METHOD AND APPARATUS FOR GUIDING ELECTRIC CURRENT

Abstract: A method for generating and guiding an electric pathway from one electrode (5) to another (6), comprises the steps of: 1) providing at least two electrodes between which it is desired to drive the electric current, said electodes being immersed in a medium; 2) providing a voltage between said electrodes, wherein said voltage is lower than the electrical breakdown voltage of said medium; 3) applying a laser pulse along at least a section of the path where it is desired to create the electric pathway, the energy of said laser pulse being sufficient to generate a plasma within said medium along said pathway; 4) continuing to apply a voltage or current, after the end of the laser pulse, of a magnitude sufficient to sustain the electric discharge; and 5) optionally injecting a gas at at least a location along the predetermined pathway between the electrodes.

Description: METHOD AND APPARATUS FOR GUIDING ELECTRIC CURRENT Field of the Invention The present invention relates to the wireless conduction of electric current. More particularly, the invention relates to the guiding of electric current through a laser- induced plasma. Background of the Invention Lasers have been used in the art in order to generate conductive paths in otherwise non-conductive media. Particularly, plasma has been generated, using laser energy, so as to create an ephemeral conductive pathway. This has been effected, for instance, in US Patent No. 5,179,279, in which a laser beam is focused onto a metallic plasma forming foil, which is positioned near the conductive surface so that the plasma touches the foil at a spot. Once physical contact is created, electric current may flow theretrough. Another example is found in US Patent No. 5,202,623, which describes a laser- activated plasma chamber for non-contact testing, in which plasma is generated in air, within a chamber, and the plasma then exits the chamber due to its self-generated pressure, to impinge onto the surface with which it is desired to establish electric contact. The generation of plasma, which impinges on a surface to create a conductive path therewith, is exploited, e.g., in US Patent No. 5,587,664, of the same assignee hereof, for the non-contact inspection of electric parts. Laser guiding of plasma was also used in a discharge machining apparatus, as is described in US Patent No. 5,496,983. The basic physical effect exploited by the present invention involves forming a conductive pathway, in open atmosphere, between a metallic tip and another metallic surface, e.g., a metallic pad that is a part of a printed circuit board (PCB). This is an example of an important use of the invention. By transferring some electrical current between two such metallic tips, through the corresponding pads, the electrical resistance of the net that connects the two pads is measured. The electrical current is conducted by gaseous plasma. The phenomenon of creating gaseous plasma by electric voltage is called 'Electrical Discharge.' Normally, electrical discharge in air requires very high voltage (above 30 kV/cm, or 1200 V in the experimental setup that will be described below, if the tip-to-pad distance is 0.4 mm), and is very 'dynamic' and aggressive. It is very difficult to control its direction. For the purposes of the present invention a method is needed to establish the conducting pathway by using much less voltage, in order not to damage the surface to which the electric current is directed, e.g., the PCB. A very narrow pathway and a good control over its direction are needed, in order to probe very small and dense features of some PCBs. Another related phenomenon, wherein gaseous plasma is generated by laser light, is called Optical Discharge' or 'Optical Breakdown.' The electric field of the light ionizes the gas. The plasma is generated at the beam focus and fades out quickly after the laser pulse ceases. Huge peak power is required for this effect, and there is a lower limit on the pulse length, to enable the discharge to evolve. Of course, all the above discussion of physical phenomena is not meant to limit the invention in any way or with reference to any particular theory, and is provided only for background purposes. The prior art has concentrated mainly on generating a conducting pathway by ablating a metallic plasma from the target conductor, which ablated metal generates, under the conditions employed in the art, a metallic plasma which is of conductive nature. This approach, while useful in some cases, suffers from some severe drawbacks: The metallic plasma is very short living and difficult to control. It also requires a relatively high amount of laser energy that is sufficient to produce substantial amount of metallic plasma, and therefore requires relatively powerful and expensive lasers. Of course, plasma can also be generated and conducting pathway can be established by applying a high voltage, which exceeds the breakdown voltage, but this method is significantly disadvantageous for the purposes of the present invention, as was explained above. It has now been surprisingly found, and this is an object of the present invention, that it is possible to obtain a precise and finely controlled electrical conducting pathway, using much lower laser energy than used by prior art methods, and to maintain this pathway open, long after the laser pulse has ceases. Hence, in the present invention, the laser serves as a trigger and a guide to the conducting pathway. It is an object of the present invention to provide a method for precisely and accurately guiding an electric pathway from one electrode to another. It is another object of the invention to provide such a method for guiding an electric pathway, which can be implemented using low-energy and low-cost laser equipment. It is a further object of the invention to provide a method for triggering a plasma-based conductive pathway, which is not laser-sustained, using a laser pulse as a trigger. It is yet another object of the invention to provide a method for maintaining such a conducting pathway after the end of the laser pulse. It is still another object of the invention to provide apparatus for carrying out the method. Other objects and advantages of the invention will become apparent as the description proceeds. SUMMARY OF THE INVENTION In one aspect, the invention is directed to a method for generating and guiding an electric pathway from one electrode to another, comprising the following steps: a) providing at least two electrodes between which it is desired to drive the electric current, said electrodes being immersed in a medium; b) providing a voltage between at least two of said electrodes, wherein said voltage is lower than the electrical breakdown voltage of said medium; c) applying a laser pulse along at least a section of the path where it is desired to create the electric pathway, the energy of said laser pulse being sufficient to generate a plasma within said medium along said pathway; d) continuing to apply a voltage, after the end of the laser pulse, of a magnitude sufficient to sustain the electric discharge; and e) optionally injecting a gas to at least a location along the predetermined pathway between the electrodes. Preferably, the gas employed is a gas which is favorable to electric discharge, e.g., Helium, Neon, Argon, Xenon, etc. The term "electrode", as used herein, means any conducting element to which a potential can be applied with reference to a second conducting element. This term includes, for instance, surfaces of electric elements, e.g., printed boards or other interconnecting media such as multichip modules, flat panel displays, integrated circuits etc., as well as thin electrodes used for the sole purpose of generating the current. By "breakdown voltage" it is meant to indicate the voltage that, when applied between the two electrodes, causes an electric discharge between the electrodes. According to the invention, the breakdown voltage is not to be reached, since it may result in uncontrolled and non-directional surges of electric current. Additionally, the breakdown voltage is quite high, above 1000 V in a typical setup to which the invention refers. A typical set-up to which the invention refers is such wherein the electrodes are in open atmosphere and the distance between them is up to about 1 mm. According to a preferred embodiment of the invention, the voltage between the electrodes before contact is established is in the range of 100 - 500 V. It decreases to 5 - 50 V when the laser fires and contact is established, and lasts during the conduction phase. This, as will be appreciated by the skilled person, is substantially less than the aforementioned breakdown voltage. The electric current obtained during the process, according to the abovementioned preferred embodiment of the invention, is up to several amperes. The current can vary during the discharge process, but is nevertheless measurable, and it is easily used for testing purposes, such as those described in US 5,587,664. As stated, according to the invention it is possible to use low-energy lasers. According to a preferred embodiment of the invention, for instance, the energy supplied by the laser during the process is in the range of 1 - 500 μj. The gas to be injected along tconductive pathway may be any suitable gas, which is favorable to the generation of plasma, and which is suitable for the purposes for which the pathway is provided. When operating in air, for instance, illustrative and non- limitative examples of suitable gases include helium, argon, neon, xenon and other inert gases. The invention is further directed to apparatus for generating and guiding an electric pathway from one electrode to another, comprising: (A) Two electrodes which define two extremities of an ephemeral electric pathway; (B) An electric circuit to supply an under-electric breakdown voltage between said two electrodes; (C) A laser, coupled with optical means to direct a laser beam along at least a section of a predetermined pathway; (D) An electric circuit, which may be the same as, or different from the electric circuit defined under (B) above, to sustain electric discharge induced by the laser beam; and (E) Optionally, means for directing a flow of a desired gas to at least one location along the predetermined pathway between the electrodes. By "ephemeral electric pathway" it is meant to indicate a pathway which exists and is functional only as long as laser light and/or electric voltage are supplied at a level sufficient to cause electric current to be conducted along it. Brief Description of the Figures The above and other characteristics and advantages of the invention will be more readily apparent through the following detailed description of preferred embodiments thereof, with reference to the appended figures, wherein: Fig. 1 is the "Paschen curve", which gives the characteristic Breakdown Voltage for various gases; Fig. 2 is the Electric Discharge I-V curve; Fig. 3 is a photograph showing the knee-shaped discharge obtained in the experimental setup detailed below; Fig. 4 schematically shows a typical setup of a system operating according to one preferred embodiment of the invention; Fig. 5 schematically shows an electric circuit that may be employed in the system; Fig. 6 shows a setup according to another preferred embodiment of the invention, in which the apparatus is utilized for the testing of electrical circuits and two equivalent heads are used; Fig. 7 shows typical voltage and current signals of the setup shown in Fig. 6; Fig. 8 shows results which illustrate the laser energy E required to establish a contact, as a function of the distance X between the electrodes at the same electrical parameters, as obtained in Example 2; and Fig. 9 shows the voltage drop ΔV on a contact in the conduction phase obtained in Example 3, while varying the distance X between the electrodes at the same laser parameters. Detailed Description of Preferred Embodiments Taking for example a PCB pad as a representative surface to which the electric current will be directed from the metallic tip, the laser pulse impinges the pad. It heats the spot and ablates some atoms from the metallic surface, while ionizing most of them. The amount of ablated material is a function of the pulse characteristics (Energy, Peak Power, Power density), the laser wavelength (Shorter wavelength is more efficient for ablation), and pad material. In particular, the part of the laser energy that is absorbed by the pad is a function of the wavelength and the material. The laser pulse also interacts with the gas. The electric field of the light can ionize the gas itself. This effect is also a function of the laser characteristics and the gas type. For this effect, longer wavelength is more efficient. High peak power is required but there is also a lower limit for the pulse length, because of the time it takes the breakdown to evolve. The well-known 'Paschen curve' (Fig. 1) gives the characteristic Breakdown Voltage for various gases. This is the voltage required to initiate electric-discharge between two electrodes in a gas. This voltage is significantly reduced by all the abovementioned effects, if they take place at the suitable time in-between the two electrodes. The breakdown voltage is much lower for specific (inert) gases, like argon, than in air, as is shown in Fig. 1. Therefore, according to a preferred embodiment of the invention, the discharge is assisted by injecting a gas into the desired pathway. The breakdown voltage depends also upon the shape and material of the electrodes. Yet, several hundred volts are required to ignite the discharge, which may extinguish quickly, because of recombination of the plasma. A voltage must be further applied to sustain the discharge. The amount of voltage depends on the specific type of discharge that takes place. The characteristics I-V curve of plasma-discharge, classifies the different types (Fig. 2). In the high-current limit of the curve (denoted by H) there is the arc-discharge range. In this type of discharge, the high current density maintains the discharge. Thus, a very low voltage is required. Fig. 3 is a photograph taken in the experimental setup described below. The result obtained is quite surprising: the electric discharge takes place not at a straight line, but rather follows a knee-shaped path. This photograph illustrates how the method of the invention influences the direction of the electric current and permits to direct it to the desired location with high precision. Arrow "A" shows the path of the laser light, and arrow "B" shows the orientation of the tip and gaz nozzle. An apparatus according to one embodiment of the invention, which is suitable for the testing of electrical circuits, is schematically shown in Fig. 4. The apparatus is constructed as follows: A laser beam from a Laser diode pumped, Q-switched, Nd:YAG laser 1 is coupled to an optical fiber 2 via coupling optics 3. The light leaving the fiber is focused through a focusing lens 4, and is then directed to the predetermined pathway (not shown), impinging on the target conductor 5 which functions as the distant electrode. A hollow metallic tip 6 serves both as the near electrode and as the gas injector, through which a gas (in this particular preferred embodiment of the invention, argon gas) is injected through a gas inlet 7. The tip and the gas jet are aligned to meet the predetermined pathway. The electrical circuit 8 is connected to the two electrodes (5 and 6). It supplies a voltage pulse, synchronized with the laser pulse by means of control circuit 9. It further supplies low voltage to maintain the plasma contact. Providing circuits such as electrical circuit 8 or control circuit 9 is well within the scope of the skilled engineer, and such circuits are therefore not described herein in detail, for the sake of brevity. An electric circuit is described briefly below, with reference to Fig. 5. The optical fiber employed in the experimental setup used to generate the data discussed below had a diameter of 50μ. Therefore, it is possible to direct the electric current to a very small area. Of course, the diameter of the optical fiber used will be adjusted, in each case, to meet the specific energy and precision requirements of a given system, as will be apparent to the skilled person. The electrical circuit is schematically shown in Fig. 5. The voltage from the high voltage power supply 11, and the sustaining power supply 12, are switched on synchronously (SI, S2), and are joined together through the diodes 13. The current source and the coupling transformer assembly 14 serve to control the discharge current. In another preferred embodiment of the invention, shown in Fig. 6, the apparatus is utilized for testing of electrical circuits. In this specific setup, two equivalent heads 21 are used. The beam from the laser 22 splits between two fibers 23 using a suitable beam splitter 24, and the electrical circuit 25, with doubled voltage, is connected between the two tips. The two heads are directed to create contacts with the two opposites of a net on the circuit under test 26. In this way, an electrical circuit is closed through the net. The net is tested by analyzing the characteristics of the electric signal that is transfer through it. Typical voltage and current signals of this setup are presented in Fig. 7. The upper trace is the voltage, negative on this figure. The peak voltage is about 850 V. When the laser fires, discharge occur (as indicated by the arrow at "D"), the voltage falls to some 50 V, and a current of about 1.2 A flows for several μsec. The current keeps flowing until the shutdown of the voltage, while the laser pulse ceases at about 15 nsec after the discharge. The apparatus described above was used in a series of experiments described in the examples to follow. Example 1 Using the experimental setup described above the resistance of a net was measured. The voltage drop on a contact itself was measured to be 25 V. The gross voltage drop between the tips and the current through the net in the conduction phase were measured. The net resistance is obtained by subtracting the voltage drop on the contacts from the measured gross voltage, and dividing the result by the measured current. Example 2 Using the experimental setup described above, the laser energy that is required to establish a contact was measured, while varying the distance between the electrodes at the same electrical parameters. The results are shown in Fig 8. Example 3 Using the experimental setup described above, the voltage drop on a contact in the conduction phase was measured, while varying the distance between the electrode at the same laser parameters. The results are shown in Fig 9. All the above description of preferred embodiments has been provided for the purpose of illustration, and is not intended to limit the invention in any way. Many modifications can be effected in the method of the invention. For instance, different laser types can be used. Different gases in different pressure can be employed, for both the surrounding gas and the gas being injected. Furthermore, many different setups can be provided for different purposes, using many different arrangements, electrodes, distances between electrodes, numbers of electrode pairs, etc., all without exceeding the scope of the invention.

Claims: Claims 1. A method for generating and guiding an electric pathway from one electrode to another, comprising the steps of: a) Providing at least two electrodes between which it is desired to drive the electric current, said electrodes being immersed in a medium; b) providing a voltage between said electrodes, wherein said voltage is lower than the electrical breakdown voltage of said medium; c) applying a laser pulse along at least a section of the path where it is desired to create the electric pathway, the energy of said laser pulse being sufficient to generate a plasma within said medium along said pathway; d) continuing to apply a voltage or current, after the end of the laser pulse, of a magnitude sufficient to sustain the electric discharge; and e) Optionally injecting a gas at at least a location along the predetermined pathway between the electrodes. 2. A method according to claim 1, wherein the distance between the electrodes is up to about 1 mm. 3. A method according to claim 1 or 2, wherein the voltage between the electrodes during the ignition process is in the range of 100 - 500 V. 4. A method according to any one of claims 1 to 3, wherein the voltage between the electrodes during the conduction phase is in the range of 5 - 50 V. 5. A method according to any one of claims 1 to 4, wherein the electric current during the process is up to 10 amperes. 6. A method according to any one of claims 1 to 5, wherein the energy supplied by the laser during the process is in the range of 1 - 500 μJ. 7. Apparatus for generating and guiding an electric pathway from one electrode to another, comprising: (A) Two electrodes which define two extremities of an ephemeral electric pathway; (B) An electric circuit to supply an under-electric breakdown voltage between said two electrodes; (C) A laser, coupled with optical means to direct a laser beam along at least a section of the predetermined pathway; (D) An electric circuit, which may be the same as, or different from the electric circuit defined under (B) above, to sustain electric discharge induced by the laser beam; and (E) Optionally, means for directing a flow of a desired gas to at least one location along the predetermined pathway between the electrodes. 8. Apparatus according to claim 7, wherein the distance between the electrodes is up to about 1 mm. 9. Apparatus according to claim 7 or 8, comprising means for generating between the electrodes a voltage, during the ignition process, in the range of 100 - 500 V. 10. Apparatus according to any one of claims 7 to 9, comprising means for generating a voltage between the electrodes during the conduction phase, in the range of 5 - 50 V. 11. Apparatus according to any one of claims 7 to 10, wherein the electrodes are suitable to withstand and electric current of up to 10 amperes. 12. Apparatus according to any one of claims 7 to 11, comprising a laser suitable to supply energy in the range of 1 - 500 μJ. 13. Apparatus for generating and guiding an electric pathway from one electrode to another, substantially as described and illustrated. 14. A method for generating and guiding an electric pathway from one electrode to another, substantially as described and with particular reference to the examples.

Inventors: BANITT DAVID, BEN-SHLOMO MOSHE, BLAU PINHAS, LEVY YAKOV, BANITT, DAVID, BEN-SHLOMO, MOSHE, BLAU, PINHAS, LEVY, YAKOV
Applicants: BANITT DAVID, BEN SHLOMO MOSHE, BLAU PINHAS, EXSIGHT ELECTRO OPTICAL SYSTEM, LEVY YAKOV, BANITT, DAVID, BEN-SHLOMO, MOSHE, BLAU, PINHAS, EXSIGHT ELECTRO-OPTICAL SYSTEMS LTD., LEVY, YAKOV
Application year: 1998
Main classification: G01R1
IPC codes: G01R1