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Post by Professor Lake Shore on Jan 31, 2017 17:33:37 GMT -5
In a CPX or CRX probe station, do you have any guidance on how to avoid contamination of a delicate sample during cool down to helium temperatures?
For helium temperature operation, residual gases in the probe station chamber will condense and freeze on the coldest surfaces, and, under normal operating conditions, the sample stage is often the fastest cooling stage of a cryogenic probe station. You can avoid contamination on your sample by maintaining the sample stage at room temperature during station cool down. An effective way to minimize the condensation on the sample, this procedure is often required when measuring a surface-sensitive sample, as well as any device where maintaining low contact resistance is critical. This is accomplished by allowing the radiation shield stage to cool first so that the majority of residual gas is attracted to it and not the sample mounted on the sample stage. Specifically, the temperature controller, which is used to precisely regulate sample stage and radiation shield temperatures, will keep the sample stage warm while the remainder of the refrigerator cools to base temperature, at which point, any residual gas in the chamber will condense on the 4 K shield stage. The step-by-step procedure for doing this can be found in Chapter 5 of the probe station manual. This feature is only available on CPX and CRX probe station models.
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Post by Professor Lake Shore on Jan 31, 2017 17:30:04 GMT -5
When characterizing a sample with delicate contact electrodes in a probe station, which type of probe tip material is best to use and ensure the best electrical contact performance?
Lake Shore offers three different types of probes distinguishable by their probe tip material. For delicate or softer contact materials (such as gold), beryllium copper (BeCu) probes are recommended. These are the softest probe tips offered, providing conventionally the lowest contact resistance and the smallest amount of deformation to the contact metallization. Over time, the BeCu material is known to form a semi-insulating layer which can diminish contact performance; see Section 6.2.7 of your probe station manual for maintenance protocols for this probe material. This probe material is available on both standard DC/RF and continuously variable temperature (CVT) probes. At the other extreme, for aluminum, refractory metals, and other material contacts that develop oxides, a standard ZN50 tungsten probe will best puncture hard oxide material layers to make electrical contact with underlying layers and will not dull as quickly as softer probe materials. Because the spring member on the CVT probe reduces the pressure at the tip apex, tungsten CVT probes may have difficulty punching through hard insulating layers and are not recommended for oxidized contacts (such as aluminum).
A good intermediate solution between beryllium copper and tungsten is a paliney probe tip material offered on Lake Shore’s standard DC/RF probes. A paliney probe offers low contact resistance but a little bit stronger of a probe tip material (while also being the least reactive material and are least likely to form resistive oxides, especially at high temperatures).
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Post by Professor Lake Shore on Jan 31, 2017 17:26:36 GMT -5
I don’t want to damage the tips of the probe as well as my sample by needlessly trying to make electrical contact with a sample under test in my probe station. When landing a probe for the first time, how can I tell when sufficient contact is made?
First, always remember to raise the probe tips before cooling or warming the system or applying vacuum, and when moving probes in the x or y direction. When using a standard (non-CVT) ZN50R probe, follow this procedure to manipulate a probe to the sample and make contact:
1. Swing the microscope away from the viewport. 2. Use the z-axis micrometers to raise all probes 3 to 4 mm above the sample (failure to do so will potentially cause damage to the probe tip or scratch the sample surface). 3. Position the probe tips over the sample or landing pads using the x-axis hand dial and y-axis micrometer. 4. Swing the microscope over the viewport. 5. Adjust the microscope to fill the monitor with the sample image and focus at the height of the landing pads or landing surface. 6. Use the z-axis micrometer to move the probe tip up and down until the tip begins to come into focus (at this point, the tip is only 30 to 60 μm away from the sample). 7. Continue lowering the probe slowly, stopping to position it as needed so the tip lands on the outside edge of the landing pad. 8. Once it lands on the pad (which is indicated by a forward movement known as skating), continue lowering it until it skates on the pad by a consistent amount. A typical amount of skating is 20 to 25 μm, which is roughly the same as two scale graduations on the z-axis hand dial. 9. The desired position of the probes with respect to the edge of the pads and the desired amount of skating should be determined and used as a lab standard to ensure consistent results. For more on this, watch this “Landing a DC Probe” video. If you are using a CVT probe, however, the procedure is rather different. The very important difference is that no skating occurs (and, if it does, skating can destroy the probe). Instead, the spring member and probe wire will flex. Users should watch for this flexing movement when beginning to make contact. Too much downward Z-travel after making contact can result in too much force on the probe tip and cause damage. However, too little downward Z-travel can result in too little force exerted, reducing the variable temperature performance. Too little force can also cause poor or inconsistent electrical contact. The step-by-step procedure can be found here. For additional help landing a CVT probe, watch this video.
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Post by Professor Lake Shore on Jan 31, 2017 17:21:35 GMT -5
Can you provide some general tips on temporarily mounting probe station samples?
There are three methods commonly used: greases and adhesives, tapes, and clamping. With a specified operating range of 1 K to 300 K, a very thin layer of vacuum compatible (low vapor pressure) grease, such as Apiezon N grease, can be used as a coupling agent between device and the top surface of a clean sample holder. Below ~ 210 K, the grease solidifies and can affix a device to the sample stage. When it comes to Apiezon N grease – more is not better. At cryogenic temperatures, a thin coating can fill the microscopic voids in the two surfaces for better thermal contact. However, on-cooling, excess grease will act as an insulating layer between device and stage. Apiezon N grease liquefies near 316 K and is not recommended for higher temperature operation. Depending on application, silver paint, carbon-based adhesives, silver epoxy, or high-temperature grease can also be used. Because greases and adhesives often need to be cleaned from a sample, a vacuum compatible tape with silicone adhesive offers a cleaner alternative. 3M brand Kapton tape, placed on the corners or edges of a sample, works well for cryogenic applications because it will retain its adhesive properties to as low as 4 K. For certain measurements, a double-sided adhesive tape can be used between the sample and sample holder. You may need to experiment with different types of double-side tape to find one that does not harden or peel away at low temperatures. While convenient, tape may not perform optimally for the lowest temperature applications. Tape on the corners may provide insufficient clamping force, while double-sided tape can act as an additional insulating layer between device and stage. When possible, Lake Shore recommends clamping the device to the sample stage. Users often make simple clamping fingers to fit their sample and hold them down with M3 screws in the tapped holes intended for the lifter tool. When used in conjunction with thermal grease, mechanical clamping of the device provides the best thermal contact between the sample and sample holder.
Regardless of the method used, be sure to wear nitrile gloves when handling anything inside the probe station (hand oils will contaminate the surfaces, resulting in poor vacuum and thermal performance).
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Post by Professor Lake Shore on Jan 31, 2017 17:16:48 GMT -5
I am looking for information about calibrating my VNA for microwave measurements in a Lake Shore probe station. Are there any general recommendations on calibration methods?
To calibrate out the dispersion and losses of the probes as well as probe station and network analyzer cabling, most network analyzers support a SOLT (short-open-load-through) calibration. For probing measurements, a CS-5 on-wafer calibration substrate (available from Lake Shore) is used as the reference standard for the calibration. Before performing the calibration at a given sample stage temperature, GSG probes should be planarized to the calibration substrate. Once the prober has been calibrated, the frequency-dependent S-parameters of your wafer-level DUT can be determined. The SOLT calibration method is sufficient for most conventional microwave on-wafer probing applications below 20 GHz. For higher-frequency measurements, however, other calibration methods may be necessary. Above 20 GHz, the SOLT method is extremely sensitive to consistent probe placement and contact pressure in order to achieve an accurate calibration. For better S-parameter accuracy at higher frequencies, Lake Shore recommends TLR (thru-reflect-line) or LRRM (line-reflect-reflect-match) on-wafer calibration techniques to reduce the impact of probe placement errors. These calibrations can be carried out using the WinCal software package in conjunction with calibration substrates from Cascade Microtech. However, the LRRM calibration method is more sensitive to the variation in load standard; for cryogenic measurements, the temperature-dependent load resistance should be measured and compensated in the calibration calculation. For more information, visit the Cascade Microtech website or contact Lake Shore.
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Post by Professor Lake Shore on Jan 31, 2017 17:12:49 GMT -5
What does Lake Shore do in their probe stations to minimize any differences between device temperature and sample stage temperature?
This is a common concern. Because of heat loads from thermal radiation and heat conduction through the probe arms, as well as the thermal resistance between the device and sample stage, the actual temperature of the device can deviate from the sample stage temperature. A Lake Shore probe station includes several features to minimize these effects, including radiation shields that reduce the thermal radiation on the sample and the thermally anchored of the probes at or near the sample stage. Lake Shore testing has shown that at the superconducting transition temperature of niobium (9.3 K), the thermal gradient between the sample stage and a probed device can be less than 0.5 K. For more on this, read this app note.
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Post by Professor Lake Shore on Jan 31, 2017 17:09:51 GMT -5
For measuring transport properties of graphene in my probe station, are there any general recommendations on how to prevent contamination of my samples when characterizing under vacuum conditions?
The high-vacuum option accelerates the pump-down time and significantly lowers the chamber pressure when compared with the compact turbo pumping system – helping to minimize contamination of sensitive samples. Whether using the standard or high vacuum options, a full-range vacuum gauge is needed to measure the chamber pressure from atmosphere to operating pressures. The full-range gauge uses a combination of Pirani (for high pressure) and Cold Cathode (for low pressure) gauges. Device measurements have shown that surface-sensitive materials like graphene and other 2D materials can be contaminated by phenomena originating from the Cold Cathode vacuum gauge. At low pressures, residual gasses are ionized by the vacuum gauge and a small portion of the ions can spread into the chamber – exposing graphene or similar samples to these chemical species and potentially changing the sample surface. To prevent this Cold Cathode ion gauge-induced contamination from occurring, it is necessary to unplug the gauge before the gauge switches from the high-pressure Pirani mode to the low-pressure ionization mode, specifically at the point when the chamber reaches 10-2 mbar pressure (as indicated by the pump controller or digital readout on the side of the pump). The turbo pump itself will continue to operate, but with gauge disabled, it will no longer be ionizing gasses in the chamber.
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Post by Professor Lake Shore on Jan 31, 2017 17:07:03 GMT -5
How fast can a probe station cool down from room temperature to 4.3 K, and then how fast does it warm up?
It depends on the probe station and its configured options. In Lake Shore’s flagship station, the CPX, which is a flow station, you actually can go from room temperature down to base temperature in around an hour and 15 minutes. That same station takes approximately 70 minutes to warm back up from the base temperature. The larger systems are going to take a little bit longer. For example, the systems with the integrated vertical field magnets, take a slightly longer to cool down and longer to warm back up again because of the heat capacity of the magnet.
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Post by Professor Lake Shore on Jan 31, 2017 17:04:54 GMT -5
In what applications should one choose an isolated sample holder over a grounded sample holder?
An isolated holder has an electrically-isolated, thermally-conductive sample mounting surface that isolates the sample from chamber ground and is recommended for measuring samples in which a conductive path to the chamber must be broken – like doped semiconductor wafers. In contrast, the grounded holder that ships with each probe station is for more conventional measurements; it suits the needs of most device applications, especially when samples are patterned on highly insulating substrates. It is referred to as grounded because the back side of the sample is held at system ground.
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Post by Professor Lake Shore on Jan 31, 2017 17:03:21 GMT -5
What is the purpose of the grid pattern on the sample holder?
The purpose of the grid pattern on the grounded, coaxial, and triaxial holder is to keep air from getting trapped under the sample, such as creation of an air pocket that can form under a layer of vacuum grease used to mount a sample to the sample holder. On a flat surface sample holder, this trapped air can cause a virtual leak, with the air leaking out over time to negatively affect the measurement.
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Post by Professor Lake Shore on Jan 31, 2017 17:02:20 GMT -5
I seem to be experiencing signal issues with my triax cabling and ZN50 probe blade – loss of continuity, shorting between the signal connector and guard electrode, or higher than normal noise. What may be causing this?
The micro-miniature cabling used in the cryogenic probe station is extremely delicate and mishandling of the cable when exchanging probe blades can damage the internal conductors. Damage is typically caused when the cable behind the SMA nut is not held steady when attaching or detaching the SMA connector the ZN50 probe. The friction from the turning SMA nut will apply a torque to the cable and the resulting twisting action will break or distort the cable conductors. A broken center conductor in the cable usually manifests as a loss of or intermittent continuity between the center conductor and probe blade. Similarly, a damaged guard electrode on the triax cable can short the guard to the center conductor or break the guard path. In the case of a broken guard path, higher than normal noise or current transients may be observed. To verify whether damage has occurred, the first step is to carefully measure the continuity between the feedthrough electrodes and the probe blade. Excess force applied to the probe blade can cause damage. If the resistance is not a fraction of to a few ohms, the cable must be replaced (the procedure to replace the cable is described in the reconfiguring cables section of Chapter 5 in the probe station user manual). The second step is to check isolation between the signal, guard and ground; this is most easily achieved by measuring the resistance between each electrode of the triax feedthrough. Damaged cables will often have megaohm-level resistances between one or more of the electrodes and need to be replaced.
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Post by Professor Lake Shore on Jan 31, 2017 17:00:15 GMT -5
Why do I have to lift and re-land a probe tip with every temperature change?
If you’re not using ZN50-CVT probes (which are specially designed to flex and compensate for variable temperature changes and adjust for probe travel during sample measurements), lifting a probe tip and re-landing the tip once a temperature settles is necessary during temperature cycling because you need to account for probe tip movement caused by the probe arm expanding as the sample stage warms. Significant ramping of sample stage temperatures may result in contact quality changes during device measurements, possibly shifting the tip enough to leave its contact pad. How much movement depends on the amount of change in temperature. When warming from 4.2 K to room temperature, standard probe tip movement can be as much as 400 μm. Therefore, to ensure more reliable continuous variable temperature measurements, the ZN50-CVT probes from Lake Shore should be used. When landed properly and used with proper contact pads, these CVT probes virtually eliminate the inconvenience of material contraction issues due to temperature changes.
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Post by Professor Lake Shore on Jan 31, 2017 16:58:20 GMT -5
Is it normal to see condensation forming on the vacuum chamber window of a probe station?
As it has line-of-sight to the sample stage, the window can radiatively cool below the dew point. Particularly in higher-humidity environments, the external viewport window can develop condensation during low-temperature operation. Excess condensation can make probe placement more difficult and is an indication that the window needs to be cleaned or treated with anti-fog solution. However, if actual frost is beginning to form on the window or lid, the vacuum level in the chamber may be too high, indicating an issue with the vacuum pump or chamber vacuum seals. Contact Lake Shore Service for further guidance.
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Post by Professor Lake Shore on Jan 31, 2017 16:55:44 GMT -5
Is it OK to change the factory default settings on the probe station’s temperature controller?
It is not recommended. The configuration is probe station specific and altering that configuration can cause costly permanent damage to the probe station.
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Post by Professor Lake Shore on Jan 31, 2017 16:53:41 GMT -5
With Lake Shore probe stations, what is the lowest temperature that the chuck can go?
The chuck temperature gets to as low as 1.6 K in the CPX probe station configured with the ultra-low temperature option.
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