Advanced mm-Wave Load-Pull Measurements
The ramp-up of 5G mm-Wave technologies comes with substantial enhancements in connectivity, revolutionizing smart cities, the internet of things, vehicle to everything (V2X), and more.
One of the fundamental requirements of such 5G devices is to maximize their performance by optimizing power and/or efficiency of the contained amplifiers and transistors. This is done by measuring the performance characteristics of the device under test at different impedances that are systematically changed using load-pull tuners.
FormFactor has partnered up with Focus Microwaves and Keysight Technologies to deliver a fully integrated solution for accurate on-wafer mm-Wave load-pull measurements.
Challenges of mm-Wave Load-Pull Measurements
To find the point of optimum performance of a device, it is essential to maximize the tuning range over the Smith chart. An increased tuning range provides more characterization data and enables higher certainty in defining the point of maximum performance.
The tuning range is affected by the measurement path which traditionally included a big tuner, a coupler, a cable and a probe (Figure 2). These components will always increase insertion loss which reduces and/or shifts the tuning range to a certain area of the Smith chart, especially at higher frequencies. The use of flexible cables can additionally cause impedance shifts.
Consequently, keeping the electrical measurement path as short as possible and highly stable is preferred to maximize characterization accuracy.
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Figure 1: Draft of a Smith chart. The blue area marks the available tuning range that can change with the measurement setup. |
New Approach with Reduced Measurement Path
The new DELTA tuners from Focus Microwaves are specifically designed for on-wafer integration, which allows them to be located as close to the wafer as physically possible, without the need for a coupler and a cable. FormFactor’s Infinity probes can be directly connected to the test port of the tuner, which minimizes insertion loss and maximizes gamma, allowing a very large cover of tunable impedances over the Smith chart.
Another key benefit of the DELTA tuner setups is the shortened electrical delay resulting in reduced impedance skew for modulated signals, making wideband modulated load pull possible with passive tuners.
Figure 2: Traditional tuner with cable.
Figure 3: Shortened measurement path: FormFactor’s Infinity probe is directly connected to the DELTA tuner from Focus Microwaves.
Performance Results - Tuning Range
Figures 4, 5 and 6 show the optimized tuning range at three different frequencies that are typically used for test of 5G devices. The setup uses the DELTA tuners from Focus Microwaves with FormFactor’s Infinity Probes, fully integrated into a SUMMIT200 probe station and with the N5291A frequency extenders from Keysight.
Independent of the setup, insertion loss gets higher with frequency which leads to a slightly shrinking tuning range. The below results show an excellent tuning range from Γmax= 0.84 at 28 GHz to Γmax= 0.7 at 96 GHz.
 Figure 4: Tuning range at 28 GHz: Γmax= 0.84, Γmax_360deg= 0.79 |
 Figure 5: Tuning range at 50 GHz: Γmax= 0.79, Γmax_360deg= 0.74 |
 Figure 6: Tuning range at 96 GHz: Γmax= 0.7, Γmax_360deg= 0.63 |
The above charts show the tuning range of a single probe tuner. A useful application of multiple probe tuners is single frequency high Gamma tuning.
If one slug is used as a prematching section, the second slug can tune around it and reach very high reflection factors in a selected area of the Smith chart. Reflection factors of 0.85 or greater can be met (VSWR > 10:1 at DUT reference).
If that is still not enough, these tuners can be used in hybrid mode with an active injection signal to bridge the gap and increase the tuning range even more.
Accurate Probing of Small Pads - High Resolution with Perfect Fit
Smaller pad geometries come with several advantages like saving valuable wafer space and reduced pad parasitics. Many devices today have pads as small as 30 μm. To properly see those pads and probe them accurately with RF probes, a high-resolution microscope system is essential.
FormFactor’s load-pull setup integrates a high-resolution microscope – either the eVue or the SlimVue – enabling highest visibility of contact pads as small as 30 μm with micron level accuracy. The dedicated high magnification, high resolving power objective perfectly fits between the tuners so that the probes can come together as close as needed without hitting the microscope, even if there are only 100 or 200 μm separation. This makes it possible to probe even very small transistors with minimal probe-to-probe distance and calibrate on standards with small calibration structures. Standard LRRM calibration substrates can be used and there is no need of special calibration standards.
Load-pull measurement setup: FormFactor’s Infinity probes are connected directly to the tuners. This minimizes insertion loss and maximizes Gamma. The setup integrates a high power microscope objective, enabling easy visibility of contact pads as small as 30 μm with micron level accuracy. FormFactor’s patented eVue Crash Detection protects your valuable equipment.
Safety Features
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eVue Crash Protection FormFactor’s patent-pending eVue Crash Detection protects your valuable equipment from expensive damage, even when probes are in contact. When accidently hitting the tuner or probes, the microscope will instantly stop all movements and retract the objective.Velox Software Fence Additional software fences in the Velox probe station control software avoid a collision between the tuners and the microscope objective by defining a limit of how much the microscope can be moved. |
Coaxial Calibration with Highest Phase Stability
With any tuner system, on-wafer calibration down to the probe tips is needed. Additionally, the end of the cable of the Keysight N5291A frequency extender needs to be calibrated to the input of the tuner. This coaxial calibration characterizes the insertion loss of the tuner in different locations around the Smith chart.
FormFactor’s millimeter-wave load-pull solution uses a short 10 cm cable from the Keysight N5291A frequency extender to the tuner input. This maximizes the available power. The cable is semi-rigid to prevent phase change error due to cable movement.
To connect a coax standard to the end of the cable, the Keysight N5291 frequency extender can easily unclamp and slide back via quick release mechanism. For thru calibrations the extenders can be lifted off and brought together to add a thru standard between the two cables.
To connect a coax standard to the end of the cable, the Keysight N5291 frequency extender can easily unclamp and slide back via quick release mechanism. The cable is semi-rigid and provides highest phase stability.
Full Thermal Range – Exclusive RF TopHat with Probe Window
With 5G devices powering the vehicle to everything (V2X) market there is a need for EMI and light-tight testing at a wide temperature range, including tests down to -40°C without a build-up of frost and condensation.
With FormFactor’s patented RF TopHat (available for CM300xi, SUMMIT200, Summit 12000 and Elite 300), the solution has full thermal capability from -60°C to +125°C. The RF TopHat was designed to minimize cable lengths, prevent stiction which impacts motorized positioner accuracy, and maintain a dark, shielded and frost-free measurement environment.
The exclusive I.T.O. coated TopHat window allows easy setup and tracking of probes without having to open the MicroChamber. The window is always shielded, always frost-free and includes a cover for dark measurements. The patented FlexShield interface enables resistance-free and highly accurate probe positioning.
The setup can be converted between TopHat and non-TopHat to meet different test requirements.
RF TopHat with I.T.O. coated window for easy setup of probes in an EMIshielded and frost-free environment. A cover enables dark measurements. |
The patented FlexShield interface enables resistance-free and highly accurate probe positioning. |
Performance Results – Tuning Range
A straight, very rigid and short cable that connects the tuner with the probe inside the TopHat provides very high phase stability. Figures 8 and 9 show that the impact of this cable on the tuning range is minimal.
The setup uses the DELTA Tuners from Focus Microwaves with FormFactor’s Infinity Probes, fully integrated into a SUMMIT200 probe station with MicroChamber and RF TopHat, and with the N5291A frequency extenders from Keysight. In this example measurement at 28 GHz, Γmax ranges from 0.84 without TopHat, to 0.74 with TopHat. The compromise for a fully shielded environment is only less than 0.1.
Figure 8: Tuning range at 28 GHz without RF TopHat. Γmax= 0.84, Γmax_360deg= 0.79 |
Figure 9: Tuning range at 28 GHz with RF TopHat. Γmax= 0.74, Γmax_360deg= 0.7 |
Unique Ergonomic Concept - Completely Modular Solution for Easy Swapping Between Bands
FormFactor’s engineers constantly strive for maximum ergonomy and ease of use. Consequently, our RF probe positioners are completely modular. You can simply choose between a manual or programmable positioner body and add a specific arm. The arms are equipped with a dovetail that makes it easy, safe and fast to change between different measurement applications.
Our RPP404 and RPP504 positioners have been developed specifically for high-performance RF, mm-Wave and terahertz measurements, to give the highest positioning resolution for the most accurate and repeatable probe positioning and measurement performance. With 0.3 μm resolution even the smallest errors can be recognized and corrected. Multiple positioners can be located not only in the east and west, but also the north and south locations.
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| Digital Micrometer Upgrade Kit for RPP404. Separates RF probes a precise known amount. This is particularly useful when performing TRL calibrations which require different lengths of line. |
Industry-Leading High-Performance Probes
FormFactor’s Infinity Probe is unique in its architecture that delivers industry leading performance. It combines extremely low contact resistance on aluminum pads with unsurpassed RF measurement accuracy to provide highly reliable, repeatable measurements. The Infinity Probe reaches this performance level through the combination of FormFactor’s proprietary thin-film technology and coaxial probe technology.
