The Current-In-Plane Tunneling (CIPT) method is a way of measuring the properties of a tunnel junction by placing a set of probes on a full wafer/large sample without conventional techniques for structuring of tunnel junctions. This greatly speeds up development and testing. Also, these non-invasive measurements are local in nature due to the very small (micron sized) probe spacing, which can be used for homogeneity control as well as characterization of a parameter varying over a wafer sized sample.
In CIPT measurements, the (sheet) resistance of a sample is measured using a four-point-probe method. A current is sent through a sample by two probes (I+ and I-), and from the induced voltage drop between two other probes (V+ and V-) the resistance of the sample can be calculated. In the figure below (a) represents the top view of the sample, while (b) represents a cross section of the sample.
If the sample is a tunnel junction, a part of the current will flow through the top electrode, while another part will tunnel through the barrier and flow through the bottom electrode. The amount of current tunneling through the barrier can be changed by changing the distance between the probes, as is shown in the figure below.
When the probes are placed very close together the current will not `have time’ to tunnel down and will mainly flow through the top electrode. When the probes are very widely spaced, the current will divide proportionally between the top and bottom electrode. Therefore, the resistance of the tunnel junction will be higher for smaller probe spacing, and fall off to a lower value at larger spacing as schematically shown in the graph above. By measuring the resistance of the tunnel junction for different probe spacing (by selecting different electrodes on the CIPT probes), a number of data points along such a curve is obtained. Fitting the data yields the characteristic properties of the tunnel junction.
In the figure above the y-axis represents sheet resistance, while the x-axis represents the distance between probes used for each of the measurements. From this figure the following important parameters can be estimated.
R(T) = Resistance of the top electrode
R(B) = Resistance of the bottom electrode
RA Product = Indication for the resistance of the tunnel barrier
λ = Characteristic length
TMR = The relative change in resistance between the top and bottom electrode