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Increasing Demands on Wafer Test Probes

Over the years, as IC technology advanced across the entire fabrication environment, the physical size of the transistor had become smaller and smaller.  While Moore’s Law may not continue indefinitely [2], the march toward IC size reduction and higher component density still continues. This means that the available pad size for signal and power delivery has decreased over time, and the number of test sites per unit area of the IC wafer has increased.  As test site density increases, the distance between adjacent probes - their “pitch” - decreases, necessitating the use of smaller diameter probe designs.

Current Carrying Capacity (CCC)

Compounding the difficulty of manufacturing these tiny probes is that they must also support current levels similar to larger, previous generation wire probes.  This means that current density increases quickly, inversely proportional to the square of the diameter.  For example, if the diameter of the probe decreases by half, all other things remaining equal, the current density in the probe increases by a factor of 4x.  This presents challenges in both the mechanical and electrical aspects of the design. The ability to simultaneously support amperage and a mechanical load is termed “current carrying capacity” (CCC).

Semiconductor test engineers use CCC values as an approximation of how much current they can pass through a needle or probe card for a specified duty cycle.  The two most common methods for measuring CCC   are the International Sematech Manufacturing Initiative (ISMI) method, and the Maximum Allowable Current (MAC) method.

ISMI CCC Testing

ISMI CCC testing measures the spring force exerted by a probe at increasing current densities, as described by Daniels [3].  Typically, the test needle is compressed to a manufacturer-specified overdrive, and the compression force in the absense of applied current is measured.  Constant current is then applied for 2 minutes, stopped, and after a cooling cycle of 10 seconds the remaining force is measured.  This cycle is repeated with stepwise current increases until a substantial drop in force is observed.  The CCC limit is conventionally defined as the current at which the remaining force is reduced by 20%.

Single Probe MAC CCC Testing

Single probe MAC CCC testing looks for permanent deformation of a probe at levels of appled current, as opposed to measuring the force exerted.  A description by Kazmi et al. [4] characterizes this approach as “[CCC is the] maximum current a probe can handle without permanently deforming (stresses remain below the yield strength) at its fully loaded state for a repeated number of cycles.”  In the simplest embodiment, a test needle is compressed per manufacturer-specified overdrive, current cycled, force removed, and the probe height measured.  The CCC of a single probe is thus defined as the current at which the needle does not return to its original position once the current and compression are withdrawn. 

Multiple Probe MAC CCC Testing

Multiple probe MAC CCC testing was offered as a somewhat more complex approach by Cassier et al. [5], where a small array of 10 probes was used.  Each probe experienced a unique current level and duty cycle before its free height was measured.  The change in probe planarity was examined as a function of current on a log-log scale.  MAC was defined as the current at which the line of best fit coincided with some allowable change in planarity (0.1 μm in this case).  Yet more sophisticated approaches designed to gather both MAC and ISMI CCC data from multiple probes have been proposed as well [6].   

Other Demands on Wafer Test Needles

In addition to CCC requirements, the probe needles must have high tarnish resistance to minimize contact resistance (CR) variation and ensure signal integrity for the lifetime of the probe.  In some cases, adhesive transfer from the DUT to the probe tip can raise the CR to unacceptable levels.  In these cases, abrasive mechanical cleaning treatments are often used periodically to remove the transferred material and prolong the useful life of the probe.  Typical needle life is in the millions of touchdowns. 


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