Method and apparatus for detecting photoacoustic signal

• US 5,136,172 A
• Filed: 08/14/1990
• Issued: 08/04/1992
• Est. Priority Date: 08/16/1989
• Status: Expired due to Term

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Kitamori, Takehiko, Nakata, Toshihiko, Kembo, Yukio, Sawada, Tsuguo
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): LEE, JOHN R
• Application Number:
  US07/567,319
• Time in Patent Office: 721 Days
• Field of Search: 250/572, 356/432, 356/432 T, 356/444, 356/445, 73/602, 364/553, 364/822, 364/825, 364/827
• US Class Current: 250/559.39
• CPC Class Codes: G01N 2021/1765 Method using an image detector and processing of image signal
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/88 Investigating the presence of flaws or contamination

Abstract

A method and apparatus are disclosed for detection of surface defects and internal defect information of a sample, such as a semiconductor device, using the photoacoustic effect. Operationally, an intensity-modulated laser beam is provided having a predetermined desired frequency. The intensity-modulated laser beam is focused on the sample thereby inducing said photoacoustic effect inside the sample which is detected in two-dimensional directions of the sample so as to compose a two-dimensional photoacoustic image of the excited sample. Surface and internal information of the sample is then extracted from the two-dimensional photoacoustic image and an inverse filtering factor is in turn computed on the basis of a thermal impulse response of the particular sample for compensating degradation of the resolution of the actual photoacoustic image obtained. Lastly, the computed inverse filtering factor is then applied to the detected photoacoustic image to arrive at an image having greatly improved resolution.

Claims

1. A photoacoustic method for detecting two-dimensional information of a sample, the method comprising the steps of:

intensity-modulating a light beam emitted from a light source to provide an intensity-modulated beam having a desired frequency;

focusing said intensity-modulated beam on the sample thereby generating a thermal distortion due to a photoacoustic effect inside the sample and scanning the focused intensity-modulated beam over the sample within a plane in two-dimensional x-y directions in the plane;

detecting the thermal distortion due to the photoacoustic effect in the two-dimensional x-y directions of the sample using a transducer apparatus and composing a two-dimensional photoacoustic image p(x,y) of the sample;

providing a two-dimensional thermal impulse response h(x,y) of the sample comprising a two-dimensional transfer function representing a two-dimensional minute displacement of the sample surface generated by propagating a heat wave, generated at an infinitely small point inside the sample, through the sample;

calculating a two-dimensional complex Fourier transformed image P (μ

,ν

) from said two-dimensional photoacoustic image p(x,y) and a two-dimensional complex Fourier transformed image H(μ

,ν

) from said two-dimensional thermal impulse response h(x,y), μ

,ν

being spatial frequencies in the x and y directions respectively and calculating an ideal two-dimensional photoacoustic image q(x,y) by performing a two-dimensional inverse complex Fourier transformation in accordance with ##EQU14## by using said two-dimensional complex Fourier transformed image P(μ

,ν

) and said two-dimensional complex Fourier transformed image H(μ

,ν

), whereby said ideal two-dimensional photoacoustic image q(x,y) is compensated for degradation of the resolution of said two-dimensional photoacoustic image p(x,y); and

extracting a two-dimensional modulated frequency component contained in said ideal two-dimensional photoacoustic image q(x,y) calculated, by referencing said desired frequency modulating said light beam, whereby said two-dimensional information of the sample is detected in accordance with said two-dimensional modulated frequency component extracted.

2. The method according to claim 1, wherein said step of providing said two-dimensional thermal impulse response h(x,y) of the sample comprises calculating h·

S(x,y) according to;

##EQU15## where β

comprises a coefficient of thermal expansion of sample, α

comprises a light absorption coefficient of sample, 1 comprises a range of light absorption region V_op, P/2f comprises the energy of said light beam in one cycle, ρ

comprises the density of said sample, c comprises the specific heat of said sample, A comprises an irradiated surface area of said sample, and S(x,y) comprises a two-dimensional temperature distribution.

3. An apparatus for detecting a surface or internal two-dimensional information of a sample using a photoacoustic signal, the apparatus comprising:

a light source emitting a light beam;

modulating means for intensity-modulating the light beam emitted from said light source providing an intensity-modulated beam having a desired frequency;

focusing-scanning means for focusing said intensity-modulated beam on the sample thereby generating a thermal distortion in the sample due to a photo-acoustic effect inside the sample and scanning the focused intensity-modulated beam over the sample in a surface in x and y directions in the surface;

detecting means for detecting the thermal distortion due to the photo-acoustic effect in two-dimensional directions of the sample and composing a two-dimensional photoacoustic image p(x,y) of the surface of the sample;

impulse response providing means for providing a two-dimensional thermal impulse response h(x,y) of the sample comprising a two-dimensional transfer function representing a two-dimensional minute displacement of the sample surface generated by propagating a heat wave, generated at an infinitely small point inside the sample, through the sample;

calculating means for calculating a two-dimensional complex Fourier transformed image P(μ

,ν

) from said two-dimensional photoacoustic image p(x,y) detected by said detecting means and a two-dimensional complex Fourier transformed image H(μ

,ν

) from said two-dimensional thermal impulse response h(x,y) provided by said impulse response providing means, μ

, ν

being spacial frequencies in the x and y directions respectively and calculating an ideal two-dimensional photoacoustic image q(x,y) by performing a two-dimensional inverse complex Fourier transformation in accordance with ##EQU16## by using said two-dimensional complex Fourier transformed image P(μ

,ν

) and said two-dimensional Fourier image H(μ

,ν

) calculated, whereby said ideal two-dimensional photoacoustic image q(x,y) is resolution compensated for degradation of said two-dimensional photoacoustic image p(x,y); and

extracting means for extracting a two-dimensional modulated frequency component contained in said ideal two-dimensional photoacoustic image q(x,y) calculated by said calculating means, by referencing said desired frequency modulating said light beams, whereby said surface or internal two-dimensional information of the sample is detected in accordance with said two-dimensional modulated frequency component extracted.

4. The apparatus according to claim 3, wherein said impulse response providing means comprises calculating means for calculating said two-dimensional thermal impulse response h(x,y) of the sample in accordance with h·

S(x,y) according to;

##EQU17## where β

comprises a coefficient of thermal expansion of sample, α

comprises a light absorption coefficient of sample, 1 comprises a range of light absorption region V_op, P/2f comprises the energy of said light beam in one cycle, ρ

comprises the density of said sample, c comprises the specific heat of said sample, A comprises an irradiated surface area of said sample, and S(x,y) comprises a two-dimensional temperature distribution.

11. The apparatus according to claim 3, wherein said focusing means comprises first confocal optical means to converge said intensity-modulated excitation light into a fine intensity-modulated excitation light spot on the sample surface and first removing means for removing high order diffraction light components around said fine intensity-modulated excitation light spot.

12. The apparatus according to claim 3, wherein said detecting means comprises light interference detection means for detecting an interference intensity signal caused by said generated thermal distortion, said light interference detection means comprising an interference light source means or emitting an interference light, a second confocal optical means for irradiating the sample surface by converging the interference light into a fine interference light spot whereby the fine interference light spot and the intensity-modulated excitation file light spot substantially occupy the same sample surface position, second removing means for removing high order diffraction light spot, and an optical detection means for detecting a focused interfered light that is reflected from the irradiated sample surface and which contains the interference intensity signal, said optical detection means including a third removing means for removing high order light diffraction components from the focused interfered light.

5. A photoacoustic signal detecting method comprising the steps of:

intensity-modulating a light beam emitted from a light source to provide an intensity-modulated beam having a desired frequency;

focusing said intensity-modulated beam on a sample thereby inducing a photoacoustic effect inside the sample;

detecting the photoacoustic effect in three-dimensional directions of the sample so as to compose a three-dimensional photoacoustic image of the sample;

extracting surface and internal information of the sample from said three-dimensional photoacoustic image;

computing, on the basis of a pure thermal impulse response and a thermoelastic impulse response of the sample, an inverse filtering factor for compensating degradation of the resolution of said photoacoustic image, said thermoelastic impulse response of the sample representing an inherent characteristic property of the sample in which a change in temperature at an infinitely small point inside said sample is converted into a minute displacement of the sample surface; and

applying said computed inverse filtering factor for said detected photoacoustic image.

6. A photoacoustic signal detecting method according to claim 5, wherein said pure thermal impulse response is provided by a transfer function representing the process in which a heat wave generated at an infinitely small point inside the sample propagates through the sample until it is converted into an internal temperature distribution of the sample.

7. A method for detecting surface or internal three-dimensional information of a sample through use of photoacoustic signal, the method comprising the steps of:

intensity-modulating a light beam emitted from a light source to provide a plurality of intensity-modulated beams obtained by changing frequency so that each of the intensity-modulated beams have a different desired frequency;

focusing each of said intensity-modulated beams on a sample in a first direction thereby generating thermal distortions due to photoacoustic effects inside the sample corresponding to each of different desired frequencies and scanning the focused intensity-modulated beams over the surface of said sample in second and third directions, the second and third directions being orthogonal to each other and to the first direction;

detecting each of the thermal distortions due to each of the photoacoustic effects in two-dimensional directions of the sample corresponding to each of different desired frequencies by an interferometer optical system so as to compose a three-dimensional photoacoustic image p(x,y,f) of the surface of the sample;

providing a three-dimensional pure thermal impulse response h_t (x,y,z) of the sample comprising a three-dimensional transfer function representing a three-dimensional internal temperature distribution generated by propagating a heat wave, generated at an infinitely small point inside the sample, through the sample and a three-dimensional thermoelastic impulse response h_e (x,y,z) of the sample comprising a three-dimensional minute displacement of the sample surface generated in accordance with said three-dimensional internal temperature distribution;

calculating a three-dimensional complex Fourier transformed image P(μ

,ν

,δ

) from said three dimensional photoacoustic image p(x,y,f), a three-dimensional complex Fourier transformed image H_t (μ

,ν

,γ

) from said three-dimensional pure thermal impulse response h_t (x,y,z) and a three-dimensional complex Fourier transformed H_e (μ

,ν

,γ

) from said three-dimensional thermoelastic impulse response h_e (x,y,z), μ

,ν

,γ

,δ

being spatial frequencies in second (x), third (y), first (z) and first (-z) directions respectively and calculating an ideal three-dimensional acoustic image q_e (x,y,z)·

q_t (x,y,z) by performing a three-dimensional inverse complex Fourier transformation in accordance with;

##EQU18## using said three-dimensional complex Fourier transformed image P(μ

,ν

,δ

), said three-dimensional complex Fourier transformed image H_t (μ

,ν

,γ

) and said three-dimensional complex Fourier transformed image H_e (μ

,ν

,γ

) calculated, whereby said ideal three-dimensional photoacoustic image q_e (x,y,z)·

q_t (x,y,z) is resolution compensated for degradation of said three-dimensional photoacoustic image p(x,y,f) and,

extracting a three-dimensional modulated frequency component corresponding to each of different desired frequencies contained in said ideal three-dimensional photoacoustic image q_e (x,y,z)·

q_t (x,y,z) calculated by referencing each of said desired frequencies modulating said light beam, whereby the surface or internal three-dimensional information of the sample is detected in accordance with said three-dimensional modulated frequency component extracted.

8. An apparatus for detecting three-dimensional information of a sample using a photoacoustic signal comprising:

a light source emitting a light beam;

modulating means for intensity-modulating said light beam emitted rom said light source to provide a plurality of intensity-modulated beams obtained by changing frequency so that each of the intensity-modulated beams has a different desired frequency;

focusing means for focusing each of said intensity-modulated beams on the sample in a first direction (z) thereby generated thermal distortion due to photoacoustic effects inside the sample corresponding to each of different desired frequencies and scanning relatively the focused intensity-modulated beams over the sample in a second direction (x) and a third direction (y);

detecting means for detecting each of the thermal distortions due to each of the photoacoustic effects in two-dimensional directions of the sample corresponding to each of different desired frequencies by a transducer system so as to compose a three-dimensional photoacoustic image p(x,y,f) of the surface of the sample;

impulse response providing means for providing i) a three-dimensional pure thermal impulse response h_t (x,y,z) of the sample comprising a three-dimensional transfer function representing a three-dimensional internal temperature distribution generated by propagating a heat wave, generated at an infinitely small point inside the sample, through the sample and ii) a three-dimensional thermoelastic impulse response h_e (x,y,z) of the sample comprising a three-dimensional transfer function representing a three-dimensional minute displacement of the sample surface generated in accordance with said three-dimensional internal temperature distribution;

calculating means for calculating a three-dimensional Fourier transformed image P(μ

,ν

,δ

) from said three-dimensional photoacoustic image p(x,y,f), a three-dimensional complex Fourier transformed image H_t (μ

,ν

,γ

) from said three-dimensional pure thermal response h_t (x,y,z) and a three-dimensional complex Fourier transformed image H_e (μ

,ν

,γ

) from said three-dimensional thermoelastic impulse response h_e (x,y,z), μ

, ν

, γ

, δ

being spatial frequencies in the second (x), third (y), first (z) and -first (-z) directions respectively and calculating an ideal three-dimensional photoacoustic image q_e (x,y,z)·

q_t (x,y,z) by performing a three-dimensional inverse complex Fourier transformation in accordance with;

##EQU19## using said three dimensional complex Fourier transformed image P(μ

,ν

,δ

), said three-dimensional complex Fourier transformed image H_t (μ

,ν

,γ

) and said three-dimensional complex Fourier transformed image H_e (μ

,ν

,γ

) calculated, whereby said three-dimensional photoacoustic image q_e (x,y,z)/q_t (x,y,z) is resolution compensated for degradation of said three-dimensional photoacoustic image p(x,y,f) and dimensional photoacoustic image p(x,y,f) and,

extracting means for extracting a three-dimensional modulated frequency component corresponding to each of different desired frequencies contained in said ideal three-dimensional photoacoustic image q_e (x,y,z)·

q_t (x,y,z) calculated by said calculating means, by referencing each of said desired frequencies modulating said light beam, whereby the three-dimensional information of the sample is detected in accordance with said three-dimensional modulated frequency component extracted by said extracting means.

9. The apparatus according to claim 8, wherein said impulse response providing means comprises calculating means.

10. The apparatus according to claim 8, wherein said focusing means comprises first confocal optical means to converge said intensity-modulated excitation light into a fine intensity-modulated excitation light spot on the sample surface and first removing means for removing high order diffraction light components around said fine intensity-modulated excitation light spot.

13. The apparatus according to claim 8, wherein said detecting means comprises light interference detection means for detecting an interference intensity signal caused by said generated thermal distortion, said light interference detection means comprising an interference light source means for emitting an interference light, a second confocal optical means for irradiation the sample surface by converging the interference light into a fine interference light spot whereby the fine interference light spot and the intensity-modulated excitation fine light spot substantially occupy the same sample surface position, second removing means for removing high order diffraction light components around said fine interference light spot, and an optical detection means for detecting a focused interfered light that is reflected from the irradiated sample surface and which contains the interference intensity signal, said optical detection means including a third removing means for removing high order diffraction components from the focused interfered light.