Explanation
The method of measuring spreading resistance demands an equipment capable of reproducing the same point contact conditions measurement after measurement. The Universal Probe incorporates a kinematic probe guidance system, a controlled velocity of descent, an adjustable and controlled load.
The measured resistance R(sr) is given by R(sr)=V(sr)/ I(sr) where I(sr) is the measuring current which flows through the sample, and V(sr) is the associated voltage drop measured between the centre probe and non-current carrying probe. In the three probe system for a uniform material the spreading resistance of a contact is related to the resistivity (rho) of the material by R(sr)=rho/4a where a is the EFFECTIVE radius of contact.
Practical Facilities on the Universal Probe
The tension gauge on the left should be set at zero load, and the lever (4g in fig. 2) pulled down to operate the camshaft (48 in fig. 8). The signal lead should be the one marked SRL which ensures that the current source and digital voltmeter are appropriately connected – viz measuring current through probes 27 and 28, voltage drop measured across probes 28 and 29. Thus, the spreading resistance probe is 28, whic lags behind 27 and 29 so that its contact is mechanically undisturbed by the other two. The load on these should be set to 45g.
General
The principal application of Spreading Resistance Measurements is depth profiling. A sample of the wafer is bevelled at an angle of 1 degrees or less, and the probe stepped along the bevel. If X is the movement of the stage carrying the specimen, d is the depth into the structure, and theta the bevel angle then X=d sin(theta).
The actual resistivity of the sample can only be deduced by using the spreading resistance probe to measure a standard wafer of known resistivity and deriving the sample resistivity by proportion.
NOTE: The three point spreading resistance measurement technique – including the preparation of samples and probes has been the subject of many papers and symposiums under the auspices of the National Bureau of Standards Gaithersburg, Maryland, USA.
Four-Point-Probes is a division of Bridge Technology. To request further information please call Bridge Technology at (480) 988-2256 or send e-mail to Larry Bridge at: sales@bridgetec.com
