Cascade Microtech : 3 Key Considerations to Delivering Reliable, Accurate RF Measurement Results

Characterization of modern RF devices and integrated circuits is associated with serious challenges. Engineers are called upon to deliver high-precision measurement results, dealing with architectures and multi-functional ICs that continuously decrease physical dimensions, and are required to continuously increase operation frequencies and level of integration complexity.

Here are 3 key common challenging points to consider in order to achieve consistently credible and accurate RF on wafer measurement results:

1. Contact performance. Consistency of RF data strongly depends on the contact repeatability, reliability and stability for both calibration and measurements. Unreliable and inconsistent contact leads to re-contacting and re-measurement of the device under test (DUT), as well as repeated calibration and significant increases in test time. Moreover, unnecessary touchdowns on the calibration standards and DUT pads decrease the lifetime of the calibration substrate, destroy DUT pads and make further probing and/or bounding difficult or impossible.

Current technologies for semiconductor development utilize different contact pad metallization materials. RF probes must provide reliable and consistent contact on all types of contact pad material-from soft and very low-resistant gold, to aluminum, which builds a highly-resistive, hard-to-break-through oxide layer under atmospheric conditions creating challenges of its own.

2. Consistent calibration and measurement boundary conditions. Modern microwave integrated circuits implement various design types of transmission media: microstrip (MS), coplanar waveguide (CPW), grounded-CPW (or GCPW) and some others. The unique feature of the CPW design is that all conductors are on the same side (on the top) of the substrate. This attribute simplifies manufacturing, scaling, characterization and integration of CPW circuits.

CPW design is common for semi-insulating, high-resistive III-V substrates (e.g. GaAs). The resistivity of the wafer material sufficiently isolates the DUT from possible impact of the metal chuck (such as parasitic coupling and cross-talk). However, special care of the chuck surface must be taken to provide consistent electro-magnetic boundary conditions across the entire wafer for repeatable and location-independent measurement results.

With an increase of measurement frequency of about 20 GHz, parasitic coupling of the electro-magnetic energy through the surface of the metal chuck can also affect the accuracy of the probe-tip calibration. To suppress it, calibration substrates can be placed on a special RF absorbing holder. However, it elevates the calibration substrate much higher above the wafer surface increasing the risk of unexpected probe damage.

3. Consistent calibration and measurement results. Obtaining consistent calibration and measurement results require having dedicated RF operators with hands-on experience. It is common to see universities and research institutes to not having dedicated RF operators for measurement systems on hand. Thus, measurement setup is often shared between different groups, with multiple users that access the test system infrequently. This might sometimes lead to inaccurate probe alignment, inconsistent probe placement and contact force on both calibration standards and the DUT. As a result, measurement data obtained for the same device can vary significantly from operator to operator, and between different measurement and calibration sessions for the same operator.

Our EPS200RF probe system package is an uncompromised 200 mm manual solution that quickly supports every engineer in achieving the highest level of accuracy and confidence in measurement results, while tackling these and other RF measurement challenges with its unique features. It delivers flexibility and enables probing on cutting-edge technologies as a complete solution - ready for your application - right out of the box! To learn more about the EPS200RF and its outstanding features, visit our website and download the technical review (PDF).