Does sample size matter?
When I am approached by customers to make measurements for them, I have a list of questions to try and guide the discussion and to determine whether an AFM measurement is feasible or even a good idea. One of the questions on my initial list is what are the dimensions of the sample? Of course the sample dimensions could be a show-stopper for the AFM (for example, a car engine would not fit into the AFM), but I’m really asking because I need to know which kind of AFM I could use.
This car engine won’t fit into an AFM, but parts of it will
AFM’s are pretty much divided into large sample and small sample instruments. The distinction mainly comes from whether the sample or the tip is moved during the scanning motion. Large sample instruments are typically tip-scanning because it is not easy to move large and heavy samples tiny displacements on the order of nanometers. Small sample instruments are typically sample-scanning where the sample is moved in x and y under the tip.
Why the 2 different designs? Why not make all AFMs as large sample instruments where the tip does all the scanning and the sample is stationary? Historically, the design of a large sample instrument where the tip does all the moving results in a noisier instrument because the tip is doing all the moving and all the detecting! The extra noise from tip-scanning instruments really only affects the highest resolution measurements, those that are trying to push the limits of resolution – e.g. atomic scale resolution. The noise floor on tip scanning instruments are continually improving, but if you are going to conduct very high resolution measurements, you may want to opt for a sample scanning instrument.
So how small is small? Again, remember that sample scanning instruments are moving the sample in x and y under the tip, so the size and sample have to be limited. Typically, the sample can only be approximately 1cm in diameter and only a few mm thick. This is suitable for thin films, graphene, 2D materials, cryotomed polymers, and any kind of material that can be cut to that size. But the size constraints can definitely be a limiting factor.
Small piece of highly ordered pyrolytic graphite (HOPG) mounted on a puck for a small sample AFM
How big is big for the tip-scanning instruments? You would be surprised as to what you can stick under an AFM. So although a whole car engine won’t fit, I myself have thrown car engine parts under the AFM to look for thin (100nm) friction reducing and wear-protecting films. But basically, if it fits under the tip, you can image it! A lot of the application of these instruments that can handle large samples is for wafers in the semiconductor industry which can come in diameters upto 200mm. Vertical clearance can be a few inches, which is a vast improvement over a few mm for sample scanning instruments. Most vendors offer both kinds of instruments; a recent entry to the market of large sample instruments is the Jupiter XR AFM from Asylum Research.
Another important design advantage of tip-scanning instruments is the ability to do environmental (fluid and temperature) measurements. These kinds of accessories are much easier to design for in a tip-scanning instrument since the real estate underneath the sample is available in contrast to a sample scanning instrument where the piezos and independent sensors are underneath the sample. Thus, in a tip scanning instrument you can easily put a thermal stage directly under the sample. You also don’t have to worry if fluids from your sample leak.
No matter what your sample size and what instrument you use, you are still limited by the dimensions that can be imaged by the AFM. Depending on the instrument, the maximum scan size might only be 30um or be as high as only 100um!
Until next time