Abstract:
A probe for testing integrated circuits at microwave frequencies employs a tapered coaxial transmission line to transform the impedance at the probe tips to the impedance of the test instruments. Mechanically resilient probe tip structures allow reliable probing of non-planar circuits and the elastic probe body allows large overprobing without damage to the test circuit. Novel insulator structures for the coaxial line allow easy and accurate assembly and high performance.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to a probe device for testing integrated circuits such as amplifiers, signal processors, mixers, filters, and analog to digital converters. The probe device may be employed for testing integrated circuits before separation from the semiconductor wafer, after separation, after the circuit has been placed into a package or after insertion into a circuit board. In general, the probe device is intended to transfer signals to and from a circuit during testing and to provide an impedance transformation to match the circuit impedance to that of the test instruments. 
     BACKGROUND OF THE INVENTION 
     Testing of integrated circuits (IC&#39;s), also called chips or dies, is an important part of the design and manufacture of the circuits. Initial testing is usually performed while the chips are still held together as parts of a semiconductor slice or wafer. Such testing requires a microprobe device that contacts the test pads on the chip and provides connections to the instruments employed for testing the circuits. Standard testing instruments for such circuits operate at a 50 ohm impedance level. Most integrated circuits operate at impedance levels that differ from 50 ohms. The impedance mismatch between the integrated circuit and the testing instruments makes accurate testing difficult and in some cases impossible. Fortunately methods for impedance matching are very well known in the electrical engineering art. There are reactive networks described for example on pages 206 to 215 of The Radio Engineer&#39;s Handbook by Terman, McGraw-Hill, 1943 (Reference 1). Another reference is Radio Engineering, third edition by Terman, McGraw-Hill, 1947 (Reference 2) where pages 100 to 104 describe the same reactive networks referred to above and pages 104 to 109 describe transmission line methods including tapered transmission lines, quarter wave matching sections and shorted stubs. A third reference is The ARRL Antenna Book, Published by The American Radio Relay League, Newington, Conn., 1994. The use of transformers to match impedances is also well known. 
     Another testing difficulty occurs when the chip to be tested has balanced input or output circuits. This presents a problem because the testing instruments are almost universally unbalanced with system ground on the external shield of 50 ohm coaxial cables and with the test signals on the center conductors. Fortunately circuits to convert from balanced to unbalanced modes (commonly referred to as Baluns) are also well known. See for example page 690 of the reference 2 above, and pages 26-9 to 26-13 of Ref. 3. Although application of the techniques of impedance matching are well known, particular structures for implementing those techniques in ways that are advantageous in the microprobing environment may not be obvious. 
     This specification describes an impedance matching probe employing a tapered transmission line that includes mechanically resilient contacting tips and structures to allow impedance matching over a broad frequency range and large impedance ratios. 
     OBJECTS OF THE INVENTION 
     This invention is directed toward probe apparatus for matching unequal impedances over a broad frequency range. A coaxial transmission line (cable) having a cylindrical center conductor separated from a surrounding outer conductor by an insulator has dimensions that provide an impedance to match a first impedance at a first end of the line and a different impedance to match a second impedance at a second end of the line. Mechanically resilient electrically conducting structures on the first end of the line are adapted to contact a device to be tested and a connector structure on the second end of the line is adapted to connect with test instruments. The insulator between the inner and outer conductors may be air, a solid dielectric, a dielectric foam, a dielectric powder or a more complicated dielectric structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows three different probe bodies. FIG. 1 a  has a tapered inner conductor, FIG. 1 b , a tapered outer conductor and FIG. 1 c  both tapered inner and outer conductors. 
     FIG. 2 shows the details of the probe tip structure. 
     FIGS. 3 a ,  3   b  and  3   c  show alternate methods for mounting the probe. 
     FIGS. 4 a ,  4   b  and  4   c  shows alternate types of insulators. 
     FIG. 5 a  shows a form for molding a particular insulator structure. 
     FIG. 5 b  shows the resulting insulator installed in a probe body. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 a  shows a cross-sectional drawing of a probe body that consists of a coaxial line with a center or inner conductor  12  surrounded by an insulating layer  13  which in turn is surrounded by an electrically conducting, uniform diameter shield  14 . Insulator  13  may be air, solid dielectric or foam or a more complex structure that will be described later. To provide impedance matching, the diameter of the center conductor  12  is tapered so that the diameter is larger at one end of the probe body than at the other. Alternatively, the inner conductor may be uniform in diameter and the outer shield may be tapered as shown in FIG. 1 b , or both inner conductor and outer shield may be tapered as shown in FIG. 1 c . The inner conductor  12  extends out beyond the end of the shield  14  and the insulating layer  13  at the right and is suitably shaped to make contact with a device to be tested, also called a device under test (DUT). An electrical connector may be attached to the left end of the coaxial line for connection to testing instruments. Alternatively, connection to the test instruments may be made by connection to a coaxial line or a strip line or a coplanar line or other transmission line that then connects to the test instruments. The dimensions of the tapered coaxial line at the connector end are chosen to provide the desired impedance match to the testing instruments. The probes depicted in FIGS. 1 a ,  1   b  and  1   c  would transform a higher impedance at the DUT on the right to a lower impedance (usually 50 ohms) at the left end that connects to the test instruments. To transform a lower impedance at the tips, the spacing between the inner conductor and the outer shield would be smaller at the tip end than at the connector end. The tapered line structures shown in FIGS. 1 a ,  1   b  and  1   c  are inherently broad band and can match impedances over a much wider frequency range than other impedance matching structures. Such tapered line structures are useful only above a cutoff frequency that depends on the length of the line, the impedance ratio at the two ends and the form of the taper. Although the drawings show a linear taper, a nonlinear taper of the proper type provides additional advantages. In particular, a taper that yields an impedance that varies exponentially along the length of the line affords the largest bandwidth for any probe length and impedance ratio. (see reference 1, page 197) 
     FIG. 2 shows the details of the probe extensions that make contact with the DUT. The solid and dashed lines in FIG. 2 indicate the position of the probe tips and the DUT before and after making electrical contact. Electrical conductors  21  and  23  are mechanically and electrically connected to the shield  14  and are arranged to extend beyond the end of shield  14  and are also suitably shaped to make contact with the DUT. Advantageously,  21  and  23  should be mechanically resilient in the vertical direction as indicated by the double arrow  24  to allow for unavoidable variations in the vertical placement of the tip ends  21  and  23  and to allow for unavoidable variations in the tilt of the DUT. Advantageously, the spacing  25  between  21  and  23  and the center conductor extension  22  should be set so that the impedance of the tip structure matches the impedance of the coaxial line at the connection between the tips and the coaxial line. An additional increase or decrease in impedance at the DUT may be achieved by increasing or decreasing the spacing  25  toward the tips that touch the DUT. In addition,  21 ,  22  and  23  should be relatively rigid in the horizontal directions so that the tip spacing is held fixed during probing and the contacting points of  21 ,  22  and  23  may be placed accurately near the center of the contact pads  26 ,  27  and  28  of the DUT. For these reasons, although conductors  21 ,  22  and  23  could be ordinary needles, advantageously they may be formed as thin sheets tapering to a small contacting point. The contacting points may have small flattened surfaces where they meet the DUT so the points do not damage the contact pads  26 ,  27  and  28 . The center conductor extension  22  may also be thinned as shown to form a mechanically resilient tip or it may be formed into a more rigid shape. As described in our U.S. Pat. No. 4,871,964, with the more rigid center conductor,  21  and  23  are offset vertically with respect to the center conductor so that as the DUT is raised the contact points of  21  and  23  make contact to the DUT before the center conductor does. As the DUT is raised further,  21  and  23  flex to then allow the center conductor to make contact with the DUT. As the DUT is raised further (“overprobing” in testing jargon) the probe body (the coaxial line) flexes to limit the forces applied to the probe pads of the DUT. Also, although the conductor  22  in FIG. 2 is illustrated as one piece with inner conductor  12 ,  22  may be made of a different conducting material and attached to  12  by suitable bonding, brazing or other method. Although two contacting elements  21  and  23  are shown in FIG. 2, it is to be understood that only one tip is necessary to connect the shield  14  to the DUT. The double connection is advantageous in that the two elements  21  and  23  partially shield the signal on the inner conductor extension  22  to reduce radiation and capacitive and magnetic coupling to other parts of the DUT and the surrounding space. The tips that make contact with the probe pads of the DUT may be simply tapered as indicated by  21   a.  Alternatively, the tip portion may be narrowed and bent downward as indicated by  22   a.  The advantage of this type of tip is that as the tip wears during use, the contact area remains substantially constant. An additional type of contacting tip indicated by  23   a  is a short pillar of material different from that of the spring structure  23  and welded, brazed or soldered to  23 . Such a contacting tip may be advantageous for probing difficult to contact DUTs such as those employing aluminum metalization for the probe pads  26 ,  27  and  28 . 
     The contact elements  21  and  23  may be formed separately and attached to the shield  14  by slitting shield  14  and soldering, brazing or welding as indicated by the fillet  29 , or they may be attached in a variety of other ways such as simply attaching without slitting the shield or by sectioning a short length of the probe body to form a shelf on which the tip structures are then fastened as indicated in U.S. Pat. No. 5,506,515. The tips may also be held by a membrane and attached as indicated in U.S. Pat. No. 4,894,612. In addition, the tip structures  21  and  23  may be fashioned from the shield material itself by metal forming methods. 
     When the insulator  13  is a non-rigid material such as to be described in connection with FIGS. 4 and 5, it may be necessary to include a rigid electrically insulating insert  13   a  at the tip end as indicated in FIG.  2 . Otherwise the stress on the inner conductor resulting from the force of the probe tip contacting the DUT may force the inner conductor  12  away from its proper position inside the outer shield  14 . A short section of inner conductor  12  may have a reduced diameter so that the rigid insert  13   a  and the reduced diameter section of the inner conductor  12  form a short section of transmission line that matches the impedance of the probe body where it meets the insert  13   a.  The rigid insert  13   a  may be manufactured separately and inserted into the probe body or it may be formed in place by flowing a liquid insulator into the space and allowing it to harden. 
     As shown in FIG. 3 a , the probe device may be mounted on a probe card  31  by soldering or adhesive fastening  32  or as shown in FIG. 3 b  by mounting via a bracket  33 . Alternatively the probe device may be mounted alone or with other probe devices on a separate holder or bracket  34  that may be mounted on a standard probe station via mounting holes  35  as shown in FIG. 3 c . These mountings may or may not include a standard connector  37  to facilitate connection to the test instruments. 
     The distance  36  between the probe tips and the probe mounting is chosen to provide the proper mechanical stiffness so that as the DUT is raised and the tips make contact with the DUT, the probe body, which is the coaxial line, may flex and the proper probing forces are exerted between the contacting tips and the probe pads of the DUT. 
     Details of the probe construction are shown in FIG.  4 . It is advantageous that the inner conductor  12  be held securely in a fixed position inside the shield  14 . One method FIG. 4 a  is to hold the inner conductor in its proper position while an insulator  41  in liquid form is flowed into the space between the inner and outer conductors. The insulator may be liquified by heating and then solidified by cooling or it may be a thermosetting compound that is liquid at ordinary temperature and solidifies as the result of chemical changes. An alternative method FIG. 4 b  is to fill the space with a fine, electrically insulating powder  42 , or the inner conductor may be wrapped with a tape or thread made from solid or foamed or fibrous material. In other methods FIG. 4 c  an insulating filament or tape of more complex structure may be wrapped around the inner conductor or a series of insulating toroids may be placed at intervals over the inner conductor. The filament or toroids may be held in place by adhesive or by grooves cut into the outer or the inner conductor. Advantageously, the filament, tape or toroids may have a structure as indicated in FIG. 4 c.  The thin fingers or fibers are flexible so that although the ends of the thin fingers extend inward to a smaller diameter than that of the inner conductor  12 , the fingers flex to allow the insertion of the center conductor into the shield  14 . A particularly simple way of providing an insulator similar to that shown in FIG. 4 c  is to manufacture a fine cloth such as velvet with a relatively long nap. A rectangle of the proper size and shape can then be cut from the material, rolled up and inserted into the shield  14  with the nap inward. The inner conductor  12  can then be inserted by forcing the ends of the nap to spread. The materials of the cloth should have low electrical loss and the nap should be mechanically stable to keep the inner conductor in a fixed position inside the shield  14 . 
     The insulator may also be formed separately in a mold, removed from the mold and then inserted into shield  14  after which the inner conductor  12  is inserted into the insulator. If the insulating material is an elastic foam, the hole to receive the inner conductor may be undersize so that as the inner conductor is inserted, the hole spreads, securely grasping the inner conductor. 
     A more complicated insulator structure may also be molded as indicated by the cross-sectional drawing of FIG. 5 a.  Here, a screw-shaped inner portion  51  of the mold has a core  52  that is tapered to correspond with the taper of the inner conductor of the coaxial probe. A sleeve  53  forms the outside of the mold with a space  54  between the inside surface of the sleeve  53  and the outside of the screw-shaped inner portion  51 . With  51  and  53  held in place, an insulating material in liquid form is injected into the space between  51  and  53 . After the insulator has solidified, the insulator is removed from the mold by turning (unscrewing)  51  out of the insulator and then removing the insulator from the inside of  53 . The insulator is then inserted into the shield  14  of the probe and the inner conductor  12  is inserted as shown in FIG. 5 b . The space  57  is air filled and therefore has a lower dielectric constant than the insulating material. Advantageously, the space  54  (FIG. 5 a ) which gives rise to the insulating web  56  that holds the insulator together in one piece should be as large as possible at the large end of center conductor  12 . This produces the smallest air space and produces the lowest possible impedance at the low impedance end of the probe (on the left) because the dielectric constant of the insulator is larger than that of air. At the high impedance end of the probe (on the right), a large fraction of the space is filled with air so the impedance there is larger than it would be if the space were filled with insulator. Therefore, this strategy allows the largest impedance ratio for a given ratio of large to small diameters of the center conductor  12 . Similar insulator structures may be constructed to provide an insulator for the probe bodies shown in FIGS. 1 b  and  1   c.    
     Although we have only described the preferred embodiments of our invention, those skilled in the art may devise variations that do not significantly differ from the spirit and scope of our invention. For example, although we have indicated a straight probe body, the body could be curved or even coiled in order to fit the probe into a smaller space. The outside surface of  12  and/or the inside surface of  14  may be plated with a thin coating of highly electrically conductive metal to reduce the electrical losses in the probe. Throughout the figures we have indicated axially symmetric structures with circular crossection whereas any combination of square, rectangular, oval or other forms could be employed for either or both the inner or outer conductors. Also, it is not necessary for the inner conductor to be in the geometric center of the outer conductor but may be offset if required. In cases where the DUT presents an impedance that is not entirely resistive, that is one that includes a reactive component, an appropriate capacitive or inductive element may be included in the tip structure or at the connection to the test instruments to provide the proper conjugate match. In FIG. 2, the probe pads  26 ,  27  and  28  are shown as residing in a common plane. It is obvious that in a situation where the probe pads of the DUT are significantly nonplanar, for example with stripline, the tip structures  21 ,  22  and  23  may be displaced vertically to accommodate the non-planar configuration of the DUT. Furthermore, in the place of or in addition to the tip structures  21  and  23 , a separate probe needle may be employed and connected to the outer conductor  14  with a low inductance conductor in a manner analogous to that described in FIG. 2 of our U.S. Pat. No. 5,373,231.