and the transceiver unit 60, etc. In addition, the first irradiation point and the second irradiation point will be described later with reference to FIG. 3 . The averaging process will also be described later.

Although not shown in the figure, the transceiver unit 60 includes a transmitter, an amplifier that amplifies the received signal received by the ultrasonic probe 20, an A/D converter that converts the received signal from an analog signal to a digital signal, and the received signal. Signal processing section for signal processing, etc.

The scan control unit 51 is connected to the mechanical control device 77 so that input and output are possible. The scanning control unit 51 controls the scanning position of the ultrasonic probe 20 through the mechanical control device 77 , the X-axis scanner 71 , the Y-axis scanner 72 and the Z-axis scanner 73 , and receives the current position of the ultrasonic probe 20 from the mechanical control device 77 Scan location information.

The output side of the mechanical control device 77 is connected to the X-axis scanner 71 , the Y-axis scanner 72 and the Z-axis scanner 73 . The output side of the encoder 21 of the ultrasonic probe 20 is connected to the mechanical control device 77 . The mechanical control device 77 detects the scanning position of the ultrasonic probe 20 according to the output signal of the encoder 21, and controls it through the X-axis scanner 71, the Y-axis scanner 72 and the Z-axis scanner 73, so that the ultrasonic probe 20 Become the indicated scanning position. The mechanical control device 77 receives the control instructions of the ultrasonic probe 20 from the scanning control unit 51 and responds to the scanning position information of the ultrasonic probe 20 .

The piezoelectric element 22 has electrodes mounted on both sides of the piezoelectric film, and is made of zinc oxide (ZnO), ceramics, fluorine-based copolymer, or the like. The piezoelectric element 22 emits ultrasonic waves from the piezoelectric film by applying a voltage between two electrodes. Furthermore, the piezoelectric element 22 converts the echo (reception wave) received by the piezoelectric film into a voltage generated between the two electrodes, that is, a reception signal.

Water 11 is poured into the water tank 10, and the subject 15 is placed in the water 11 in a submerged state. The water 11 in the water tank 10 is a liquid substance that is a propagation medium required to efficiently propagate the ultrasonic waves emitted from the opening surface at the lower end of the ultrasonic probe 20 (ultrasonic probe) into the subject 15 . The subject 15 is, for example, a semiconductor package including a wafer, a multilayer structure (or a built-up structure), or the like.

The ultrasonic probe 20 is immersed in the water 11 that fills the water tank 10 and is arranged to face the upper part of the subject 15 at a predetermined distance in the Z direction.

The ultrasonic probe 20 can be freely moved in the XYZ direction by the scanning unit 70 . For example, the ultrasonic probe 20 irradiates the subject 15 with ultrasonic waves while scanning along the X-axis direction at a predetermined speed from a starting point (one end point) to an end point (the other end point) of the subject 15 . When the ultrasonic probe 20 reaches the end point, the probe is moved in the Y-axis direction by a predetermined amount, and scans in the X-axis direction from the starting point to the end point in the opposite direction at a predetermined speed.

Based on this movement, the ultrasonic probe 20 scans a predetermined measurement range on the surface of the subject 15, sends ultrasonic waves, and receives reflected echoes at a plurality of preset measurement points within the measurement range. Defects in the internal structure included in the measurement range are imaged and inspected.

As shown in FIG. 1 , the noise removal process of the reflected wave in this embodiment is characterized by using two processes (matching processing) in the matching processing unit 56 and processing (averaging processing) in the averaging processing unit 57 . processed in stages.

In the averaging processing unit 57 of FIG. 1 , noise is removed by an additive averaging method (described in detail later) that adds the reflected waves obtained from a plurality of irradiation points and divides them by the number of irradiation points after addition. (Noise removal step). Noise is generated randomly and uncorrelated in time in each reflected wave, but the interface echo is acquired at the same time. Therefore, if these reflected waves are added together, the interface echo becomes n times (n is the number of irradiation points after addition), taking advantage of the fact that the noise becomes smaller. And, by averaging the results, the interface echo becomes clear.

FIG. 3 is an irradiation point arrangement diagram, showing the arrangement of the first irradiation point and the second irradiation point. In FIG. 3 , the irradiation point for which the pixelation information is obtained (pixelated irradiation point) is represented by ● as the first irradiation point, and ○ represents the other irradiation points (non-pixelated irradiation point) as the second irradiation point. In this embodiment, by averaging processing, addition and averaging processing are performed as follows.

The control unit 50 selects one or more irradiation points adjacent to the first irradiation point and sets the second irradiation point used in the averaging process. The effect of sharpening is obtained by using adjacent illumination points because the summed reflected waves have similar properties to each other. That is, in the ultrasonic imaging device 100, in most cases, the scanning pitch of the scanner is set to be smaller than the focus size of the ultrasonic wave. As a result, the difference in acquired reflected waves caused by a difference of one pitch is extremely small. If the added reflected waves have similar properties to each other, they will not cancel each other out due to the addition. On the other hand, random noise becomes smaller by adding up in time, so a sharpening effect can be obtained. In the embodiment, when P0 is set as the first irradiation point in FIG. 3 , one or more second irradiation points for averaging processing are selected from P1 to P8. In addition, the greater the number of selections, the clearer the pixelated information can be obtained, but since the processing time also becomes longer, the setting is made taking into account the overall processing time.

FIG. 4 is a diagram showing an example of averaging processing (noise removal processing). For example, the averaging process based on eight second irradiation points (P1 to P8) adjacent to the first irradiation point (P0) is performed as follows. The reflected waves in P0 and P1 to P8 are stored in the storage unit 58 as the time axis value (the time based on the time when the reflected wave is obtained) and the time series data of the signal intensity (time, signal intensity) (see FIG. 1 ) . Add the signal strength of the time series data of P0 and the signal strength of the time series data from P1 to P8 at the same time, and divide it by the number of irradiation points after adding this value (the number of the first irradiation point is the same as the number of the second irradiation point). The sum of the number of irradiation points) is 9 for averaging. This processing is performed on the signal intensity at all moments in the time series data to remove noise and clarify the interface echo of the reflected wave at the first irradiation point, thereby obtaining the signal intensity.

The averaging process in this embodiment has been described above, but in actual processing, the time element is taken into consideration. That is, it is necessary to deal with the influence of fluctuations in the direction of the time axis, that is, jitter. In the reflected wave obtained by irradiating the object with ultrasonic waves, slight changes occur in the time axis direction of the waveform. That is, these small changes in the time axis are inherent to each reflected wave. Therefore, when the reflected wave from the pixelated irradiation point is used as the basis, the reflection wave from the non-pixelated irradiation point used in the averaging process is generated. Phase shift. If the averaging process is performed in a state where a phase shift occurs, the waveforms of the interface echoes that should generate pixelated information will not be properly added.

FIG. 5 is a diagram showing an example of matching processing. The waveform data 32 is the waveform before the matching process is performed, and the waveform data 33 is the waveform after the matching process is performed. In this embodiment, the matching process shown in FIG. 5 is used. The reflected wave from the pixelated illumination point is used as the template signal, so that the phase of the reflected wave from the non-pixelated illumination point is consistent with the template signal. Specifically, the peak of the interface echo from the object joint interface in the reflected wave from the non-pixelated irradiation point that is the object of the averaging process and the peak of the interface echo from the object joint interface in the template signal are Adjust the position of the time axis in a consistent manner to eliminate phase deviation.

However, this method can cope with the situation where the signal strength of the interface echo from the object joint interface is large enough, but when the signal strength is small, it is difficult to extract the interface echo as the object. Therefore, in this case, the surface echo that can be extracted first among the reflected waves is used. That is, the position of the time axis is adjusted so that the peak of the surface echo in the reflected wave from the non-pixelized irradiation point that is the object of the averaging process coincides with the peak of the surface echo of the average signal, and the phase difference is eliminated. offset.

In addition, in the matching process, whether to use the interface echo from the object joint interface or the surface echo is determined based on whether the signal strength of the interface echo is above a preset threshold. When the signal strength of the interface echo is above the preset threshold, the interface echo is used for matching processing. When the signal strength of the interface echo is less than the preset threshold, the surface echo is used for matching processing. . The threshold is set to an appropriate value through experimentation. In addition, as shown in FIG. 1 , the matching process is performed as a preprocessing of the averaging process.

FIG. 6 is a flowchart showing the noise removal processing of reflected waves. First, the control unit 50 determines the number and position of the second irradiation points, and selects the second irradiation point (process S51). The control unit 50 determines whether the processing of all the first irradiation points has been completed (processing S52). If the processing of all the first irradiation points has not been completed (processing S52, No), the control unit 50 proceeds to processing S53. When the processing of the irradiation point is completed (Yes in processing S52), the process proceeds to processing S57.

The control unit 50 aligns the probe with the irradiation position, transmits and receives ultrasonic waves to the object (process S53), and stores the reflected waves as time series data in the storage unit 58 (process S54). Then, the control unit 50 performs matching processing on the first reflected wave and the second reflected wave (processing S55), and then performs averaging processing (processing S56), and returns to processing S52.

In process S57, the image generation process of all the first irradiation points is performed. Specifically, pixelation information is generated by converting the signal strength of the sharpened interface echo into a positive integer value. These processes are repeatedly performed on the entire inspection range of the target object to visualize the joint interface.

In process S54, when the object is irradiated with ultrasonic waves and all the second irradiation points are located in front of the first irradiation point, when the ultrasonic transmission and reception processing for the first irradiation point is completed, the second irradiation point is Click to perform matching processing, and then perform averaging processing.

In addition, when the second irradiation point is also located after the first irradiation point, matching processing and averaging processing are performed at the time when the ultrasonic transmission and reception processing for all related second irradiation points is completed.

FIG. 7 is a diagram showing waveform data 31A after averaging processing. FIG. 7 shows the result of performing matching processing and averaging processing on the waveform data 31 shown in FIG. 2 . The signals of surface echo and interface echo originating from the sample (from the subject 15) will not disappear even if average processing is performed. On the other hand, noise components that do not originate from the sample and are randomly generated in time are averaged and their intensity is reduced.

FIG. 8 is a diagram showing an image before averaging processing. FIG. 9 is a diagram showing an image after averaging processing. In the image generated without performing the matching processing and averaging processing shown in Figure 8, the defect is imaged in the field of view, but the surrounding noise level is high relative to the brightness value of the defective part, so it is visually recognized The performance and contrast become lower.

In contrast, in the image generated by performing the matching process and averaging process shown in Figure 9, the signal of the defect does not disappear and the surrounding noise level is reduced, so higher visibility and contrast can be obtained. .

As mentioned above, the ultrasonic imaging device 100 of this embodiment has the following characteristics. (1) An ultrasonic imaging device 100, characterized by: irradiating ultrasonic waves to a multi-layered object to obtain reflected waves at the joint interface of the object, and generating an image of the joint interface based on the signal strength of the reflected waves. , and has a matching processing unit 56 that compares the second reflected wave from the second irradiation point (for example, P1 to P8) in the object with respect to the second reflected wave from the first irradiation point (for example, P0) in the object. Correcting the phase shift of the first reflected wave; and a pixelated information generation processing unit (for example, the averaging processing unit 57), which removes noise from the first reflected wave based on the first reflected wave and the phase-corrected second reflected wave. The signal is generated, and based on the signal strength of the interface echo representing the waveform from the bonding interface in the first reflected wave after removing the noise, pixelated information that returns the location of the interface echo in the bonding interface is generated. In this way, in an ultrasonic imaging device, the waveform after removing noise is used to generate pixelated information, thereby generating high-precision image information of the joint interface of the object formed by stacking multiple layers.

(2) In (1), when the signal intensity of the interface echo of the first reflected wave is greater than or equal to a threshold value indicating a predetermined value, the matching processing unit 56 performs a peak portion of the interface echo of the second reflected wave. Processing in which the phase is consistent with the phase of the peak of the interface echo of the first reflected wave is performed to make the surface echo of the second reflected wave occur when the signal intensity of the interface echo of the first reflected wave does not exceed the threshold. The phase of the peak part is consistent with the phase of the peak part of the surface echo of the first reflected wave.

(3) In (2), the pixelation information generation processing unit performs an additive average on the first reflected wave and the second reflected wave after phase correction with the first reflected wave.

(4) In (1), the matching processing unit 56 selects one or more irradiation points at predetermined positions from the irradiation points adjacent to the first irradiation point to set the second irradiation point (for example, process S51, see FIG. 3 ).

(5) A method for removing noise from reflected waves, which is characterized in that it is a method for removing noise from reflected waves in the ultrasonic imaging device 100, which irradiates an object formed by laminating multiple layers with ultrasonic waves to obtain the joint interface of the object. The reflected wave is used to generate an image of the joint interface based on the signal strength of the reflected wave. The noise removal method of the reflected wave includes: a phase correction step (for example, processing S55, refer to Figure 5), which is performed by The first reflected wave from the first irradiation point in the object and the second reflected wave from the second irradiation point in the object are matched to correct the phase shift of the second reflected wave relative to the first reflected wave; and noise removal Step (for example, process S56, see FIG. 4), which removes noise from the first reflected wave based on the first reflected wave and the second reflected wave that is phase-corrected through the phase correction step.

In addition, the present invention is not limited to the above-described embodiment, and includes various modifications. In addition, the above-mentioned embodiment is described in detail in order to explain the present invention easily, and is not necessarily limited to having all the described configurations. In addition, regarding part of the components of each embodiment, addition, deletion, and replacement of other components can be performed.

10:sink 11:water 15: Subject (object) 20: Ultrasonic probe 21:Encoder 22: Piezoelectric element 31,31A,32,33: Waveform data 50:Control Department 51:Scan control department 52: Transceiver control department 53:Image generation department 56: Matching processing department 57: Averaging processing unit (pixelation information generation processing unit) 58:Storage Department 60:Sending and receiving department 70: Scanning Department (Mechanical Department) 71:X-axis scanner 72:Y-axis scanner 73:Z-axis scanner 77: Mechanical control device 100: Ultrasound imaging device P0: The first irradiation point P1~P8: The second irradiation point

FIG. 1 is a diagram showing the structure of the ultrasonic imaging device according to this embodiment. Figure 2 is a diagram showing interface echo and noise as reflected waves. FIG. 3 is an irradiation point arrangement diagram, showing the arrangement of the first irradiation point and the second irradiation point. FIG. 4 is a diagram showing an example of averaging processing. FIG. 5 is a diagram showing an example of matching processing. FIG. 6 is a flowchart showing the noise removal processing of reflected waves. Figure 7 is a diagram showing waveform data after averaging processing. FIG. 8 is a diagram showing an image before averaging processing. FIG. 9 is a diagram showing an image after averaging processing.

10:sink

11:water

15: Subject (object)

20: Ultrasonic probe

21:Encoder

22: Piezoelectric element

50:Control Department

51:Scan control department

52: Transceiver control department

53:Image generation department

56: Matching processing department

57: Averaging processing unit (pixelation information generation processing unit)

58:Storage Department

60:Sending and receiving department

70: Scanning Department (Mechanical Department)

71:X-axis scanner

72:Y-axis scanner

73:Z-axis scanner

77: Mechanical control device

100: Ultrasound imaging device

Claims (6)

An ultrasonic imaging device, characterized in that it irradiates an object formed of a plurality of layers with ultrasonic waves to obtain reflected waves at the joint interface of the object, and generates an image of the joint interface based on the signal strength of the reflected wave. and has: a matching processing unit that corrects the phase shift of the second reflected wave from the second irradiation point in the object with respect to the first reflected wave from the first irradiation point in the object; and A pixelated information generation processing unit that removes noise from the first reflected wave based on the first reflected wave and the phase-corrected second reflected wave, and based on the representation in the first reflected wave after removing noise from the above The signal strength of the interface echo in the waveform of the bonding interface generates and returns pixelated information of the location of the interface echo in the bonding interface. The above matching processing department When the signal intensity of the interface echo of the first reflected wave is greater than or equal to a threshold indicating a predetermined value, the phase of the peak portion of the interface echo of the second reflected wave is adjusted to the phase of the first reflected wave. The processing of the phase consistency of the peaks of the above-mentioned interface echo, When the signal intensity of the interface echo of the first reflected wave does not exceed the threshold value, the phase of the peak of the surface echo of the second reflected wave is adjusted to the phase of the surface echo of the first reflected wave. The processing of the phase consistency of the peak part of the echo. The ultrasonic imaging device of claim 1, wherein The pixelation information generation processing unit performs an additive average on the first reflected wave and the second reflected wave that has been phase-corrected with the first reflected wave. The ultrasonic imaging device of claim 1, wherein The matching processing unit selects one or more irradiation points at a certain position from the irradiation points adjacent to the first irradiation point to set the second irradiation point. A method for removing noise from reflected waves, which is characterized in that: it is a method for removing noise from reflected waves in an ultrasonic imaging device. A multi-layered object is irradiated with ultrasonic waves to obtain reflected waves at the joint interface of the objects. , and generate the image of the above-mentioned joint interface based on the signal intensity of the above-mentioned reflected wave, The above-mentioned noise removal methods for reflected waves include: The phase correction step is to correct the above-mentioned second reflected wave by performing matching processing on the first reflected wave from the first irradiation point in the above-mentioned object and the second reflected wave from the second irradiation point in the above-mentioned object. The shift relative to the phase of the first reflected wave; and a noise removal step that removes noise from the first reflected wave based on the first reflected wave and the second reflected wave that has been phase-corrected by the phase correction step, When the signal intensity of the interface echo representing the waveform from the bonding interface of the first reflected wave is greater than or equal to a threshold value representing a predetermined value, in the matching process, the interface echo of the second reflected wave is performed. The phase of the peak of the wave is consistent with the phase of the peak of the interface echo of the first reflected wave, When the signal strength of the interface echo of the first reflected wave does not have a magnitude greater than or equal to the threshold, in the matching process, the phase of the peak of the surface echo of the second reflected wave is matched with the phase of the surface echo of the second reflected wave. 1. The process of making the peak of the above-mentioned surface echo of the reflected wave consistent in phase. For example, the method for removing noise from reflected waves in claim 4, wherein In the above-mentioned noise removal step, the above-mentioned first reflected wave and the above-mentioned second reflected wave after performing phase correction with the above-mentioned first reflected wave through the above-mentioned matching process are added and averaged. For example, the method for removing noise from reflected waves in claim 4, wherein In the above-mentioned matching process, one or more irradiation points at predetermined positions are selected from irradiation points adjacent to the above-mentioned first irradiation point to set the above-mentioned second irradiation point.