Nanoindentation – comparing AFM and instrumented indenters
I started this blog by calling it “AFM vs. instrumented nanoindenters.” But I really didn’t like that title because I don’t like to pit these 2 methods against one another and I don’t really view it as a competitive landscape. The goal of today’s blog is to try and outline the various pros and cons of either approach to nanoindentation. In a previous blog I described the load vs. depth measurements nanoindentation to determine elastic and plastic properties of the substrate.
First, let’s make sure we all understand what an instrumented indenter (IIT) is. These are instruments that designed for the purpose of nanoindentation. Typically they involve a transducer with a diamond tip where the transducer is confined primarily to motion in z (up and down), and it can very sensitively monitor the force as a function of displacement in an accurate, controlled manner. There are many manufacturers of nanoindenters but some common ones include Nanomechanics, Hysitron (now Bruker), Anton Paar (formerly CMS) and Keysight Technologies.
IITs are different than AFMs in some very important ways. In an AFM, the diamond tip is mounted onto a cantilever and the cantilever is what governs the up/down motion and the force sensing. The cantilever has multiple degrees of freedom as it is lowered into the surface, and so its motion is much more complicated than the clean up/down z motion in an IIT. You can imagine that as the tip deforms into the surface the cantilever can torque or the tip even slide significantly. These are not motions that are desirable when trying to analyze the tip-sample contact. Furthermore, the force is detected through the motion of the cantilever, which is detected by optical beam deflection.
In an instrumented indenter, the force can be imposed in different ways, but typically through either electrostatic or electromagnetic forces. The displacement can also be measured in different ways, but a common method used in commercial instruments is through a capacitive gauge. So the force and displacement are measured much more accurately, and with a cleaner z motion (only up/down, no convolution from cantilever twisting), than in the AFM.
Instrumented indenters also often come with powerful and sophisticated automation capabilities over large areas. This is especially important as nanoindentation benefits from statistics of many measurements. Being able to automate hundreds of indents over large areas and under varied conditions is essential to using nanoindentation as a practical tool. AFM certainly has some automation capability, but it is usually limited to areas that piezos can handle (e.g. 100um on large sample instruments.)
Where AFM has a distinct advantage over IIT is in dimensions. An AFM can probe much smaller areas both laterally and in z than an instrumented indenter. The instrumented indenter typically employs diamond tips of various geometries, the most common of which is a Berkovich tip. The radius of a Berkovich tip is 100nm, around an order of magnitude larger than that of a conventional silicon tip.
Image of a Berkovich tip
Also the minimum vertical displacement is typically a few nanometers (although of course it depends on the properties of the material). So if your features are on the order of that size or less, IIT will struggle. For thin film measurements there are further complications because of the general rule of thumb that nanoindentation should be made no deeper than 10% of the total film thickness to avoid measuring the substrate properties. That means on a 10nm ultrathin film, the measurement should be done at depths less than 1 nm, which is challenging if not impossible for many nanoindenters. However, that is a length scale that is very comfortable for AFM.
Which tool you use to determine the nanomechanical properties of your sample should first depend on the dimensions of your sample. If they are suitable for an instrumented indenter, the IIT will provide more reliable, accurate, and robust measurements. But if you need to probe very small or very thin samples, conducting these measurements with AFM is certainly doable, provided you are aware of the uncertainties and assumptions associated with the cantilever motion and sensing.
Dalia Yablon, Ph.D.