Probes, spreading resistance

One technique is referred to as the "spreading resistance" method. In this procedure, a wafer is fractured and the edge containing the film is beveled, as shown in Figure 15. Then, a two-point probe is used to measure resistivity at a sequence of points traversing the interface between the substrate and the epi film. By relating the local resistivity to carrier concentration, one is able to deduce the concentration of dopant atoms over the epi layer. This technique is effective for even highly-doped layers. 

Scanning force microscopy (SFM) 87 Scanning mode 155 Scanning probe microscopy (SPM) 94 Scanning spreading resistance microscopy (SSRM) 171... 

Scanning force spectroscopy (SFS) Force-distance curves Amplitude-distance curves Phase-distance curves Frequency-distance curves - Kelvin probe spectroscopy - Scanning capacitance spectroscopy Full-resonance spectroscopy (FRS) AFAM resonance spectroscopy (AFAM-RS) Scanning spreading resistance spectroscopy (SSRS)... 

Here we see that the resistance is independent of the electrode separation. The physical reason is that the major part of the voltage drop occurs in the immediate vicinities of the tips of the electrodes, and we may think of the resistance as the sum of a spreading resistance from the source electrode and a convergence resistance to the drain electrode. From this we conclude that 2-point probe measurements will not be very reliable or reproducible, because they will depend so sensitively on the small sample of material at the electrode tips. On a pliable material the form of indentation and contact area will be especially difficult to control with precision, and, in addition, we recognise that the material will be most distorted by the electrode pressure in just that region to which the resistance measurement is most sensitive. Therefore 2-point volume-resistivity measurements are not recommended. 

Figure 1 Illustrates the experimental procedure used In making spreading resistance measurements. Two probes are carefully aligned and then stepped across the bevelled surface of a semiconductor sample at each point, the probes are lowered onto the sample surface and the resistance between the two probes Is measured and plotted. The technique Is referred to as the spreading resistance technique because the dominant resistance of a point contact diode occurs In a very small volume beneath the probe, where the current rapidly spreads out Into the sample. Spreading resistance profiles are usually computer-processed to yield resistivity or dopant concentration profiles.

Figure 2 shows a typical automatic spreading resistance system. It consists of a mechanical apparatus to operate the probes and step them across a bevelled test chip and an electronics sub-system to acquire, process and plot the data. 

Figure 2. An ASR-lOOC/2 Automatic Spreading Resistance Probe.

Figure 4 is a scale drawing of a spreading resistance probe tip and the nominal contact size for a 10 gram probe load. The probe tips are a hard tungsten-osmium alloy they have a... 

The Improvement in spreading resistance profiles obtained through controlling probe penetration is Illustrated In Figure 5. 

A further example of the depth resolution possible with properly conditioned spreading resistance probes is shown in Figure... 

Figure 3. A pair of standard spreading resistance probe arm assemblies with probe pins attached, shown In measurement position on a bevelled test sample.

Figure 4. Scale drawing of a spreading resistance probe tip and typical microcontact clusters produced at various degrees of probe conditioning. Reproduced with permission from Ref. Copyright 1984 American Society for Testing and Materials.

This discussion of the mechanical and electrical properties of spreading resistance contacts makes It clear that the probes used differ from any others previously used In either point-contact diode work or In semiconductor electrical measurements. 

Therefore, we make resistivity measurements by first generating calibration curves on known-reslstlvlty samples of the same type, orientation, and surface finish as the test specimens to be profiled. Calibration curves are generated for a particular pair of probes at a particular time, using known-reslstlvlty samples of the highest quality available. This calibration procedure Is a particularly noteworthy characteristic of the spreading resistance technique. It means that spreading resistance Is a comparison method, and that Its ultimate accuracy Is therefore limited only by the calibration material available. Fortunately, It s now possible to obtain complete sets of calibration samples from the National Bureau of Standards. 

Figure 8. Spreading resistance and carrier concentration profiles of a boron lon-lmplant Into an N-type substrate as measured with controlled low penetration, 10 gram probes ...

First, SIMS and NDP both show an Increase In boron concentration at the surface, due to a redistribution of boron into a thin oxide grown during the annealing process. This boron is not electrically active and is therefore not seen by the spreading resistance probe. 

Hall, and C-V, four-point probe and spreading resistance measurements to some extent provide a measure of the net impurity concentration of dopants in SiC ([ND - NA] or (Na - Nd]). In addition, Hall measurements provide a method for obtaining the mobility of the net carriers. These measurements have been applied to both n- and p-type SiC and its various polytypes. In this Datareview, we will report on the mobilities for most of the SiC polytypes under various growth conditions. 

Hydrogen-enhanced thermal donor formation was studied in p-type Czochralski material after exposure to H plasma and post-hydrogenation annealing. The H diffusivities at between 350 and 450C were determined via spreading resistance probe measurements, and were described by ... 

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