Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R1/00—Details of instruments or arrangements of the types included in groups G01R5/00 - G01R13/00 and G01R31/00
  • G01R1/02—General constructional details
  • G01R1/06—Measuring leads; Measuring probes
  • G01R1/067—Measuring probes
  • G01R1/073—Multiple probes

Definitions

• the present invention relates to improvements in or relating to probes and in particular, to coplanar waveguide probes which can be used to measure the performance of integrated circuits.
• On wafer circuit testing For relatively low frequency silicon integrated circuits "on wafer” circuit testing is carried out by using conventional probe cards, as is well known in the art.
• the inductance of the individual probes and the inter-probe capacitances inherent to the needle-like structures prevent meaningful measurements at frequencies much above 10MHz.
• Some recent generations of silicon integrated circuits are capable of functioning at frequencies above 2 GHz and gallium arsenide circuits can achieve frequencies of 18 GHz and above. It can be seen, therefore, that there is a real need for wafer test equipment capable of reliable and meaningful radio frequency (r.f.) measurements to at least 18 GHz.
• r.f. radio frequency
• a probe system must be used which can provide a low loss, closely matched (low input voltage standing wave ratio) signal path from the measurement equipment to the r.f. input and output connecting pads of the integrated circuits under test.
• a coplanar waveguide probe comprising a dielectric substrate, a ground electrode and a signal electrode arranged on a surface of the substrate in spaced relationship, an electrically conductive layer arranged on a further surface of the dielectric substrate, and electrically conductive means for electrically connecting the ground electrode to the electrically conductive layer.
• the electrically conductive layer may extend into contact with the ground electrode so as to provide the electrically conductive means.
• the conductive means for electrically connecting the ground electrode to the electrically conductive layer may comprise a via hole, containing electrically conductive material, extending through the dielectric substrate spacing the ground electrode from the electrically conductive layer.
• the conductive means for electrically connecting the ground electrode to the electrically conductive layer may comprise a via hole, containing electrically conductive material, extending through the dielectric substrate spacing the ground electrode from the electrically conductive layer in combination with the electrically conductive layer extending into contact with the ground electrode.
• the electrically conductive material contained in the via hole may comprise conductive expoxy.
• the electrically conductive material contained in the via hole may comprise a coating of metallic material.
• the electrically conductive material contained in the via hole may comprise a filling of metallic material. Benefically the conductive layer comprises a metallised layer.
• the metallised layer may comprise gold.
• the metallic material contained in the via hole may comprise gold.
• the conductive means for electrically connecting the ground electrode to the electrically conductive layer comprises a plurality of via holes each containing electrically c nductive material, the cross sectional area of any via hole being dependent upon the spacing of the via hole from the tip of the probe.
• the ground electrode does not extend the length of the probe and the end thereof remote from the tip of the probe is of a predetermined tapered shape.
• the probe comprises a pattern of signal electrodes and ground electrodes, a ground electrode being disposed on and spaced from either side of each signal electrode and wherein the outermost ground electrodes are in contact with the electrically conductive layer extending around the dielectric substrate and the remaining ground electrodes are electrically connected to the electrically conductive layer by means of via holes containing electrically conductive material.
• the dimensions and spacing of the ground and signal electrodes are such that the probe exhibits a characteristic impedance of approximately 50 throughout its length.
• the dielectric substrate may comprise alumina.
• Figure 1 illustrates a generally schematic plan view of a coplanar waveguide probe
• Figure 2 illustrates a generally schematic end view of a known coplanar waveguide probe
• Figure 3 illustrates a generally schematic plan view of a coplanar waveguide probe in accordance with the present invention.
• Figure 4 illustrates a generally schematic cross sectional view through the line X-X of the probe shown in Figure 3.
• coplanar waveguide permits the relatively large dimensions of transmission lines required to interface with 50 SL characteristic impedance measuring equipment to be transformed to the typical 100 to 150 micron feature sizes encountered on integrated circuits.
• a coplanar waveguide probe 2 comprises a planar pattern of metal ground electrodes 4 and signal electrodes 6 printed on a dielectric substrate 8, such as an alumina substrate.
• the ground and signal electrodes 4, 6 are disposed on the substrate 8 such that each signal carrying electrode 6 is placed symetrically in the space between two ground electrodes 4, as shown in figure 1.
• the tip of the probe may be up to about 10 wavelengths away from the shorted ground connections and so the potentials of the ground electrodes at the probe tip may not be equal.
• this problem has been alleviated by bonding wire or tape loops 10 between the ground electrodes 4, as shown in figure 2 or by the use of nickel channel bridges extending between the ground electrodes (not shown).
• the wire loops 10 or nickel bridges are very costly and very time consuming to fabricate and, furthermore, the wire loops 10 are very susceptible to mechanical damage during handling and/or use.
• such wire loops 10 or nickel bridges may be formed at a compromise distance from the probe tip. The ground shorts may, therefore, still be a few wavelengths from the probe tip.
• the potentials of the ground electrodes 4 at the probe tip may not be as closely matched as is possible in spite of the expensive attempts to remedy this problem, giving rise to measurement errors.
• Errors may also arise from radiation through the back of the substrate, as shown in figure 2.
• the electromagnetic fields generated between the ground and signal electrodes 4, 6 are not wholly confined in the dielectric medium of the substrate 8 and hence, there is r.f. radiation from the back of the substrate.
• the resultant leakage can give rise to measurement errors and may degrade the isolation between the measurement channels.
• the probes may be exposed to and be susceptible to stray r.f. radiation from other wafer processing equipment, such as ion implanters, r.f. sputtering apparatus etc. This exposure of such probes to stray r.f. radiation may give rise to further errors in measurement.
• the coplanar waveguide probe 2 in accordance with the invention comprises a pattern of spaced ground and signal electrodes 4, 6 on the surface of a dielectric substrate 8.
• An electrically conductive layer such as metallisation layer 12 which may comprise gold, is provided as a backing layer to the dielectric substrate 8.
• the metallisation layer 12 is grounded by connection to the ground electrodes 4. In the embodiment illustrated by figure 4 this is achieved by electrically conductive means such as a wrap 14 of electrically conductive material around each side edge 16 of the dielectric substrate 8.
• the wraps 14 may be formed as extensions of the metallisation layer 12.
• the inclusion of the grounded back metallisation layer 12 confines the r.f. fields in the probe and prevents any radiation from the back ot the dielectric substrate 8.
• the inter channel isolation of the probe 2 is dramatically improved when compared to known devices.
• the back metallisation layer 12 also provides a convenient means of providing a common potential for the ground electrodes 4 which can not contact the metallisation -layer by means of the wraps 14.
• the dielectric substrate 8 is of the relatively thin thickness, typically about 0.25 mm, and hence, a very short ground return path may be achieved by drilling via holes 18, such as by laser drilling, from the front to back surface of the dielectric substrate 8 as shown in figure 4.
• the electrically conductive means may then be located within the via holes 18, such as a filling of conductive epoxy resin 20, to form the ground return path between the metallisation layer 12 and the ground electrodes 4.
• via holes 18 may be drilled through the dielectric substrate 8 for each ground electrode 4 to provide ground return paths throughout the lengths of the ground electrodes, as shown in figure 3, thereby ensuring that the ground electrodes have virtually equal potential to each other throughout their lengths.
• a laser drilled via hole may be positioned as close as is practicably possible to the tip of the probe 2 to ensure that the ground electrodes 4 have common potential at the tip where interconnection with the integrated circuit under test occurs.
• the size of the via holes 18 may be increased as the width of the ground electrodes 6 increases with spacing from the probe tip.
• the provision of the metallisation layer 12 enables the ground electrodes 4 to extend only a limited length of the probe 2. This is because in the region 22 shown in figure 3 the spacing between the signal electrodes 6 and the ground electrodes 4 to maintain the 50 SI- characteristic impedance is large when compared to the thickness of the dielectric substrate 8. Heruse, the isolation between the signal electrodes 6 and the metallised layer 12 is less than the isolation between the ground and signal electrodes. Therefore, from the region 22 to the probe interface with the test equipment connectors, the waveguide is, effectively, formed by the signal electrodes 6 in combination with the metallised layer 12. This interchange between the ground electrodes 6 and the metallised layer 12 as the effective waveguide ground electrode can be smoothed by providing the ground electrodes 6 with end portions of predetermined tapered shape, as shown in figure 3.
• a coplanar waveguide probe in accordance with the present invention provides, when compared to known devices, improved electrical isolation and probe radiation performance and hence, improved measuring accuracy. Furthermore, the probe is easier to fabricate than known designs and is more robust in use.
• the metallisation layer 12 preferably comprises gold but any electrically conductive material may be used.
• the electrically conductive means in the via holes 18 may comprise a coating or filling of metallic material, such as gold.
• the probe may have any number of signal electrodes.
• the ground return paths to all ground electrodes may be provided exclusively by means of via holes and not the combination of via holes and conductive wrap arounds, as described.
• the metallisation layer 12 may not comprise the exterior backing layer of the probe. Further layers may be provided over the metallisation layer.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Leads Or Probes (AREA)

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• 1985
  • 1985-05-02 GB GB858511169A patent/GB8511169D0/en active Pending
• 1986
  • 1986-05-02 EP EP19860903448 patent/EP0221172A1/de not_active Withdrawn
  • 1986-05-02 JP JP50275786A patent/JPS62502709A/ja active Pending
  • 1986-05-02 WO PCT/GB1986/000249 patent/WO1986006495A1/en not_active Application Discontinuation

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