DISTURBANCES emitted by ICs are mainly due to IC near fields. The identification of these fields would greatly benefit IC development. Near-field probes are usually used for this purpose.
The smallest available near-field probes that have been developed to examine electronic modules are presently around 1mm to 2mm in size.
This resolution, however, is not sufficient to examine ICs. ICs have much finer conducting networks.
Only micro-probes in the µm range can be used to examine ICs in terms of pin, bonding wire, lead frame or chip.
The following paper presents results from examinations that were carried out with probes of 150µm in diameter.
The characteristics and resolutions of these probes are investigated more closely through measurements on common mode and differential mode models. A sample IC is used to compare direct current and near field measurements.
Measuring devices: micro-probes
Practical experience with EMC measurements has shown that the electric field is also important alongside the magnetic field.
Field-specific probes allow the designer to detect and thus analyse the fields separately.
The ICRH-S and ICRH-H probes are used for the H-field in these near-field examinations.
The measuring coil of the H-field probes has an inner diameter of approximately 150µm and an outer diameter of approx. 250µm.
The coil's orientation is the distinguishing feature between both probes: ICRH-S is vertical and ICRH-H is horizontal.
Both probes are shielded against E-field. The E-field attenuation is approximately 20dB.
The voltage induced in the coil is amplified by 30dB within the probe. The measuring range is between 16kHz and 3GHz.
The E-field is measured using the ICRE probe with a probe head size of 150μm x 35µm.
The E-field probe works with the same amplifier and measuring range as the H-field probes.
Near field measurements on a 25µm round conductor, round twin conductors and an IC
The ICRH-S, ICRH-H and ICRE field probes are used to measure the near field first on a round conductor and then on two round twin conductors at various heights above the conductors. The transmission factor can be determined on the round conductor.
These measurements are relative to the horizontal and the vertical distance as well as to the current that flows through the round conductor.
The probes are guided by an IC S 103 scanner.
Measurements on a round conductor
The transmission factors in the frequency range between 16kHz and 3GHz are determined on the round conductor.
The measurement is based on the following parameters: the round conductor has a diameter of 25µm and a termination of 50Ω.
The probe coil's low end is adjusted to a height of 20µm above the round conductor. The round conductor is supplied with 107dBµV (0dBm) via the tracking generator of the R3132 spectrum analyser.
All further measurements on the round and twin conductors are carried out at this supply.
Increments of 0.05mm are chosen to measure the transfer characteristics. The ICRH-S, ICRH-H und ICRE characteristics are shown on Langer EMV’s website.
Measurements on simulated neighbouring lines
The fields of two parallel conductors (twin conductors) are scanned to analyse the resolution and behaviour of the probes more closely.
The conductors are operated in differential or common mode and their distance is varied.
In addition, the field is examined as a function of the vertical measuring position.
The probe detects measurable differences between the fields of the conductors as a function of the height.
Differential-mode current measurement
The distance set between the conductors can be easily read from the measured peaks.
Each round conductor remains clearly identifiable even at a distance of 0.1mm between the twin conductors.
If the height of the probe above the twin conductors is also considered, it can be seen that the round conductors can still be distinguished with increasing height.
Common-mode current measurement
The peak can again be used as an indicator for the set distance. Both magnetic fields merge into one if the distance between the twin conductors decreases to less than 0.3mm.
If the height of the probe above the twin conductors is also considered, it can be seen that a distinction is no longer possible with the twin conductors spaced at 0.6mm as of a height of 0.2mm.
Measurement on an IC example
An application-oriented wiring is chosen for the measurements on the IC (64-PIN QFP). The E-field in the area of the pins 53 to 59 is measured with the ICRE probe.
The E-field is not detectable in the frequency range from 10MHz to 1000MHz. It thus has no effect on the H-field measurement in this example.
Measurements with ICRH-S above pins 53 to 59 show that the spectra measured reach a maximum above the VDD pin. A contraction is visible between VDD and VSS.
The differential-mode measurements on the twin conductors suggest that the disturbance travels from the VDD PIN to the VSS PIN via the blocking capacitor and thus forms a differential-mode system. The stray field is responsible for the field configuration above the other pins.
The current is measured on the VDD pin so that the results of the H-field measurements can be compared.
The current has to be measured at a very low impedance to avoid any measurement degrading. The ammeter probe 622 is used because it has an internal resistance of 1 _ and is thus suitable for measurements on voltage supply pins.
The direct measurement of the current produces a wide spectrum, dark blue curve. A relationship is clearly visible by comparing the spectra of the current and the field measurements.
The H-field distribution above the chip is determined with ICRH-H in an additional surface measurement with the ICS 103 scanner.
Lower field strengths in the form of lower induced voltages are present because the distance to the bonding wires is larger than in the measurements directly above the pins. A light red and an orange range of highest field strengths are visible in the VDD-pin area.
The yellow ranges represent the field above the lines to and from the other supply pins.
The measurements of the current spectra on the other supply pins confirm that the field spectra above the IC correlate with those of the current.
These H-field measurements thus allow the designer to trace the current paths on the IC in the near field.
The current paths of the IC can be traced through H-field measurements in the near field.
Measuring IC samples of different stages of development allows the designer to analyse and determine how their modifications affect the IC in the near field range.
Conclusions can thus be drawn and continuous modifications made during the development process. These probes open up new possibilities in the field of IC technology.
* Georg Dude works at Langer EMV-Technik GmbH .