EP1579228A1 - Procede et appareil d'inspection de circuits hybrides a micro-ondes et a hautes frequences et cartes de circuit imprime - Google Patents

Procede et appareil d'inspection de circuits hybrides a micro-ondes et a hautes frequences et cartes de circuit imprime

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Publication number
EP1579228A1
EP1579228A1 EP03813704A EP03813704A EP1579228A1 EP 1579228 A1 EP1579228 A1 EP 1579228A1 EP 03813704 A EP03813704 A EP 03813704A EP 03813704 A EP03813704 A EP 03813704A EP 1579228 A1 EP1579228 A1 EP 1579228A1
Authority
EP
European Patent Office
Prior art keywords
antenna
dut
probe
test
signal
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP03813704A
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German (de)
English (en)
Inventor
Igor Shvets
Roman Kantor
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin
Original Assignee
College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin filed Critical College of the Holy and Undivided Trinity of Queen Elizabeth near Dublin
Publication of EP1579228A1 publication Critical patent/EP1579228A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/08—Measuring electromagnetic field characteristics
    • G01R29/0864—Measuring electromagnetic field characteristics characterised by constructional or functional features

Definitions

  • the present invention relates to a method of assessing the operation of a Device Under Test (DUT) at high and microwave frequencies. Further, the invention provides an antenna for use in such a method and also provides a topography recording system for use in such a method.
  • DUT Device Under Test
  • in-depth information is required on a relatively small number of DUTs.
  • the production phase one does not have to obtain in-depth information but the measurements have to be performed rapidly as a large number of DUTs have to pass through the quality control procedure.
  • Measurements require the setting up of special contact pads on the DUT thus imposing an additional design requirement.
  • any pin of a device or printed circuit board can serve as a contact pad as it is.
  • special contact pads with well defined properties have to be placed on the DUT just for the purpose of such testing.
  • a single probe usually has to make contact with the DUT at two or three locations or, as they are generally termed, points simultaneous and not just at one point as with a low frequency probe. Therefore surface roughness of the DUT or contamination can play a detrimental role.
  • the high frequency probes are much more complex, fragile and less durable than the low-frequency ones.
  • the impedance of the probe has to be matched at the test point so as not to affect the circuit's operation. This imposes an additional design restriction on the probe and also on the DUT: meaning that either the DUT has to be designed is such a way that all the test points have identical impedance or that several different probes have to be used to test the DUT at several points.
  • the high frequency/microwave probe is much more likely to affect the performance of the DUT than probes taking low frequency signals.
  • the objective of the present invention is to address these shortcomings of the test technologies at high and microwave frequencies, namely, frequencies in the range from 50 MHz to 50 GHz.
  • PCB generally an unpowered board
  • the board perturbs the field.
  • the pattern of field distortion contains information about any defects in tracks of the board.
  • This technology was developed for finding faults in unpopulated or inactive populated PCBs.
  • the field perturbation is measured by an array of electromagnetic sensors.
  • An example of such technology is described in US Pat. 5,424,633 (Soiferman).
  • a relatively similar technology is described in US Pat. 5,006,788 (Goulette et al).
  • PCB test technology should not be based on a particular resonance frequency determined by the size of the probe.
  • these coaxial antennas consist of a central conductor that protrudes for a defined length from a shielding. Because of the axial symmetry, such antennas are sensitive to the component of the microwave electric field parallel to that axis. The external field is commonly assumed to be homogeneous thus resulting in a single sensitivity coefficient, that is the ratio between the signal level induced in the antenna and the field intensity.
  • the length of the protruding conductor must not exceed the desired spatial resolution. The resolution does not just depend on this length but also on the dimensions of the shielding as surface currents in the shielding induce a secondary field and change the input signal.
  • the second reason making it unattractive to reduce the antenna size into the micrometer range is that with decreasing the antenna size, its impedance values move away from the common values of the microwave and radio frequency amplifiers. As a result, it is more difficult to couple the signal from the antenna into a preamplifier.
  • the objective of the present invention is to overcome the resolution limit determined by the antenna's dimensions and increase its resolution capability without the need for further miniaturization of the antenna.
  • the spatial resolution also depends on the gap separating the antenna and the DUT. For large separation, the resolution gets worse. Also the signal detected by the antenna depends on this separation and again the larger the separation, the smaller is the signal detected. It is, therefore, intuitively attractive to reduce the separation to a value as small as possible. This is not, however, the best course of action for many reasons.
  • the antenna starts influencing the DUT, mainly through the capacitive coupling between them.
  • the situation with a small separation between the antenna and the surface of the DUT is effectively equivalent to a capacitor between the DUT and the antenna at the point of test. The smaller the separation, the greater the capacitor's value. This capacitance depends not only on the separation between the probe and the surface, but also on the dielectric properties of the material underneath the probe.
  • the optimal separation between the probe and the sample must satisfy two criteria: it has to make it possible to achieve high resolution and low coupling between the probe and the sample.
  • the surface of PCBs contain various features (strip edges, wire bodings, etc.) the sample profile varies and the inspection area is not flat. Accordingly, the system that controls the separation between the probe and the DUT must satisfy very demanding criteria.
  • topography scanning techniques utilise very light topography probes, such as silicon cantilevers, usually manufactured by microfabrication processes.
  • a combination of the topographic and field probe is employed using microfabrication technologies whereby the functional electric or magnetic field probe is fabricated as part of the Atomic Force Microscope (AFM).
  • the topography probes are mostly based on mechanical resonance of the probe and changes in resonance conditions caused by the proximity of the device surface. These probes operate very close to the device, normally with a separation in the range of 1-50 nm at which a large capacitive coupling between the probe and DUT is virtually unavoidable.
  • the probes can be withdrawn out of the sample for measurements of various probe-sample interactions at higher distances, as described in the US Patent Specification No. 5,418,363 (Elings et al.) and various publications referred therein.
  • a probe of small mass is crucial to assure good sensitivity in scanning atomic force microscopy and shear force microscopy. Some of the probes combine the tip probe with integral small electric or magnetic field antenna.
  • US Patent Specification No. 5,936,237 (Van der Weide, D. Warren) teaches the combination of the electromagnetic probe with the probe of an Atomic Force Microscope (AFM) in a single multipurpose probe.
  • AFM Atomic Force Microscope
  • a quartz tuning fork is commonly used in Atomic Force Microscopy (AFM) and Scanning Near-Field Optical Microscopy (SNOM), for control of the distance between the probe and the sample.
  • the technique incorporates a dithered probe interacting with a surface in its proximity. The dependency of the amplitude and the phase of the probe's mechanical oscillation on the probe/sample separation is used in a feedback to keep the separation constant.
  • the tuning fork is utilized for the stabilization of the mechanical oscillations of the probe and the detection of the amplitude of the mechanical resonance.
  • the method was originally introduced by Karrai and Grober [Karrai K., Grober R. D., Appl. Phys. Lett. 66, p.
  • the oscillations in the tuning fork systems are usually excited by an external piezo- tube, bi-morph, thickness or shearing mode piezo plate, and not by the application of the signal directly on the fork electrodes.
  • the piezo-electric properties of the quartz tuning fork as a self-oscillating resonator are disregarded.
  • the reason advanced for the use of an external dithering piezo is that the quartz tuning fork resonator operates with a much lower quality factor (which drops by more than 2 orders of magnitude from their original value), caused by additional damping forces: air damping, non-elastic deformation within the system with the tip attached and drag forces of the tip/sample interaction.
  • a typical configuration has a dithering piezo such as a thickness mode piezo and a probe connected to one of the two arms of a fork and oscillating parallel to the surface.
  • a generator supplies an excitation signal to the thickness mode piezo.
  • the generator also supplies a signal to the reference input of a lock-in amplifier (LIA) through a phase shifter.
  • the detection signal is collected from the electrodes of the tuning fork crystal and supplied to the input of the LIA.
  • LIA lock-in amplifier
  • a method of assessing the operation of a device under test (DUT) at high and microwave frequencies comprising using an antenna terminating in a tip or apex to acquire microwave electromagnetic field measurements in a near field region of a test point of the DUT comprising the steps of:-
  • the invention overcomes the problem that the resistance influences the working regime of the DUT and the hitherto experienced increase of the mutual coupling between the probe and the DUT.
  • the results heretofore of the measurements have been relatively meaningless. By this separation of the probe from the DUT by a relatively large gap, there is very weak coupling between them.
  • the inclination of the antenna is substantially orthogonal to the DUT.
  • the predetermined rotation angle may be substantially 180° or may be rotated at rotation angles of substantially 120° and 240° to obtain three sets of measurements. In this way, different spatial components can be measured and, for example, with two measurements with the antenna rotated by 180° around the normal axis, a vertical and one tangential field density can be obtained, while with the three measurements, three components of the signal can be obtained. Both the amplitude and the phase of signal can be acquired by a phase sensitive VNA.
  • the inclination is of the order of 45°.
  • the sensitivity of measurement of the electrical field intensity at a particular frequency is defined by
  • ⁇ U is the voltage difference of antenna signal measured for two positions of the antenna displaced along antenna axis
  • M is the displacement of the test positions
  • E is the component of the electrical field intensity of the microwave field parallel to the antenna axis
  • the sensitivity constant (S) is determined by a calibration measurement in a well defined field standard and is subsequently used to determine the real value of the electrical field intensity of a DUT during a test. It will be appreciated that microwave property of the DUT can be one of the amplitude, phase or frequency of the voltage detected by the antenna.
  • test position is at least spaced-apart from the DUT by a distance greater than the widest lineal dimension of the tip of the antenna facing the DUT.
  • the separation between the tip of the antenna and the test point of the DUT is between 1 ⁇ m and 100 ⁇ m.
  • the siting of the antenna comprises:-
  • the topography probe In order to measure the offset distance the topography probe is brought into a focus point of a long focal length microscope and the antenna are brought to the same focus point to measure the offset between the positions of the probes.
  • test position for a number of similar DUTs is recorded, averaged and used to provide the test position for subsequent similar DUTs.
  • a plurality of DUTs which have been determined to function correctly in practice are measured at one or more test points and the resultant measurements recorded as acceptable measurements for subsequent DUTs measured at these test points.
  • the invention provides an antenna for use in the method defined above, comprising a coaxial shielding and a protruding conductor therefrom in which the length by which the conductor protrudes from its shielding substantially exceeds the greatest lineal dimension of the shielding adjacent the conductor to isolate the effects of the shielding from the DUT.
  • the antenna is a coaxial antenna and in which the length by which the conductor protrudes exceeds, by at least a factor of two, the external diameter of its coaxial shielding and by at least the same factor, the smallest dimension of the feature at the DUT that needs to be resolved.
  • this factor is at least three.
  • the invention provides a topography sensing system for use in the method defined above comprising:-
  • an excitation generator having means to operate at the resonance frequency of the oscillator with the probe
  • the invention provides a topography sensing system for determining the vertical height above a DUT in the method described above, comprising:-
  • a probe being supported in the holder in a rest position and freely movable upwards within the holder on the tip contacting portion of the topography;
  • the holder comprises a bored tube and the probe is a stiff rod mounted within the tube.
  • Fig. 1 is a general schematics of the apparatus according to the invention
  • Fig. 2(a) is a diagrammatic view of an electric field antenna used in the invention
  • FIG. 2(b) is an enlarged view of portion of the antenna encircled in Fig. 2(a),
  • Fig. 3 is a graph illustrating the effect of antenna displacement
  • Fig. 4 is a plan view of a DUT used to test the invention
  • Fig. 8 is a schematic view of an antenna calibration unit
  • Fig. 9 illustrates operation as required to measure horizontal components of the field according to the invention
  • Fig. 10(a) and Fig. 10(b) show results obtained by using inclined coaxial antenna as illustrated in Fig. 9,
  • Fig. 11 is a view similar to Fig. 2 of a loop antenna according to the invention.
  • Fig. 12 is a diagrammatic view of one distance control system according to the invention.
  • Fig. 13 is a view similar to Fig. 12 of a further distance control system according to the invention.
  • Fig. 14 shows the response of a topography probe according to the invention.
  • Fig. 15 is a view of a topography sensing system for fast measurement of the elevation of the DUT surface.
  • Figs. 1 and 2 there is illustrated apparatus, indicated generally by the reference numeral 1 , for the inspection of high frequency properties of a DUT 2, above which is mounted a field antenna, indicated generally by the reference numeral 3.
  • the field antenna 3 which is illustrated partly by interrupted lines is illustrated in more detail in Figs. 2(a) and (b) and comprises an antenna case 4, shown by interrupted lines in Fig. 1 , mounting within it a conditioning RF preamplifier 5, only illustrated in Fig. 1.
  • a conditioning RF preamplifier 5 mounted within the antenna case 4 various signal conditioning devices for transmission to the input of an acquisition instrument, in Fig. 1, a vector network analyser 6.
  • the field antenna 3 further comprises antenna coaxial shielding 7 mounting a central protruding conductor 8 having an apex or tip 9.
  • the output signal from the antenna 3 is connected by a transmission line 11 to the vector network analyser 6.
  • the DC bias for the preamplifer 5 is coupled from an external source by a bias coupler 10.
  • the high frequency signal which energizes the DUT 2 is provided by an RF output of the VNA and coupled by a signal line 12.
  • the apparatus 1 further comprises a topography sensing system, indicated generally by the reference numeral 15.
  • the topography sensing system 15 is one based on measuring shear forces when a tip is dithered parallel to a surface such as a DUT surface.
  • the topography sensing system 15 comprises a tuning fork or quartz crystal oscillator 16 connected directly to a probe 17.
  • the topography sensing system 15 further comprises a circuit, including a generator 18, a lock-in amplifier 19 and a conditioning feedback amplifier 20. There is also provided a piezo- actuator 21 for fine adjustment of the probe's 17 position relative to the DUT 2 and further motorised positioning stages 22 and 23 for the probe 17.
  • the motorised positioning stage 22 is a vertical Z axis positioner and the motorised positioning stage is a two stage horizontal X and Y axis positioner.
  • the apparatus 1 further includes a control computer 24.
  • the topography of the DUT surface is measured by the topography sensing system 15.
  • the vertical position of the probe 17 is adjusted by the piezo-actuator 21 to keep the separation between the apex or tip of the probe 17 and the DUT 2 in the range of shear force interaction.
  • the motorised positioning stage 22 and the piezo-actuator 21 are used for vertical displacement of the probe with the latter being used for fine operation.
  • the motorised stage 23 operates in two horizontal directions.
  • the topography probe 17 is removed and the antenna 3 placed in position and driven in accordance with the topographic data, by the piezo- actuator 21 and the motorised stages 22 and 23 so that the tip 9 is correctly positioned at required distance from the DUT 2.
  • the antenna 3 is positioned above and in the near field region of a test point of the DUT 2 and, as illustrated in Fig. 2 orthogonal thereto, the DUT 2 is energised and then the microwave signal induced by the DUT 2 is measured and recorded.
  • the signal corresponds to the electrical component of the microwave field.
  • the antenna 3, as described below, is moved relative to this position in a vertical or an inclined orientation. As described below, in a first method, it is moved orthogonal to the DUT 2.
  • Such a configuration allows low perturbation of the signals in the DUT 2 as the shielding body is relatively distant from the measured sample, otherwise that bulky part of the antenna 3 is bound to induce perturbation if brought into proximity of the DUT. If the length / of the protruding conductor 8 is too short and thus the distance between the antenna coaxial shielding 7 and the signal line, i.e. the width of the strip conductor of the DUT 2, the shielding 7 starts affecting the signals within DUT 2 and changes its performance.
  • the effect of moving the antenna 3 a small distance ⁇ / towards the DUT 2 needs to be considered.
  • the strength of the field is greatest close to the circuit surface and decays with increasing distance from the surface.
  • the signal level can be approximated as a sum of the field contributions acting along the protruding conductor 8.
  • the signal levels are described as the induced currents / ⁇ , h at the input to the coaxial shielding as the impedance of such short antennas are high in comparison with the input impedance of the subsequent network and therefore the antenna functions as a current source.
  • Geometry and the position of the protruding conductor 8 in the middle region B does not change with the antenna displacement and therefore the contribution from the same external field remains substantially unchanged, I - I 2
  • the field intensities rapidly decay with the increase of the distance above the DUT 2, the field strength in the region C, and its contribution to the overall signal can be supposed to be negligible, / - I 2 - 0.
  • the measured signal difference can be calculated as follows.
  • the signal level depends only on the field solution and changes in the boundary conditions at the apex 9 of the protruding conductor 8. As these changes are limited to the region ⁇ / of the displaced antenna apex, the measured signal and the resolution of the measurement method are determined by the displacement Al. In this way the field surrounding the conductor apex can be isolated and measured which improves the spatial resolution of the microwave field mapping.
  • the scanning measurement of a DUT 2 demonstrates the benefit of the invention.
  • the protruding conductor 8 of the antenna 3 had a length of 1 mm and a diameter of 8 ⁇ m.
  • the DUT 2 was in the form of a PCB surface capacitor with a small separation gap between its fingers is illustrated in Fig. 4.
  • figures 5(a) and 5(b) represent scanned field images of the normal electric field acquired for two different antenna with DUT separations of 5 ⁇ m and 12 ⁇ m respectively.
  • Fig 5(c) is the difference of these signals. It can be clearly observed that there is a significant resolution enhancement for the signal difference.
  • the signal difference corresponds to the local electric field intensities surrounding the antenna apex only.
  • the signal difference also reveals weak local field intensities close to the signal lines of the bottom port of the capacitor, otherwise masked by strong background signals. These signals are induced by background fields acting along the whole length of the protruding conductor above the displaced apex of the antenna.
  • Fig. 6 there is illustrated a line scan A-A' across the middle of the capacitor as indicated in the Fig. 5.
  • Considerable enhancement in amplitude spatial contrast is achieved when the amplitudes of the signals at 5 ⁇ m and 12 ⁇ m separation are subtracted from each other, this difference is represented by a solid line.
  • the vertical axis presenting the signal level is logarithmic (in dB) The signal difference is naturally significantly smaller than each of the two signals.
  • the signals presented in Fig. 5(a) and 5(b) are vectors (represented by complex amplitudes of electric field intensities E and measured voltages U) and they are characterised not only by the magnitude but also by phase. Therefore, their difference is also a vector that is in turn characterised by phase.
  • Fig. 7 which corresponds to the same cross- section as presented in Fig. 6.
  • the increase in the phase contrast is caused by the fact that phase of the localised microwave field close to the DUT, represented by the signal difference, varies significantly as opposed to the phase of the average field at greater distances, as acquired by the entire antenna length and presented by the individual measurements.
  • the antenna is connected to the preamplifier 5 and in a typical .embodiment the antenna and the preamplifier form mechanically a single component.
  • the measured voltage difference AU corresponding to the two antenna positions after its conditioning and transmission to the input of the acquisition instrument (VNA) is proportional to the antenna displacement Al.
  • / is the amplitude of the component of the electric field intensity of the microwave field parallel to the antenna axis.
  • the sensitivity constant S can be determined from the calibration measurement in a well-defined field standard and can be used during the scanning process for the calculation of real values of the electric field intensity.
  • the design of this specific antenna, employed to obtain results '" shown in Figures 3, 5, 6, 7 was optimised for higher sensitivity S at frequencies close to 4 GHz.
  • the measurement of sensitivity constant S was performed using an antenna calibration unit, indicated generally by the reference numeral 50, using a well-defined field standard as presented in Fig. 8.
  • the term 'field standard as used in this specification is substantially a circuit with well-defined geometry so that the surrounding field generated by the field standard, can be calculated (without measurement).
  • a calibration unit When used for probe calibration, such a device can be called a calibration unit as the detected signal can be compared with the calculated theoretical field intensity.
  • the field standard represents a 50 ⁇ transmission line consisting of a cylindrical conductor 51 placed above ground plane 52. A microwave signal of known amplitude is coupled to one port 53 of the line and the other port 54 is terminated by a 50 ⁇ load 55 to avoid reflection of the signal.
  • the level of the acquired signal depends not only on the signal induced in the apex of the conductor but also on the efficiency of its matching to the input of the coaxial line, the properties of the preamplifier and the transmission of the signal to the acquisition system (usually a VNA).
  • the se ' hsitivity of the antenna may effectively limit its resolution and make it dependent on minimal detectable field intensities.
  • ⁇ / the apex of the protruding conductor functions as a near ideal current source and one of the main factors influencing the sensitivity is the matching of such a high- impedance source to the input of the coaxial line and subsequently to a preamplifier.
  • the signal was conditioned by an MGA-86576 MMIC preamplifier.
  • MGA-86576 MMIC preamplifier Unfortunately its standard input impedance of 50 ⁇ represents a great mismatch to the high impedance of the antenna.
  • the input impedance of the protruding conductor has to be adjusted by an impedance matching circuit.
  • a relatively simple matching scheme was chosen which uses the antenna coaxial input line as a quarter-wavelength transformer. Thanks to its relatively high characteristic impedance of 120 ⁇ and by setting its length to equal to ⁇ /4 (8 mm) for the frequency of interest (4 GHz), the level of the signal and the antenna sensitivity was increased by 15 dB to about 10 V/m for displacement Al - 30 ⁇ m.
  • the separation of the tip of the antenna from the DUT should not exceed the desired resolution but it should be greater than the diameter of the protruding end of the antenna to avoid direct capacitive coupling between the antenna apex and the DUT.
  • the range of some 10-100 ⁇ m is practical for protruding wire conductors with diameters of 5-30 ⁇ m.
  • the measurement process is further complicated by the fact that most DUTs are not flat and contain complex topographical features such as electronic elements, wires, air bridges etc. Therefore, it is vital to control the separation between the end of the antenna and the DUT while maintaining this distance in that range with small error.
  • Fig. 9 demonstrates how the in-plane (tangential) component of the electric and magnetic field can be measured.
  • the antenna is placed with an inclination of 45 degrees (for example) relative to the vertical axis.
  • Standard Cartesian intensities, perpendicular and parallel to the surface of the DUT can be then calculated.
  • a vertical E z and one tangential E x field intensity can be obtained.
  • R 0 exhaustive ,E 180 ⁇ are the electric field intensities before and after the probe rotation.
  • Fig. 9 shows a configuration with electric field coaxial antenna, however, loop antenna can be used for the measurements of all spatial components of the magnetic field in the same manner.
  • tangential components acquired using a 45° inclined antenna The measurements were performed above a microstrip line, the position of the strip edges are highlighted by dashed lines.
  • the antenna was rotated in two opposite directions perpendicular to the strip, effectively aligning the antenna at angles of +45° and -45° relative to the normal of the DUT plane.
  • the distance from the circuit surface was chosen to be relatively large (600 ⁇ m) as the tangential components are negligible close to the circuit surface and vanish at the conductive boundaries of the strip or ground plane.
  • a loop antenna instead of the coaxial probe.
  • This is schematically shown in Fig. 11 for two positions of a loop antenna, indicated generally by the reference numeral 30, displaced along the vertical direction by Al, where / is the length of the loop antenna, as before.
  • a shielding 31 does not affect the signal detected by the loop 32 and also does not affect performance of the DUT.
  • Subtracting two sets of data corresponding to the two positions of the antenna makes the measurement in a sense equivalent to the small loop identified in Fig. 11 by the numeral 33.
  • the signal orthogonality allows a phase-sensitive detection of the component corresponding to the mechanical vibrations only and the suppression of the excitation signal. After the conversion of the currents to voltages this detection is performed using a Lock-In Amplifier (LIA) such as SR830 (Stanford Research) where the signals are demodulated relative to a reference signal provided by an excitation generator.
  • LIA Lock-In Amplifier
  • the phase of either this reference or measured signals has to be adjusted in a phase shifter to assure a 90 degrees phase shift between the reference and probe excitation signals. This phase shift allows nearly complete suppression of such an out-of-phase signal.
  • the functional component corresponding to mechanical vibrations, is in-phase with the reference signal and it is fully demodulated.
  • the amplitude of the output signal Uo after lock-in detection can be written in the form
  • a topography sensing system again indicated generally by the reference numeral 15.
  • the tuning fork or quartz crystal oscillator 16 is mounted on a ceramic holder 36.
  • the tuning fork in this particular embodiment was a standard watch quartz crystal (such as an AEL and Euroquartz, supplied, for example, by Radionics Part No. 304-447) removed from its protective encapsulation and attached to the holder by an adhesive (EpoTek 77).
  • the resonance frequency of the fork drops from its standard value 32768 Hz to one in a range of 25-30 kHz, depending on the particular mass of the probe attached thereto.
  • the probe was produced from a glass optical fibre.
  • the probe 17 had a sharp apex 37 formed using a puling machine with C0 2 laser heating. All of these components form a quartz crystal oscillator assembly delineated by the interrupted lines and identified by the reference numeral 35.
  • the circuitry of the topography sensing system 15 comprises, as well as the lock-in amplifier 19 and generator 18, described already, a high impedance signal coupling element 40, formed by a capacitor of about 5-20pF or a resistor of 0.2-1 M ⁇ .
  • the generator also feeds a phase shifter 41 to provide a reference signal, identified by the reference numeral 44, to the lock-in amplifier 19.
  • the lock-in amplifier 19 also collects a signal, identified by the reference numeral 43, from the tuning fork 16.
  • the tuning fork which is a quartz crystal
  • the circuit consisting of elements 16, 46, 47 represents an electronic differentiator with integration factor determined by the capacitance C f of the tuning fork 16 and resistance R of element 47.
  • the functional component namely, the response I f , resulting from mechanical oscillations, is phase shifted relative to that of the excitation by an additional 90 degrees at the resonance frequency.
  • the distance control system based on the self-excitation of the tuning fork according to the present invention has a number of advantages over the state-of- the-art system utilising a tuning fork and a dithering piezo. Both systems have similar sensitivities and comparable response times. However, the system based on the self-excitation has a simpler design, as no external dithering piezo is required. Also the system with the self-excitation is highly simple to adjust. There are no requirements of phase adjustment of the detected signal, as the electric excitation signal on the quartz fork electrodes is always in phase with the excitation forces and out of phase (shifted by 90°) with the mechanical oscillations and response signal.
  • the following procedure is employed. First, the surface of the DUT is scanned using a topography probe such as described above. The topography is recorded. Then the topography probe is replaced for the antenna.
  • the topography probe and antenna are each attached to a single XYZ translation stage device which is computer controlled. First, the topography probe is brought in focus of a long focal distance microscope. Then the computer controlled mechanical translation stage removes the topography probe from the focus and brings the antenna to the same focal point of the microscope. The computer records the position offset as accomplished by the XYZ stage between the two positions.
  • the present invention may be used for scanning large size areas.
  • the size of the DUT could be in the range of some 10-200 mm.
  • the situation with the height control i.e. movement along the Z-axis normal to the DUT surface is much more complex.
  • the problem is that one needs to have a large dynamic range of Z-displacement, typically in the range of 10 mm or more and simultaneously, high resolution, down to some 1 nm.
  • the high resolution is required as the probe height control is based on SF or AF interaction that is only active in the nanometer height range.
  • piezo tubes or piezo stacks are used. They are capable of providing the required resolution of the displacement but their dynamic range is limited to a fraction of a millimetre.
  • a hybrid solution utilising both a piezo stack and a motorised translation stage is envisaged. In use, during the topography scan the motorised stage is maintained at such a position that the piezo stack is kept in close to the middle of its dynamic range. For example, if the piezo stack is displaced from the middle of its dynamic range, by more than +/- 25%, the motorized stage performs adjustment and moves the probe so that piezo stack is placed again in the middle of its range.
  • test position relative.to a datum point of a fixture for reception of the DUT and this test position will then be used for subsequent similar DUTs placed on the fixture. In this way, the antenna will not have to be accurately positioned each time using the topography probe. It is also envisaged that the test position from a number of similar DUTs could be recorded, averaged and used to provide the test position for subsequent similar DUTs.
  • the measurements will be carried out on one or more test points and the resultant measurements recorded as acceptable measurements for a subsequent DUT measured at these test points. Obviously, in this way, the speed of measurement during production will be greatly enhanced.
  • the speed at which the topography probe can be approached to the surface is limited by the response time of the feedback loop. If the topography probe approaches a surface too fast, it may crash into the surface before the feedback response to the shear-force contact.
  • the typical approach time for a single point is 15-40s, which may not be acceptable if a large number of DUTs are tested during the production phase. In such a situation an alternative fast topography sensing system is required.
  • an elevation sensing system comprising a probe in the form of a stiff rod 61 having an apex 72.
  • the rod 61 can freely move inside a holder in the form of a guiding tube 62.
  • the guiding tube 62 is a glass tube with an internal diameter in the range 70-200 ⁇ m and the rod 61 is a glass rod with a diameter in the range 50 - 180 ⁇ m.
  • An upper part of the rod is coated with a metal forming a sleeve 67 so that is not transparent to the light.
  • a fixing ring 69 is mounted below the sleeve 67 to form a stop and thus define the lowest position of the rod 61.
  • the rod 61 projects a distance S a out of the guiding tube 62.
  • an opaque screen with a size of some 0.5x0.5 mm is glued on the top of the rod.
  • Means to record displacement of the rod 61 is provided by a position sensing device.
  • the position is determined by an opto-coupler, comprising a light-emitting diode 63 (such as supplied by Farnell under Part No. SE5470), a photo-detector 64 (such as supplied by Farnell under Part No. L14F1) and optical lenses 65 and 66 to focus a light beam to a threshold position 68 and to the sensitive area of the photo-detector 64.
  • a light-emitting diode 63 such as supplied by Farnell under Part No. SE5470
  • a photo-detector 64 such as supplied by Farnell under Part No. L14F1
  • optical lenses 65 and 66 to focus a light beam to a threshold position 68 and to the sensitive area of the photo-de
  • the system 60 mounted on a vertical (Z) motorised positioning stage such as the vertical (Z) motorised position stage 22, described above.
  • the rod 61 is moved towards the DUT 2.
  • continuous monitoring of the signal detector by opto-coupler is carried out.
  • the rod apex 72 is located in its lowest rest position, determined by fixing ring 69.
  • the rod 61 is pushed upwards and at some moment determined by the threshold position 68 of its upper end, it disrupts the path of the light beam, detected by proto-detector 64.
  • the position of the vertical (Z) motorised positioning stage 22 is recorded and stored in a computer, such as the computer 24 described above.
  • the movement of the stages " is then decelerated within a short path that does not exceed the distance S a and the elevation-sensing system 60 is withdrawn by a servo-mechanical device.
  • the antenna is then displaced by offsets ⁇ X, AY, AZ +dz to position its apex at specified distance dz above the inspection point of the DUT.
  • the offset values ⁇ X, AY, ⁇ Z represent the distance between the antenna apex and the lower end, i.e.
  • This threshold position 68 would generally be marginally above the probe, i.e. the rod 61, but could equally be sited some distance above it once the distance is known, all that is required is to be able to record the position of the holder 62 above the DUT.
  • the elevation sensing system since it incorporates the vertical (Z) motorised positioning stage, also mounts the antenna.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Resistance Or Impedance (AREA)

Abstract

La présente invention concerne un procédé et un appareil d'inspection de circuits à micro-ondes et à haute fréquences tels que des cartes de circuit imprimé de test. Cette invention utilise une sonde ou une antenne (3) qui est séparée du dispositif testé (DUT) (2). Cette invention comprend un long conducteur central saillant (8) destiné à l'antenne qui fait saillie à partir de son blindage (7). Dans ce procédé, on utilise l'antenne (3) pour acquérir des mesures de champ électromagnétique micro-onde dans une région de champ proche d'un point de test du dispositif DUT (2). Généralement cette opération est effectuée à deux endroits de test avec une différence de séparation (?/) entre le sommet (8) de l'antenne (3) et le dispositif DUT. Les deux résultats de test sont calculés et enregistrés et la différence des propriétés micro-onde de ces deux tests est obtenue de façon à fournir des informations relatives au fonctionnement du dispositif DUT (2). L'antenne (3) peut être soit une antenne de champ électrique droite soit une antenne cadre. Par ailleurs, l'antenne (3) peut être inclinée à la verticale et il est ainsi possible, par la prise d'une série de mesures, d'obtenir la phase et la fréquence des courants portés par le dispositif DUT (2) lorsque celui-ci est alimenté.
EP03813704A 2002-12-20 2003-02-14 Procede et appareil d'inspection de circuits hybrides a micro-ondes et a hautes frequences et cartes de circuit imprime Withdrawn EP1579228A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
IE20020985 2002-12-20
IE20020985 2002-12-20
PCT/IE2003/000022 WO2004057355A1 (fr) 2002-12-20 2003-02-14 Procede et appareil d'inspection de circuits hybrides a micro-ondes et a hautes frequences et cartes de circuit imprime

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EP1579228A1 true EP1579228A1 (fr) 2005-09-28

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EP (1) EP1579228A1 (fr)
AU (1) AU2003303279A1 (fr)
WO (1) WO2004057355A1 (fr)

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Publication number Priority date Publication date Assignee Title
FR2885446B1 (fr) * 2005-05-09 2007-07-20 St Microelectronics Sa Sonde coaxiale, son procede de fabrication et dispositif de mesure en champ proche electromagnetique sur des systemes a distance submicrometrique
IL291884A (en) 2019-10-06 2022-06-01 Orbotech Ltd Hybrid 3D inspection system

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US5028866A (en) * 1990-05-30 1991-07-02 General Motors Corporation Method and apparatus for mapping printed circuit fields
US6173604B1 (en) * 1996-09-20 2001-01-16 The Regents Of The University Of California Scanning evanescent electro-magnetic microscope
US5900618A (en) * 1997-08-26 1999-05-04 University Of Maryland Near-field scanning microwave microscope having a transmission line with an open end
US6625554B2 (en) * 2001-06-22 2003-09-23 Hitachi, Ltd. Method and apparatus for determining a magnetic field

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AU2003303279A1 (en) 2004-07-14

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