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2) 1) Why exactly is the Iridium there [in the SCM-PIC probes]? Is it possible to just obtain a Pt coat, and would there be any advantages/disadvantages in doing so?
3) 2) What is the maximum frequency you would recommend? We will be using this with a KPFM, and the NSV manuals don’t do a good job of telling me what the maximum response time is for the DC feedback loop.
4) 3) I’ve used about 25 of the Veeco version of the SCM-PIC’s, and honestly we were not really satisfied with the tips because after scanning any sort of non uniform topography, we would begin to get double images from what I assume was the PtIr coating chipping off the tip. When I say non uniform, I mean the double imaging would occur during imaging of nanotubes between source/drain contacts, which never exceeded 15nm (and I assure you I was never tapping harder than 85% of the engagement point). Do you have any tips or recommendations on how to correct this problem or a tip that wouldn’t exhibit these issues?
1. Pt/Ir is harder than pure Pt.
2. I assume you are referring to measuring Surface Potential during the 'Lift line". In this case you want to work of course at resonance. So use whatever frequency you used for a particular cantilever to get your Tapping mode operation working.
3. Quite difficult to answer that question without seeing first-hand what is going on. If your feedback is not well adjusted you might touch features and potentially compromise your coating. If that is the case maybe try scanning slower and see if the issue goes away and work from there. It could also well be that your tip picks up loose objects from the surface due to electrostatic attraction. This is not unheard of in electrical modes and can only be minimized by keeping your surfaces as clean as possible.
Platinum–iridium alloy of platinum containing from 1 to 30 percent iridium, used for jewelry and surgical pins. A readily worked alloy, platinum–iridium is much harder, stiffer, and more resistant to chemicals than pure platinum, which is relatively soft. Platinum–iridium is also very resistant to high-temperature electric sparks and is widely used for electrical contacts and sparking points. http://www.britannica.com/EBchecked/topic/1514796/platinum-iridium
SCM-PIC is designed for contact mode based electric measurements such as conductive-AFM, where smaller spring constant can be beneficial (~0.2N/m). For KPFM, please use SCM-PIT, OSCM-PT instead, which has higher resonant frequencies (~75kHz). Note the following two points
Conductive probes with a wide range of resonant frequency can be used for KPFM: I have used ones with resonant frequency from 20kHz~500kHz.
OSCM-PT has actaully PURE Pt coating, it is prefferred over SCM-PIT (which has Pt-Ir alloy coating as SCM-PIC), as the work function of the tip is better defined for a pure metal than in an alloy (Pt~5.5eV Ir~5.3eV).
I recoomend you try SCM-PIT for tapping mode to see whether tip chipping or debris pickup is less of a problem.
Chunzeng
I was actually the one that initially started bugging Dave about this stuff a few weeks ago. Unfortunately in my email I typed SCM-PIC's when i meant SCM-PIT's. I assure you I am not using the contact tip for KPFM :)
My questions on the iridium percentage were based on the quest to use a metal that doesn't oxidize, as oxidation layers will cloud any KPFM measurement. While I initially thought a pure Pt coated tip would be great, I am concerned with the ductility of pure Pt under these tapping conditions.
My question on maximum frequency was actually related to the maximum resonant frequency of the KPFM tip. I can use long cantilevered probes with a low resonance frequency of ~75kHz, or different probes with resonance ~150kHz, or even coat my own tapping tips or buy tapping tips with metal coats that have resonance near ~300kHz. Aside from the obvious times when you choose not to KPFM at the resonant frequency, I was more curious why 75kHz is the resonant frequency of the most commonly sold non-contact metal tips out there. Is there an issue with the Nanoscope controller feedback speed? Is there some other issue where using a probe with a higher resonant frequency during electrical measurements wouldn’t be advised?
For my 3rd question, while I completely agree that its hard to diagnose a problem like over the internet without seeing any pictures (ITAR), and that it could be a cleanliness related issue, the fact is that I haven't seen this while using other tips that incorporate a Cr layer (PPP-EFM, PPP-NCHPt, PPP-NCLPt) while scanning the same surface. I personally have never worked with PtIr so I can't speak to its wettability or adhesion to silicon, but unfortunately its something we've only seen with the SCM-PIT tip. Granted, I've used more of those than any other kind so there is more data, but at the same time its an unacceptable situation for me considering how much parasitic capacitance I am already fighting.
Nabil,
I am not really sure why you are concerned about the oxidation of Ir. The normal potential (M/M2+) of Pt is 1.2V that of Ir 1.1V making them very close. Ir is also very stable and can not even be easily dissolved in aqua regia which is able to attack even Au. Another hint at the stability of Pt/Ir alloys is that fact that the original standard meter was a bar made from 90/10 Pt-Ir. Any oxidation would have of course changed the dimensions of that bar and hence made it unsuitable for that purpose. Pure Iridium tips are unfortunately difficult to make as the metal is brittle and not easy to machine. I did etch some pure Ir STM tips over 10 years ago for some echem-applications and might be able to dig up the procedure if you are interested. I only remember that it was painfully slow.
I have no exerience with the coated ppp-probes but have also used the OSCM-Pt ones with success.
Stefan
Stefan:
I apologize, but my interest in the behavior and iridium content in the probes comes purely from my own ignorance of Iridium. I've never worked with Iridium, nor do I ever see it making its way into my lab. I should also clarify that when I say oxidation I don't mean a porous oxide like FeOx that can consume an entire metal, I meant a surface oxide like that which shows up on Ti, Al and Cr.
I'm not really concerned with the stability of Ir, just with the surface oxidation of PtIr as the oxidation layers will interfere with KPFM signals. When we try to extract quantitative data from a KPFM measurement we obviously need as little interference as possible. Since I don't know much about Iridium or Iridium alloys, I don't know how prone they are to surface oxidation or when they appear after how much heating. When KPFMing HEMTs and HBTs and other MMICs, surface temperatures can get hot, so I am just trying to make sure I don't report faulty data.
From the sounds of it, I should be OK so that I only have to qualify each tip with one CPD measurement, but I was just asking because I didn't know.
Nabil
Maybe a bit late reaction, however, I think I had similar problem: The laser points to the end of the probe and part of the light reflects from the metallic contacts which leads to artifacts. Just move the laser a bit further from the end of cantilever.
1. Is Surface Potential AFM quantitative for electrical measurements?
2. How can I calibrate Surface Potential AFM to get quantitative electrical measurements (potential)?
3. How can we get the work function of the tip and how can we load that in the system?
4. We acquired our Icon in July of 2012. My guess is that we may not have Peak Force KPFM on our system. Please clarify this.
5. If I have two separate samples, can I be able to map out the potential of the two samples differently or can I only get relative measurements within one sample?
6. Can I be able to do both KPFM-FM as well as KPFM-AM with our Dimension Icon AFM?
7. I have SRAM standard sample (from Bruker) with me. Can this sample be used also as a standard for KPFM-AM and KPFM-FM? What is the potential of this sample? Does the potential depend on the type of the tip used?
8. In the webinar (KPFM by Bruker) presentation slide (attached page 11), how did they get 30mV and 105mV?Did they get that by drawing a cursor profile? Also in the same webinar (page 13), how did they get a potential of 150mV?
Hi Wilson,
Thank you for attending my webinar about AFM applications in the LED world yesterday.
KPFM (surface potential) is indeed a quantitative method. That is due to the fact that you use a feedback technique to "null" any difference in potential encountered between tip and surface. Calibration of course, as in any quantitative method for that matter, can become an issue. How do you e.g. figure out what the workfunction of your tip is? As far as I know there is no official standard available but clean gold is easily attainable; you may either use gold on glass and flame anneal it before use or some template stripped gold. The value for your tip has to be then treated as an offset in your measurements.
We released PeakForce KPFM late in 2012 and I do not know if you purchased it or not. You can check in your "experiment selector" or in the entry for the software keys if it is present. In case you did not purchase it you can always add it at a later point in time to your ICON platform. PeakForce KPFM does come with the AM and FM capability.
The SRAM reference sample we ship with the Scanning Capacitance option is not a good choice for calibrating KPFM. In the Webinar about AFM applications in the world of LEDs I did introduce KPFM and I assume your question is related to that slide. The measured voltages displayed are the output of your KPFM feedback loop. Now, because one of the materials on the sample was indeed gold, one could go ahead and use the known workfunction for gold to calibrate the system after the fact. KPFM measurements can be affected by a varying degree by the ambient conditions; that is why we also offer a 1ppm glovebox integration for ICON and MM8.
With kind regards,