Abstract:
A near electric-field probe is driven by a short laser pulse delayed through an optical delay-line for detecting vectors of near-field components of electrical signals propagating through a device under test including an electrical device or an electronic circuit based on a sampling principle. The near-field probe includes a photoconductive switch assembly including a thin semiconductor photoconductive body, at least two separated switch electrodes formed on the thin semiconductor photoconductive body, and an electrode gap formed between the two separated switch electrodes; and an optical waveguide attached to one side of the photoconductive switch assembly by using an optical adhesive, wherein the optical waveguide is partially coated with conductive material on the outer surface thereof.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scanning system, and more particularly, to an near electric-field probe capable of measuring vectorial electric field by using short optical pulse and an ultrafast photoconductive switch. 
     2. Description of the Prior Art 
     Continuing advances in the development of novel materials and modern semiconductor devices/circuits have pushed the operating frequency of quantum-electronic and opto-electronic devices/circuits over multi-hundred gigahertz frequency region and have brought the millimeter-wave circuits into real world. The integrity of the devices/circuits in turn is becoming more and more increasing. On the other hand, high-speed devices/circuits have been facing unexpected phenomena and problems related to electro-magnetic field therein. Accordingly, needs for a novel diagnostic system to cope with those kind of phenomena and problems are also increasing. Moreover, there have been needs not only for diagnosis for electrical characteristics of the devices/circuits but for spectroscopic technologies reaching even far infra-red region in order to give a diagnosis of various electric and optical characteristics of materials. 
     One of the most popular methods for the diagnosis is to measure near-field with high spatial resolution. A probe or detector for use in measuring the near electric-field is required to have wide range of bandwidth and high temporal resolution for measuring various frequency components as well as to be small enough to detect field at any measured point. Also required are a flexible polarization and a high sensitivity(signal to noise ratio) to achieve precise measurement. In addition, a loading effect and an invasiveness should be low enough to minimize the distortion due to the detector or the peripheral devices with respect to a device under test(DUT). 
     Since 1980s, a variety of scanning probe microscopes such as a scanning tunnel microscope(STM), a scanning force microscope(SFM) and a scanning optical mocroscope(SOM) have been developed. Such scanning probe microscopes have a spatial resolution up to a degree of elements, and therefore, are feasible for measurement of ultrafast high-integrated devices/circuits. The electrical measurement frequencies of such microscopes are limited depending on the electronic parts employed therein. For instance, they exhibit rather low measurement sensitivity due to a tunneling current introduced thereto as for the STM, due to deviation of a cantilever as for the SFM, and due to exploiting an evanescent-wave coupling as for the SOM. 
     In an effort to overcome the reduction in the measurement bandwidth and low measurement sensitivity resulting from employment of these peripheral devices, a photoconductive sampling technology has been developed, in which picosecond electrical signals are measured by introducing short optical pulses into an ultrafast photoconductive switch. The photoconductive sampling technology has advantages that it can measure sub-picosecond signals with high sensitivity by exploiting low frequency devices. The photoconductive sampling technology involves a photoconductive switch which is a kind of a metal-semiconductor-metal(MSM) photodetector. If a laser beam with short pulse duration(e.g., a laser beam having less than 100 fs can be generated using the mode-locked Ti:Sapphire laser) is introduced to the photoconductive switch, carriers are generated in the photoconductive region of the photoconductive switch. Measured signals can be detected from the behavior of the carriers which display the behavior in conformity to the electrical signal under measurement. An electrical pulse with very narrow duration may be generated when the lifetime of the carriers is very short. Herein, a photoconductor having carriers exhibiting very short lifetime is called an ultrafast photoconductive body. LT-GaAs(Low-temperature-grown GaAs) and SOS(Silicon-on-Sapphire) are examples of the ultrafast photoconductive body, especially with carriers having lifetime of sub-picoseconds. There may be two methods for measuring electrical signals with resolution of sub-picoseconds on the basis of a photoconductive sampling principle. One is a pump-probe measurement(PPM) and the other is a time-equivalent sampling(TES). 
     According to the PPM, a short pulse laser beam generated from a laser is splitted into two beams: a pump laser pulse beam and a probe laser pulse beam. The pump laser pulse beam is introduced onto a first ultrafast photoconductive switch(or photodetector) to generate an electrical pulse signal, while the probe laser pulse beam is introduced to a second ultrafast photoconductive switch via a fine translation stage or an optical delay-line, to detect the electrical pulse signal generated at the first photoconductive switch. Herein, where a device under test(DUT; e.g., electrical device or circuit) is placed in-between the first photoconductive switch and the second photoconductive switch, the second photoconductive switch may present an impulse response of the DUT in response to input of the electrical pulse signal from the first photoconductive switch, wherein the impulse response is regarded as a measured signal. The measured signal includes a signal presenting cross-correlation between the electrical pulse from the second photoconductive switch and the to-be-measured signal with respect to a time delay between the pump laser pulse beam and the probe laser pulse beam. That is, the second ultrafast photoconductive switch measures electrical characteristics of the DUT by sampling electrical signal proportional to the to-be-measured signal from a time delay of the first ultrafast photoconductive switch. 
     As described above, the PPM is basically a method for measuring pulse response. The TES, on the other hand, is a method for measuring ultra high frequency signal operating with respect to a single frequency in the steady state. The TES requires synchronization between repetition rate of laser pulse beams(f laser ) and the operating frequency(f m ) of the DUT. In other words, signal f IF =ABS(f m −n·f laser ) obtained by mixing the electrical signal f laser  obtained by introducing laser pulses to the first photoconductive switch(or photodetector) and to-be-measured signal with frequency of f m , is used for triggering a lock-in amplifier(or an oscilloscope or a spectrum analyzer). Then, sampling the electrical signal by activating the second ultrafast photoconductive switch with another probe laser pulse beam generated at the laser, results in signals equivalent to the to-be-measured signal at the frequency of f IF . 
     Measurement of electrical signal by the photoconductive sampling method was first proposed by D. H. Auston, and this method is also called an “On-wafer Measurement Method.”  FIG. 1  is a schematic diagram of a set-up for measuring electrical signal propagating on a conducting line by using the photoconductive sampling method. The on-wafer measurement method employs a photoconductive switch to be used as an electrical pulse signal generator  110  and an electrical signal detector  120 . More specifically, the conducting line and a first photoconductive switch  110  connected thereto are placed on the ultrafast photoconductive body and dc voltage is applied to the first photoconductive, switch  110 , a pump laser pulse beam generated from a short pulse laser system  140  is applied thereto in order to generate the electrical pulse signal propagated through the conducting line, consecutively a probe laser pulse beam is guided to pass through an optical delay-line before being applied to a second photoconductive switch  120  and a third photoconductive switch (not shown, but positioned at the side of the electrical signal generator  110 ), and finally, the electrical signal passing through or being reflected from the DUT may be detected. Therefore, according to the on-wafer measurement method, electrical characteristics of the DUT which are positioned between photoconductive switches may be detected. In order to overcome disadvantages of the on-wafer measurement method that the photoconductive switches must be associated with the DUT on the ultrafast photoconductive body, the photoconductive switches are associated with electrical contact probe to measure electrical characteristics of an DUT. This method, however, still bears limitations that it can measure only a resultant scalar voltage/current not a vectorial component which collectively constitutes electromagnetic wave. 
     Another exemplary method on the basis of the photoconductive sampling principle is a terahertz system. A so-called terahertz radiation refers to a phenomenon that when short laser pulse is introduced onto a photoconductive switch biased with dc voltage, not only electrical pulse signal is generated and propagated through an electrical line connected to the photoconductive switch in the wake of generated carriers and but also electromagnetic wave which is the derivative of the electrical pulse signal is radiated. A system exploits the terahertz radiation is called terahertz system. A terahertz signal radiated from a terahertz transmitter is measured by the photoconductive sampling principle in order to identify characteristics of various materials and to perform imaging(also called a terahertz imaging), wherein the terahertz transmitter introduces laser pulse beam onto a photoconductive switch biased with dc voltage to radiate terahertz wave and another photoconductive switch positioned on the ultrafast photoconductive body through an optical delay-line detects the responsive terahertz wave. 
       FIG. 2  is a schematic diagram of the terahertz system. As shown in  FIG. 2 , a pump laser pulse beam generated from the short pulse laser system  240  is introduced to a transmitter including a first photoconductive switch  210  which is biased with dc voltage in order to generate terahertz electromagnetic wave, and on the other hand, a probe laser pulse beam is introduced to a receiver including a second photoconductive switch  220  through the optical delay-line  250  to measure the terahertz electromagnetic wave. A terahertz electromagnetic wave that has been radiated from the first photoconductive switch  210  of the transmitter and then transmitted through or reflected on the DUT  230  is measured by the second photoconductive switch  220  of the receiver. Accordingly, the electrical characteristics of the DUT  230  positioned between the terahertz transmitter  210  and the terahertz receiver  220  can be measured. The terahertz system, however, also bears limitations that it exhibits rather low spatial resolution and is inappropriate for use in near-field measurement due to relatively large sizes of the transmitter/receiver and due to various auxiliary devices for collecting the radiated terahertz electromagnetic wave. 
     Meanwhile, when carriers are generated by introducing short laser pulse to unbiased ultrafast photoconductive switch placed in the free space, the carriers displays movements affected by ambient electromagnetic field. The movement of the carriers solely depends on the component of the electromagnetic field that is parallel to the ultrafast photoconductive switch. Accordingly, components of electromagnetic field can be distinctively observed under the photoconductive sampling principle employing the ultrafast photoconductive switch. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an electric near-field probe capable of conducting direct measurement of the electric field component inside a high-speed device under test by using a photoconductive sampling principle utilizing short optical pulse and ultrafast photoconductive switches. 
     Another object of the present invention is to provide a terahertz electromagnetic wave radiator and measurement probe for use as a terahertz transmitter and a terahertz receiver in a terahertz system. 
     Yet another object of the present invention is to provide a manufacturing method of a probe for near-field measurement. 
     According to an aspect of the present invention, there is provided A near-field probe driven by a short laser pulse delayed through an optical delay-line for detecting vectors of near-field components of electrical signals propagating through a device under test including an electrical device or an electronic circuit based on a sampling principle, comprising: a photoconductive switch assembly including a thin semiconductor photoconductive body, at least two separated switch electrodes formed on the thin semiconductor photoconductive body, and an electrode gap formed between the two separated switch electrodes; and an optical waveguide attached to one side of the photoconductive switch assembly by using an optical adhesive, wherein the optical waveguide is partially coated with conductive material on the outer surface thereof, wherein the conductive material forms at least two parallel stripe electrodes in the longitudinal direction of the optical waveguide, and the stripe electrodes are electrically connected to each other with the switch electrodes and a core of the optical waveguide is optically aligned with the electrode gap. 
     According to another aspect of the present invention, there is provided An array near-field probe driven by a short laser pulse delayed through an optical delay-line for detecting vectors of near-field components of electrical signals propagating through a device under test including an electrical device or an electronic circuit based on a sampling principle, the array near-field probe including M by N (M and N are integers larger than 1) array of near-field probes, wherein each near-field probe includes: a photoconductive switch assembly including a thin semiconductor photoconductive body, at least two separated switch electrodes formed on the thin semiconductor photoconductive body, and an electrode gap formed between the two separated switch electrodes; and an optical waveguide attached to one side of the photoconductive switch assembly by using an optical adhesive, wherein the optical waveguide is partially coated with conductive material on the outer surface thereof, wherein the conductive material forms at least two parallel stripe electrodes in the longitudinal direction of the optical waveguide, and the stripe electrodes are electrically connected to each other with the switch electrodes and the core of the optical waveguide is optically aligned with the electrode gap. 
     According to yet another aspect of the present invention, there is provided A method for manufacturing a near-field probe, comprising the steps of: (a) growing or depositing a sacrificial layer on a semiconductor substrate; (b) growing or depositing an epilayer of a thin photoconductive body on the sacrificial layer; (c) forming two separated switch electrode patterns by using a photoresistor on the epilayer; (d) forming a photoconductive switch with an electrode gap between the two separated switch electrode patterns by depositing a metal electrode on each of the switch electrode patterns; (e) removing the photoresistor and the unnecessary metal except the switch electrodes by lift-off process; (f) forming a desired pattern of a photoconductive switch assembly by selectively etching the epilayer; (g) detaching the photoconductive switch assembly from the semiconductor substrate by using a chemical etching solution; and (h) forming the near-field probe by associating the photoconductive switch assembly with one end of an optical waveguide by using an optical adhesive, wherein the optical waveguide is coated with a conductive material, the conductive material is coated on the outer surface of the optical waveguide in the longitudinal direction thereof to form two parallel stripe electrodes, the stripe electrodes and a core of the optical waveguide are aligned to switch electrodes and the electrode gap of the photoconductive switch assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will become apparent from the following description of a preferred embodiment given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a set-up for measuring electrical signal propagating on a conducting line by using the photoconductive sampling method; 
         FIG. 2  is a schematic diagram of the terahertz system; 
         FIG. 3  illustrates a near-field probe for use in measuring tangential electric field component in accordance with a first preferred embodiment of the present invention; 
         FIG. 4  illustrates a constitution of an array probe in accordance with the second preferred embodiment of the present invention; 
         FIG. 5  shows a near-field probe in accordance with the third preferred embodiment of the present invention; 
         FIG. 6  describes a near-field probe in accordance with a fourth preferred embodiment of the present invention; 
         FIGS. 7   a  to  7   k  provide a fabricating process for the near-field probe; and 
         FIGS. 8   a  and  8   b  respectively exhibit exemplary probe positioning methods for measuring a transverse near electric-field component (X-direction) and a vertical near electric-field component (Y-direction) of an electromagnetic field. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 3  illustrates a near-field probe for use in measuring tangential electric field component in accordance with a first preferred embodiment of the present invention. 
     The inventive near-field detector comprises a photoconductive switch assembly  310  and an optical waveguide  320  placed by one side of the photoconductive switch assembly  310 . 
     The photoconductive switch assembly  310  includes a thin ultrafast photoconductive body, at least two switch electrodes  314  and  316  separated from each other formed on the thin ultrafast photoconductive body and an electrode gap  318  between the separated switch electrodes  314  and  316 . 
     The thin ultrafast photoconductive body  312  has a carrier lifetime of around picosecond or shorter and composed of materials capable of absorbing wavelength of short laser pulse. For instance, the thin ultrafast photoconductive body  312  uses LT-GaAs or SOS where the laser wavelength is less than or equal to 840 nm but uses InGaAs where the laser wavelength is less than or equal to 1.6 micrometer. The switch electrodes  314  and  316  of the photoconductive switch  319  are preferably made of such materials that could withstand an HF solution such as used in thin-film removal, e.g., Chromium(Cr) or Gold(Au). The switch electrodes, however, may be made of any metal as long as they are protected by coating with material such as wax. 
     Meanwhile, the optical waveguide  320  is made of an optical fiber or a dielectric waveguide, and on the outer surface thereof is partially coated by two stripes of conductive material in the longitudinal direction, the conductive material forming two stripe electrodes  322  and  324 . The stripe electrodes  322  and  324  is made of, e.g., a conductive epoxy. Short laser pulse generated from such as a mode-locked Titanium:Sapphire laser or a mode-locked fiber optic pulse laser is introduced to a core  326  or the center of the optical waveguide. 
     The photoconductive switch assembly  310  is attached on one end of the optical waveguide  320  with an adhesive for optical use. The photoconductive switch assembly  310  may be connected either to a side on which the photoconductive switch  319  is formed or to the other side on which the photoconductive switch  319  is not formed. As the adhesive for binding the optical waveguide  320  and the photoconductive switch assembly  310 , a ultraviolet adhesive is used since the short laser pulse can pass through it. The refraction coefficient of the ultraviolet adhesive is preferably smaller than that of the thin photoconductive body  312  and greater than that of the optical waveguide  320 . When the optical waveguide  320  and the photoconductive switch assembly  310  are bound, two stripe electrodes  322  and  324  of the optical waveguide  320  are aligned with two electrodes  314  and  316  of the photoconductive switch  319 , and at the same time, the core  326  of the optical waveguide  320  is aligned with the electrode gap  318  of the photoconductive switch  319 . The two stripe electrodes  322  and  324  of the optical waveguide  320  and the two electrodes  314  and  316  of the photoconductive switch  319  which are aligned with each other, are electrically connected through a conductive adhesive such as a conductive epoxy. 
     In the present invention, though the cross-section of the optical waveguide  320  is illustrated as a circle, it may be formed as rectangle or other shapes. 
     The inventive probe of  FIG. 3  is used for measuring tangential component of the near-field with respect to the DUT, i.e., transverse and longtudinal component of the near-field. If the inventive probe is used with a moving stage in the X, Y and Z direction or a three-dimensional scanning system with respect to the electrical signal detector  120  of  FIG. 1 , it can be used in producing a three-dimensional near-field map. If, however, a DC bias is applied and laser pulse is irradiated to the ultrafast photoconductive switch  319 , the terahertz radiation occurs. Referring to FIG.  2  and  FIG. 3 , by implementing a terahertz system with a terahertz transmitter and a terahertz receiver and by introducing an electromagnetic pulses from the terahertz system to various materials, characteristics of the various materials can be measured. In addition, it can be used in constituting images of the DUT by associating the probe in accordance with the present invention with the moving stage making a non-contact move in the X- or Y-direction or the two-dimensional scanning system. Therefore, the probe illustrated in  FIG. 3  is capable of performing terahertz imaging, measuring far field components as well as performing imaging through the measurement of the near-field components. 
       FIG. 4  illustrates a constitution of an array probe in accordance with the second preferred embodiment of the present invention. 
     The array probe illustrated in  FIG. 4  is an array including M by N number (M and N are integers) of the near-field probes that is illustrated in FIG.  3 . In the array probe of  FIG. 4 , each probe includes the photoconductive switch assembly  430  consisting of the photoconductive switch  420  and the optical waveguide attached to one end of the photoconductive switch assembly  430  as shown in FIG.  3 . Therefore, detailed description as to the photoconductive switch assembly  430  and the optical waveguide are omitted herein. 
     If each probe  410  is arranged perpendicular to each photoconductive switch  420 , it can be used as a two-dimensional tangential field components measuring array probe since it separately measures near-field component in the X-direction and Y-direction at the same time. 
     In addition, the array probe in accordance with the present invention, by applying DC voltage and introducing laser pulse to each ultrafast photoconductive switch  420 , is capable of causing the terahertz radiation, and capable of inducing terahertz radiation by applying DC voltage to a single probe among the array probe and measuring reflected terahertz electromagnetic waves by utilizing the remaining probes. Therefore, provided the terahertz system employing two array probes are used, it is capable of measuring both the terahertz electromagnetic wave reflected from the DUT and the terahertz electromagnetic wave transmitting through the DUT, which ensures obtaining an image. 
       FIG. 5  shows a near-field probe in accordance with the third preferred embodiment of the present invention. 
     The near-field probe in  FIG. 5  includes, like the near-field probe illustrated in  FIG. 3 , the photoconductive switch assembly  510  consisting of the ultrafast photoconductive body  512  and the photoconductive switch  519 , and the optical waveguide  520  attached to one end of the photoconductive switch assembly  510  with the optical adhesive. Unlike the near-field probe in  FIG. 3 , however, in the near-field probe in  FIG. 5 , the cross-section of one end of the optical waveguide  520  is inclined, e.g., 45 degrees, and the photoconductive switch assembly  510  is attached on this cross-section by using the optical adhesive. 
     The outer view of the incline of the optical waveguide  520  is an oval figure when a circular optical fiber is used as the optical waveguide, and in consequence, the switch assembly  510  attached to the incline is also of the oval figure. When bound by the optical adhesive, the core  526  of the optical waveguide  520  and the electrode gap  518  of the photoconductive switch  519  should be aligned, and at the same time, the two stripe electrodes  522  and  524  of the optical waveguide  520  and the two switch electrodes  524  and  526  of the photoconductive optical switch  519  should be electrically connected. 
     The probe in accordance with the third preferred embodiment of the present invention has advantages that it can measure not only the two-dimensional tangential component of electromagnetic field but also the three-dimensional vector including a normal component of the electromagnetic field. For the purpose of a near-field measurement, the probe should be positioned 45 degrees inclined with respect to the DUT. 
       FIG. 6  describes a near-field probe in accordance with a fourth preferred embodiment of the present invention. 
     The near-field probe in  FIG. 6  includes, like the near-field probe illustrated in  FIG. 3 , the photoconductive switch assembly  610  and the optical waveguide  620  attached to one end of the photoconductive switch assembly  610  with the optical adhesive. Unlike the near-field probe in  FIG. 3 , as for the near-field probe in  FIG. 6 , one end of the optical waveguide  620  is tapered by a prescribed tapering angle to form a bevel-edge  630  and the tip of the bevel-edge  630  is inclined like the near-field probe of FIG.  5 . The inclined angle of the tip of the bevel-edge  630  is 45 degrees for measuring three-dimensional right-angled intersection components, and further, the inclined angle of the taper of the bevel-edge  630  should be smaller than or equal to 45 degrees. Therefore, the inclined surface of the optical waveguide  620  is of oval figure when circular optical fiber is used, and accordingly the photoconductive switch assembly  610  attached to the inclined surface is also of oval figure, though the size is very small. 
     The near-field probe in accordance with the fourth preferred embodiment of the present invention has a constitution that the optical waveguide  620  is cut or polished as the form of the bevel-edge  630  and the tip thereof is polished to be inclined so, and very small ultrafast photoconductive switch assembly  610  is attached thereto. Therefore, it is capable of separately measuring each near-field component in each of the X-, Y- and Z-direction near the DUT in the order of several micrometers, and in consequence, is capable of measuring three-dimensional vectors. Herein, the probe should be positioned relative to the DUT with 45 degrees. 
       FIGS. 7   a  to  7   k  provide a fabricating process for the near-field probe shown in  FIGS. 3  to  6 . The fabricating process may be applied to the near-field probe that exploits a LT-GaAs epilayer as an active photoconductive film. 
     First, as shown in  FIG. 7   a , a substrate  710 , e.g., SI-GaAs substrate is prepared, and then, a sacrificial layer  712  is layered atop the substrate. Herein, the sacrificial layer  712  is a selective-etching material such as AlAs, and a few tens of nanometers would be enough for the thickness of the sacrificial layer  712 . 
     As a next step, as shown in  FIG. 7   b , a thin ultrafast photoconductive body is formed on the sacrificial layer  712  with thickness of 100 nanometers to 2 micrometers by growing a photoconductive epilayer  714 . The photoconductive epilayer  714  is formed by growing GaAs when AlAs is used as the sacrificial layer  712  or by growing InGaAs when InP is used as the sacrificial layer  712 . The sacrificial layer  712  is utilized for isolating the photoconductive epilayer  714  from the substrate  710 , as described later. 
     As shown in  FIGS. 7   c  and  7   d , the photoconductive epilayer  714  is cleaned before a photoresistor  716  is deposited on the cleaned photoconductive layer  714 . The photoresistor  716  is patterned to form two electrode patterns  718  and  720  by etching the photoresistor  716  with a developer, e.g., AZ5214 using an image reversal photolithography. An electrode gap pattern is defined between the two electrode patterns  718  and  720  on an unetched photoresistor  716 . 
     As a next step, as shown in  FIG. 7   e , a switch electrode  722  is formed on the switch electrode patterns  718  and  720  by depositing a metal according to the method of a evaporation or a sputtering. The metal deposited on the region other than the switch electrode pattern  718  and  720  are removed together with the photoresistor  716  by using a lift-off technique. Thereby, the photoconductive switch  726  with electrode gap  724  is fabricated. 
     As shown in  FIG. 7   g , the photoresistor  728  is deposited on the photoconductive switch  726 . Next, as shown in  FIG. 7   h , the photoconductive epilayer  714  is etched out except a desired photoconductive assembly  730 . Herein, as the method for etching out the photoconductive epilayer  714  a dry etching technique by using plasma or a chemical etching using mixed solution including a diluted ammonium hydroxide acid(NH 4 OH) and hydrogen peroxide. When using the chemical etching method, the etching of the photoconductive epilayer  714  can be verified by rainbow color reflected on the scarifying layer  714  of which the refraction coefficient is lower than that of the substrate  710 . 
     According to the process above, the photoconductive switch assembly  730  will have different patterns depending on each near-field probe shown in  FIGS. 3  to  6 . That is, provided that the photoconductive switch assembly  730  uses circular optical fiber as the optical waveguide, the photoconductive switch assembly  730  is formed as a circular figure when applied to the near-field probes of  FIGS. 3 and 4 , while the photoconductive switch assembly  730  is formed as an oval figure when applied to the near-field probes of  FIGS. 5 and 6 . 
     Next, as shown in  FIG. 7   i , the photoconductive switch assembly  730  is detached from the substrate  710  by selectively etching the sacrificial layer  712  using a chemical etching solution. Herein, the chemical etching solution is selected depending on the material used as the sacrificial layer  712 . For instance, diluted hydrofluoric acid(HF) solution is used when the sacrificial layer is of AlAs and diluted hydrochloric acid(HCL) is used when the sacrificial layer is of InP. The photoconductive switch assembly  730  detached from the substrate are then be preserved in Isopropyl Alcohol(IPA). 
     Next, as shown in  FIG. 7   j , the optical waveguide  740  of  FIGS. 3  to  6  on which the stripe electrodes  742  and  744  are coated is associated with the photoconductive switch assembly  730 , to form the near-field probe as shown in  FIGS. 3  to  6 . If the photoconductive switch assembly  730  is associated with the optical waveguide  320  and  420  of  FIGS. 3 and 4 , it functions as the near-field probe or the array probe; if the photoconductive switch assembly  730  is associated with the optical waveguide  520  of  FIG. 5 , it functions as the three-dimensional near-field probe; and if the photoconductive switch assembly  730  is associated with the optical waveguide  620  of  FIG. 6 , it functions as the very small three-dimensional near-field probe. 
     As a final step, as shown in  FIG. 7   k , by using a conductive adhesive such as the conductive epoxy, the two switch electrodes  722  of the photoconductive switch assembly  730  are electrically connected to the two stripe electrodes  742  and  744  coated on the optical waveguide  740  respectively, to finally constitute the near-field probe. 
       FIGS. 8   a  and  8   b  respectively exhibit exemplary probe positioning methods for measuring a transverse near electric-field component (X-direction) and a vertical near electric-field component (Y-direction) of an electromagnetic field originated from the DUT when the DUT is introduced to the pump laser pulse beam when the near-field probe is provided. 
     Referring now to  FIG. 8   a , the photoconductive switch  820  associated with a coplanar waveguide(CPW) is formed on the ultrafast photoconductive body  810 . The probe in accordance with the present invention is capable of measuring the transverse near electric-field component (X-directional component) generated by a picosecond pulse propagating on the coplanar waveguide. If a pump laser pulse beam is introduced to the photoconductive switch  820  to which DC bias has been applied, a picosecond electrical pulse signal is generated. When a probe laser pulse beam having delayed at the optical delay-line  850  is introduced to the end point of the near-field probe  830  through the optical waveguide, carriers generated at the photoconductive switch  840  are forced to move by a transverse component at the measurement point, which leads to measurement of the transverse component at the measurement point near the coplanar waveguide. 
     The longitudinal component (Z-directional component) can be measured in the state that the probe is rotated clockwise by 90 degrees (the photoconductive switch  840  of the probe is positioned in parallel with the measured surface of the DUT or in the Z-direction) from the configuration for measurement of the transverse component shown in  FIG. 8   a  (the photoconductive switch  840  of the probe is positioned in parallel with the measured surface of the DUT or in the X-direction). On the contrary,  FIG. 8   b  illustrates a state for measuring the normal field component, and therefore, the photoconductive switch  840  is located in the perpendicular direction with the measured surface of the DUT. 
     The near-field probe in accordance with the present invention is capable of separately measuring near-field components of electrical signals existing near the region of interest above the DUT by scanning the region of interest of the DUT using an electronically controlled two-dimensional or three-dimensional scanning system and finally forming a map of the three-dimensional field components resulting from the electrical signal propagating on the DUT. 
     As described above, in accordance with the near-field probe of the present invention, sub-picosecond temporal resolution can be achieved by employing the ultrafast photoconductive switch, the space between the photoconductive switches can be limited to sub-micron, and sub-micron spatial resolution can be implemented by utilizing a single-mode optical waveguide according to a laser wavelength. 
     In addition, a high sensitivity of measurement can be achieved since the DUT can be measured by the movement of the carriers generated by the laser beam and having sub-picosecond lifetime, and therefore, less affected by a noise source. 
     Moreover, a so-called loading effect can be minimized since the carriers generated by the laser beam degenerate in sub-picoseconds and the ultrafast photoconductive epilayer with thickness of 100 nanometers to 2 micrometers is detached and transplanted by using the epitaxial lift-off (ELO) technique. 
     While the invention has been shown and described with respect to the preferred embodiments, it will be understood by the skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.