Electro-optic measuring apparatus

Abstract: An E-O probe with improved spatial resolution has a light transmissive base part (15), an electro-optic material (16) which is fixed to the base part and has an index of refraction which varies in response to an electrical field from a measured object, and a mirror (17) which is fixed to the electro-optic material and reflects an incident beam penetrating the base part and the electro-optic material. The mirror is formed to be smaller than the incident beam in diameter. The electro-optic material is formed very thin.

Description: The present invention relates to a measuring apparatus for measuring a voltage on a measured object. The invention may be applied to a voltage detecting apparatus which measures without electrical contact a voltage of a measured object, by detecting a strength of an electrical field generated from the measured object. Related Background Art There are some apparatus known as a voltage detecting apparatus which measures without electrical contact a voltage of a measured object, making use of an E-O probe. For example, see U.S. patent No. 4,618,819. In those apparatus, E-O probe is made of an electro-optic material such as LiTaO 3 which has an electro-optic effect causing an index of retraction to vary in response to an intensity of an electrical field generated from a measured object. Referring to Fig. 7, the principles of such a voltage detecting apparatus are described hereunder. The E-O probe includes an electro-optic material 1 formed as a quadrangular pyramid or the like with a flat face at the top end, and a reflecting mirror 2 attached to the flat face with the reflecting face facing the flat face, and during measuring the mirror 2 is kept in the proximity of the measured object 3. An incident beam hν1 polarized as linearly polarized, circularly polarized or ovally polarized light enters the electro-optic material 1, being directed to the reflecting mirror 2, from the other side of the material 1. A light hν2 reflected by the mirror 2 is converted to a light intensity by a polarization analyzing means and further the light intensity is measured by a photo-electric converting element. Since the electro-optic material 1 has a characteristic that an index of refraction varies in response to the intensity of an electrical field caused by a voltage at the measured object 3, the polarization status of the reflected light hν2 is changed from the incident beam hν1, and when the reflected light hν2 is measured with a polarizing beam splitter and a photo-detector, a voltage at the measured object 3 can be detected. Where, the photo-detector is of photoelectric effect such as a photomultiplier tube, a high responsivity charge coupled device and the like. As mentioned above, since a voltage of a measured object is measured optically based on an intensity of an electrical field from a measured object, a non-contact measurement is possible. This is especially efficient in such a case as measuring a signal voltage at each wire, electric line, electrode or element in a semiconductor chip and determining an operation characteristic of the semiconductor chip. Recent advances in high-density device technologies of semiconductor integration circuits are requiring an extremely narrower space between wires of the circuits. And further improvement in spatial resolution is desired so as to enable a signal voltage propagating in each wire to be measured with high precision without being affected by an electrical field from each other wire. Where λ is a wavelength of an incident beam hν1 emitted by a semiconductor laser or the like, NA is a numerical aperture of an objective lens to be used for focusing and a 0 is a diameter of the incident beam hν1 reflected by a reflecting mirror 2, the formula a 0 =(2×λ)/(π×NA) is met. It means the shorter the wavelength of an incident beam hν1 is, the smaller the diameter a 0 is, and consequently the higher spatial resolution is attained. For example, in a case of λ=780nm and NA=0.4, a 0 is 1.2µm, and a voltage at a wire in a circuit with a close space of 1.2µm or larger between wires on a semiconductor chip can be measured without any contact. Actually, this is a theoretical minimum beam diameter to be calculated based on a wavelength of an applied light and it is difficult to realize the minimum value due to aberration and wave front distortion of optical elements such as lens. As described in the formula above, the highest spatial resolution is now limited to about 1µm which can not sufficiently satisfy the requirements in response to the recent advance in fine semiconductor processing technologies. Further, no attention has been paid to the thickness of the electro-optic material 1 (the height of the quadrangular pyramid), and a relatively thick material was used. Under the circumstances, an overall polarization status of the electro-optic material 1 varies even when the tip portion of the electro-optic material 1 is not just above the place to be measured of the measured object 3, since the other part than the tip portion is also placed under the influence of the electrical field from the measured object 3 as illustrated in Fig. 8. It results in a difficulty to achieve a high spatial resolution. With a thick electro-optic material 1, a polarization status would vary in response to an electrical field from the measured object 3 not only in the tip portion of the electro-optic material of E-O probe but also in the body portion of the electro-optic material 1, even when the tip portion is not just above the measured point on the measured object 3. Microelectronic Engineering, Vol. 16, no. 1/4, pages 341 to 348 describes an electro-optic configuration which employs a pulsed solid state laser mounted on a probe station delivering 50 ps pulses at a repetition rate which is determined by an external trigger. US-A-4873485 describes a probe having a multiple quantum well structure which defines an active region sufficiently small in width that it can be placed between neighbouring lines on an integrated circuit. EP-A-541139 which is prior art falling within the terms of Art. 54(3) EPC describes an electro-optic sensor element provided with an electrically conductive tip at one side and with an electrode at the other side so that an absolute and calibrated measurement is possible. According to the invention there is provided an apparatus as defined by claim 1. In an embodiment of the present invention with the structure as described above, an actual spatial resolution is determined by a size of the reflecting mirror since an incident beam is reflected by the smaller reflecting mirror than the incident beam in diameter even when no desired spatial resolution can be attained with a beam diameter determined based on a wavelength of the incident beam (a 0 = (2xλ)/(πxNA)) . Conventionally a further improvement in a spatial resolution was very hard since a possible beam diameter is limited by a wavelength of an applied light as an incident beam, aberration and wave front distortion of optical elements. However, according to the embodiment where an incident beam is reflected by a smaller reflecting mirror in diameter than the incident beam, a spatial resolution can be improved exceeding the limit. Forming an electro-optic material as thin as the size of the reflecting mirror or a theoretical minimum beam diameter to be calculated based on a wavelength of said applied light, an electrical field from a measured object little affects an electro-optic material before the electro-optic material comes just over the measured object, and consequently a spatial resolution can be improved. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects, features and advantages of the invention will become more apparent from a reading of the following description in connection with the accompanying drawings in which: Fig. 1 shows an example of a structure of a non-contact voltage detecting apparatus with an E-O probe; Fig. 2A and Fig. 2B are drawings describing a structure of a first E-O probe; Fig. 3A and Fig. 3B are drawings describing a structure of a second E-O probe; Fig. 4A and Fig. 4B are drawings describing a structure of a third E-O probe; Fig. 5 is a drawing further describing a structure of an E-O probe; Fig. 6A and Fig. 6B show characteristic curves describing a difference due to the E-O probe thickness; Fig. 7 is a drawing describing a structure of an E-O probe of the prior art; Fig. 8 is a drawing describing a problem in an E-O probe of the prior art; and Fig. 9A and Fig. 9B are drawings describing a reflection status in a case of an extremely small diameter of an incident beam. Referring to Fig. 1, an example of structure of a non-contact voltage detecting apparatus. There are a light source 4 such as a semiconductor laser and a driving apparatus 5 to supply electricity to the light source 4 disposed. A light emitted by the light source 4 passes a collimating lens 6, a polarizing beam splitter 7 and a wave plate 8 in this order. Therefore, the incident light is polarized for example to a linearly-polarized light by the polarized beam splitter 7, and further is converted to a predetermined polarized light such as a circularly-polarized light or elliptically-polarized light by the wave plate 8. This light is focused by a focusing lens 9 and enters an E-O probe 10. An electro-optic material described later and a reflecting face are disposed at a part of the E-O probe 10 facing the measured object 11, and an index of refraction of the electro-optic material varies in response to intensity of an electrical field developed by the measured object 11 such as a wire, electric line, electrode or a semiconductor device integrated in a semiconductor integrated circuit, for example when a voltage at each point of the semiconductor integrated circuit is measured. Therefore, the reflected incident beam as mentioned above passes the focusing lens 9 and the wave plate 8 as a reflected beam, in which a polarization status is determined in response to a variation of the index of refraction, and further only an polarized light element orthogonal to the incident light is separated by the polarized beam splitter 7 and enters a photo-detector 12 such as photomultiplier. The photo-detector 12 converts it to electricity by the photo-electric effect, and the converted signal is measured by a measuring instrument 13 such as an oscilloscope. Referring to Fig. 2A and Fig. 2B, a structure of a first E-O probe 10 is described. In Fig. 2A, a base part 15 of a quadrangular pyramid shape is made of a light transmissive material such as glass and has an infinitesimal flat face 14 at the measured object side. And a thin plate shaped electro-optic material 16 is attached to the flat face 14. Further as shown in a vertical cross section shown in Fig. 2B, an infinitesimal mirror 17 is provided having a reflecting face which faces the incident side of the incident beam hν1. The mirror 17 is fixed to the tip part face of the electro-optic material 16. The rest of the tip part face of the electro-optic material 16 other than the mirror 17 is covered by a light absorbing layer 18 which absorbs light incident to the outside of the mirror 17 among the incident beam hν1. Here, a diameter w of the mirror 17 is designed to be smaller than the diameter a 0 of the incident beam hν1 at the tip part face of the electro-optic material 16. Therefore, the beam diameter of the reflected beam hν2 reflected by the mirror 17 is smaller than the beam diameter of the incident beam hν1. Referring to Fig. 9A and Fig. 9B, there is shown an E-O probe in which the formation is the same as that in Fig. 2A and Fig. 2B but a diameter of a reflection mirror is equal to or smaller than a minimum possible diaphragm diameter to be calculated based on a wavelength of an applied light, that is, a diffraction limit diameter. A diffraction limit d is given by the following formula, including a wavelength λ and a numerical aperture NA of the focusing lens. d=(2xλ)/(πxNA) For example, in the case where a semiconductor laser light of λ=780nm is focused by an objective lens of NA=0.42 and of 50 magnification, a diffraction limit is d=1.18µm. Where a diameter w of the mirror 17 is equal to or smaller than the diffraction limit, the reflected beam hν2 spreads wider than the incident beam hν1 as shown in Figs. 9A and 9B. In this case, since a part of the reflected beam hν2 does not return to the focusing lens, an amount of light to be detected by the photo-detector 12 decreases and an output signal may be weakened. However, since the output signal is obtained from the beam reflected by the infinitesimal mirror 17, a higher spatial resolution can be attained than in case of a beam diameter a 0 of the incident beam hν1. A beam diameter a 0 of the incident beam hν1 may not be able to be smaller than 1µm due to a usage of a light with a wavelength which is not attenuated in an optical system of a non-contact voltage measuring apparatus as shown in Fig. 1. Even in this case, as described above, using the E-O probe 10 as shown in Fig. 2, a diameter of a reflected beam hν2 to be modulated in the electro-optic material 16 can be about equal to a diameter w of the mirror 17 which is made smaller than the beam diameter a 0 of the incident beam hν1. Consequently, a spatial resolution can be set higher than a beam diameter a 0 of the incident beam hν1. More particularly, accompanying recent advances in infinitesimal processing technologies of semiconductors, wires or internal devices formed with a space of 1µm or smaller are not rare in a semiconductor integrated circuit. A voltage at each point can be detected with a spatial resolution of 1µm or smaller to analyze a circuit operation or an abnormality such as an electrical field concentration, by detecting the voltage without any contact. Through applications like the above, the present invention can greatly contribute to the semiconductor technologies in the future. Referring to Fig. 3A and Fig. 3B, the same numerals as in Figs. 2A and 2B are assigned to the same or corresponding parts in Figs. 3A and 3B. In Fig. 3A, a base part 15 of a quadrangular pyramid shape is made of a light transmissive material such as glass and has an infinitesimal flat face 14 at the measured object side. And a thin plate-shaped electro-optic material 16 is attached to the flat face 14. Further, as shown in vertical cross section in Fig. 3B, a light absorbing layer 20 is applied to the tip part face of the electro-optic material 16. The light absorbing layer 20 has an infinitesimal opening 19 which admits light of the incident beam hν1, and the rest of the light absorbing layer other than the opening 19 absorbs light. A reflecting mirror 21 is fixed on the other side of the light absorbing layer 20 to the tip part of the electro-optic material 16, the diameter w m of which is larger than the diameter w of the opening 19. The reflecting face of the mirror 21 faces the opening 19. Here, a diameter w of the opening 19 is designed to be smaller than the diameter a 0 of the incident beam hν1 at the tip part face of the electro-optic material 16. Therefore, the substantial diameter of the reflecting face of the mirror 21 is approximately equal to w. Consequently, the beam diameter of the reflected beam hν2 reflected by the mirror is smaller than the beam diameter of the incident beam hν1. A light incident to the outside of the opening 19 or the reflecting face among the incident beam hν1 is absorbed by the light absorbing layer 20. A beam diameter a 0 of the incident beam hν1 may not be able to be smaller than 1µm due to usage of light with a wavelength in a range of near infrared radiation in a non-contact voltage measuring apparatus as shown in Fig. 1. Even in this case, as described above, using an E-O probe 10 as shown in Fig. 3 a diameter of a reflected beam hν2 to be modulated in the electro-optic material 16 can be about equal to a diameter w. Consequently, a spatial resolution can be set higher than a beam diameter a 0 of the incident beam hν1 in a conventional apparatus. A reflecting mirror 17 or 21 as shown in Figs. 2A and 2B, Figs. 3A and 3B can be formed by applying fine processing technologies such as aluminum vacuum evaporation deposit conventionally known or the like. The light absorbing layer 18 or 20 is formed by applying fine processing technologies in semiconductor producing processes or the like. Referring to Figs. 4A and 4B, the same numerals as in Figs. 2A and 2B, Figs. 3A and 3B are assigned to the same or corresponding parts in Figs. 4A and 4B. In Fig. 4a, a base part 15 of a multi-angular (quadrangular in this probe) pyramid shape is made of a light transmissive material such as glass and has an infinitesimal flat face 14 at the measured object side. And a thin plate shaped electro-optic material 16 is attached to the flat face 14. The shape of the electro-optic material 16 is made so as to same shape of the flat face 14. Further as shown in vertical cross section in Fig. 4B, a reflecting mirror 24 is fixed to the tip part face of the electro-optic material 16. The reflecting mirror 24 has an infinitesimal reflecting face 22 which faces an incident beam hν1 and reflects the incident beam hν1. The rest of the mirror 24 other than the reflecting face 22 is of a diffused reflection face 23. Here, a diameter w of the mirror 22 is designed smaller than the diameter a 0 of the incident beam hν1 at the tip part face of the electro-optic material 16. Therefore, the beam diameter w of the reflected beam hν2 reflected by the mirror 22 is smaller than the beam diameter a 0 of the incident beam hν1. Light incident to the outside of the reflecting face 22 in the incident beam hν1 is diffused by the diffused reflection face 23, and it is not involved in the reflected beam hν2. A beam diameter a 0 of the incident beam hν1 may not be able to be smaller than 1µm due to a usage of a light with a wavelength which is not attenuated in an optical system of a non-contact voltage measuring apparatus as shown in Fig. 1. Even in this case, as described above, using the E-O probe 10 shown in Figs. 4A and 4B, a diameter of a reflected beam hν2 to be modulated in the electro-optic material 16 can be about equal to a diameter w. Consequently, a spatial resolution can be set higher than a beam diameter a 0 of the incident beam hν1 in a conventional apparatus. A separate forming of the reflecting face 22 and the diffused reflection face 23 can be realized by the following process. That is, the diffused reflection face 23 is formed first by applying fine processing technologies such as sputter etching technologies in a semiconductor producing process to the rest of the area of the electro-optic material 16 other than the prospective reflecting face 22. Then, the reflecting mirror 24 is formed all over its face by an aluminum vacuum evaporation deposit method and the like. In the E-O probes 10 shown in Figs. 2A, 2B, 3A, 3B, 4A, 4B, 9A and 9B, the electro-optic material 16 is made of a material as thin as possible. A very thin electro-optic material 16 can almost prevent the E-O probe from being affected by a lateral electrical field from a measured object 11 which is placed not just under the E-O probe 10 but a little aside as shown in Fig. 5, which illustrates a typical example of such an E-O probe 10. It enables an index of refraction of the electro-optic material 16 to vary sharply when a measured object 11 is shifted nearly just under the E-O probe 10, which further improves a spatial resolution. Preferably, the thickness should be same as the diameter of the reflecting mirror. The diameter of the reflecting mirror is made the same as the theoretical minimum value based on a light wavelength used for the apparatus and the thickness of the electro-optic material is of the theoretical minimum value based on a light wavelength. Figs. 6A and 6B show experimental results or characteristic curves describing a difference in spatial resolutions between cases of a thick electro-optic material and a thin one. In this experiment, a CW semiconductor laser is applied as a light source 4 of a non-contact voltage detecting apparatus shown in Fig. 1, a magnification of the focusing lens 9 is 10, the measured object 11 is a circuit of micro-strip line electrode with a line width B=230µm where a predetermined voltage is given, a beam diameter of the incident beam hν1 to the electro-optic material 16 is set for 3.5µm, the electro-optic material 16 is a crystal of ZnTe and a distance between the measured object 11 and the top end of the electro-optic material 16 is kept constant. The curve of Fig. 6A is for an electro-optic material 16 of 1mmx1mm and 1mm thick, and the curve of Fig. 6B is for an electro-optic material 16 of 1mmx1mm and 88µm thick. The horizontal axis indicates a distance from the point (0 point) just under the electro-optic material 16 to the center point of the measured object 11, and the vertical axis indicates a detected level by the photo-detector 12 which is normalized. Obviously from the experimental results, a detected level varies more sharply in case of a thinner electrical material 16 (the curve of Fig. 6B) than in case of a thick one. For example, the distances L1 and L2 to the measured object 11 are compared when the detected level is 50%. The spatial resolution in case of a 1mm thick electro-optic material 16 (the curve of Fig. 6A) is L1=67µm, and the spatial resolution in case of 88µm thick electro-optic material 16 (the curve of Fig. 6B) is L2=25µm, which indicates an obvious improvement in a spatial resolution by using a thinner electro-optic material 16. In the probes described above, an actual spatial resolution is determined by a size of the reflecting mirror since an incident beam is reflected by the smaller reflecting mirror than the incident beam in diameter even when no desired spatial resolution can attained with a beam diameter determined based on a wavelength of the incident beam. (a 0 =(2xλ)/(πxNA)) . Conventionally a further improvement in spatial resolution is very hard since a possible beam diameter is limited by a wavelength of an applied light as an incident beam. However, where an incident beam is reflected by a reflecting mirror having a smaller diameter than the incident beam, spatial resolution can be improved exceeding the above-mentioned limit. Even when the minimum theoretical beam diameter cannot be attained due to an aberration or a wave front distortion of the optical elements, it little affects the spatial resolution. Forming an electro-optic material very thin, an electrical field from a measured object little affects an electro-optic material before the electro-optic material comes just over the measured object, consequently a spatial resolution can be improved. From the foregoing description it will be apparent that there as been provided an improved E-O probe with high spatial resolution. Variations and modifications within the scope of the invention will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.

Claims: An apparatus for measuring a voltage on a measured object, comprising: a light source (4) for producing light of a wavelength λ; a lens (9) of predetermined numerical aperture NA for focusing said light; and an electro-optic probe comprising: a base part (15) which is substantially transparent to said light; an electro-optic material (16) attached to a first end face (14) of said base part, and having an index of refraction which varies in dependence on electric field intensity, for receiving from said light source (4) a light beam of a predetermined diameter (a 0 ) having been focused by said lens (9); and a reflective face (17) at an end face of the electro-optic material (16) for reflecting said received light beam passing through said base part (15) and said electro-optic material (16) toward said end face of the electro-optic material (16) back into said electro-optic material, wherein said reflective face (17) has a diameter (w) that is smaller than said beam diameter (a 0 ) and the electro-optic probe is arranged such that in use the reflective face is facing a measured object, characterised in that said reflective face diameter (w) is substantially equal to or smaller than the theoretical minimum beam diameter 2λ/πNA . An apparatus as claimed in claim 1, wherein said electro-optic material (16) is of a thickness about equal to or smaller than said theoretical minimum beam diameter 2λ/πNA . An apparatus as claimed in any preceding claim, wherein the predetermined diameter (a 0 ) of said light beam is about equal to said theoretical minimum beam diameter 2λ/πNA . An apparatus as claimed in any preceding claim, wherein said reflective face is adapted to provide a mirror (17). An apparatus as claimed in claim 4, wherein the probe further comprises: a light absorbing layer (18) around said mirror for preventing reflection of parts of said light beam not incident on said mirror (17). An apparatus as claimed in any of claims 1 to 3, wherein said probe comprises a mirror (21) disposed at the end face of the electro-optic material (16) to cover an extended area thereof; and a light absorbing layer (20) disposed between said mirror (21) and said end face to prevent a portion of the light beam incident thereon from being reflected, the light absorbing layer having an opening of a diameter (w) which is substantially equal to or smaller than said theoretical minimum beam diameter 2λ/πNA through which light from the light beam can pass to be reflected by the mirror (21) and thereby defining said reflective face on said mirror (21). An apparatus as claimed in any of claims 1 to 3, wherein said probe comprises: a mirror (24) disposed at the end face of the electro-optic material (16) to cover an extended area thereof; and a light diffusing layer (23) disposed between said mirror and said end face to diffuse a portion of the light beam incident thereon, the light diffusing layer (23) having an opening of a diameter (w) which is substantially equal to or smaller than said theoretical minimum beam diameter 2λ/πNA through which light from the light beam can pass to be reflected by the mirror (24), and thereby defining said reflective face on said mirror (24). An apparatus as claimed in claim 7, wherein said light diffusing layer (23) is formed by a sputter etching method. An apparatus as claimed in any of claims 1 to 5, wherein said reflective region comprises a mirror (17) formed by a metal vacuum evaporation deposit method.

Inventors: AOSHIMA SHINICHIRO, HIRANO ISUKE, TAKAHASHI HIRONORI, AOSHIMA, SHINICHIRO, HIRANO, ISUKE, TAKAHASHI, HIRONORI
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1993
Main classification: G01R1
IPC codes: H01L21, G02B1, G01R19, G01R1