Eliminating Ground-Loop Induced Noise

TestCell-generated noise used to be of minimal concern, but that is no longer the case.


As semiconductor device performance increases, especially for low power and higher speed ICs, testing low frequency 1/f, RTN and phase noise with improved signal-to-noise ratio is required. Finding and eliminating unwanted noise is required in multiple areas. Noise sources can be found inside a prober, outside a prober, and in a measurement TestCell. Historically, TestCell-generated noise was of minimal concern, because device signal levels were high. This is not the case anymore.

A traditional on-wafer measurement TestCell is defined as a connected set of equipment, including test software, instruments, probe station, thermal system, and related measurement accessories (cables, on-wafer probes) for the purpose of collecting specific types of test data.

For a typical TestCell configuration, even when all the individual equipment parts are well-designed (low noise, high quality, good EMI/ RFI shielding), after being joined together, long AC power ground-loops are formed. And when any common nearby magnetic fields pass through these ground-loops, they are converted into very small electrical signals resulting in unwanted power line frequency noise, with multiple harmonics.

This is shown in the following TestCell example, with unwanted noise (red trace) versus ground isolated condition (green trace). Here, measurement of chuck noise is done with a Keysight Oscilloscope. The STD_condition with ground loop shows more than 30 power line fundamental and harmonics noise spectrums. With the improved GRD_isolate_condition, the power line fundamental and harmonics noise spectrums are dramatically reduced. (Note: 60 Hz power line spectrum is only -20dB (= 1/10) reduced.)

Engineers and technicians often struggle in their lab to eliminate the ground-loop induced noise in a TestCell that is used for low-noise device measurement applications. For small applications this is easy, using simple battery power to eliminate unwanted power line frequency noise. And for small applications where AC power is needed, an isolation transformer can be used.

However, this technique has serious limitations when scaling up to support the size and power needs of a TestCell that includes a probe station with full on-wafer temperature support (-60..300oC) – limitations such as high cost, large physical size, complex cabling, safety issues, and increased magnetic field generation.

To address and solve this problem, FormFactor has developed a new TestCell Power Management (TCPM) system using PureLine 3 patented technology. And the CM300xi-ULN probing system is now the world’s first probe station with integrated TestCell Power Management. It not only eliminates all ground-loop induced TestCell noise, but also has a small footprint, and low field emissions, providing fully managed and filtered AC power to the entire system, prober and instruments.

A quick note on safety…

A key part of the TCPM systems design is the ULN Power Conditioning Unit (PCU) with expandable power modules, enabling unlimited instrument support with full power filtering and EMO management. The PCU provides a unified Emergency Off / Emergency Power control system for safe operation of the whole system and all instruments. It also supports instruments that do not have any special EMO capability, through a dedicated module with EMO controlled AC outputs. Safety is guaranteed through exhaustive industry standard compliance testing and full safety certifications supplied by TUV worldwide.

For more information on the CM300xi-ULN probe system, visit the website or download the data sheet here.

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