Semiconductor designers and users always want more for less. They want more circuits and functions on a chip, so that their customer can make an end-product smaller. They want more chips per wafer, so yields will be higher and the net cost to produce each device will be less. And they want larger wafers for less, so the overall costs of a semiconductor process will drop. Over time, all these things have come to pass. But one thing hasn't changed: Semiconductors still need to be tested, and that is a cost of developing an IC that can't be ignored.
Testing an IC before it is packaged (a practical approach, since the cost of the package can sometimes exceed the value of the IC) requires a wafer prober to pass signals to and from a semiconductor device. Wafer probers range from simple mechanical units to fully automatic systems with probe cards and complex interfaces to test equipment. High-frequency wafer probes, typically formed with coplanar transmission lines terminating in a fine, spring-loaded probe tip, can be useful in pinpointing circuit junctions on an IC or specific locations for injecting and extracting test signals.
Whether for DC testingto simply determine whether or not an IC is worth further testingor for full functional RF testing, it is helpful to think of the wafer probe as part of the IC. For a complex IC, a probe card often must be customized to provide necessary test and bias points, and the electrical characteristics of a wafer probe or probe card can impact the performance being measured when testing an IC.
Full functional test can be used to determine whether an IC meets minimum performance requirements, but also to sort ICs according to different levels of performance. Even within the same wafer, IC performance can vary widely, and wafer probing and testing can provide the data needed to create a wafer map on which ICs are identified by various levels of performance, such as low noise and very low noise. Later, when the semiconductor wafer is diced and individual ICs are sorted, they can be placed in different performance bins according to the wafer map, and eventually sold as products with various grades of performance.
Experienced IC designers have learned that the cost of an IC also includes the cost of testing it. The cost of test can be dropped by testing faster or less , and also by intelligent IC design, by including test structures on a wafer or using built-in-self-test (BIST) methodologies that can slash IC test costs.
Microwave ICs have come a long way in 50 years, as have the means to test them. But the importance of IC test can never be underestimated and should always be considered part of the actual design process for the IC.