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Monitoring of Probe Card Performance at Wafer Test

IP.com Disclosure Number: IPCOM000132046D
Original Publication Date: 2005-Dec-25
Included in the Prior Art Database: 2005-Dec-25
Document File: 2 page(s) / 50K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

The wafer test (also called wafer sort, wafer probe) is the test step where each individual die on a wafer is tested and bad dies are marked as bad. Chips which fail the wafer test will not be assembled and decrease the yield of a semiconductor production process. For testing a probe card is used. This card holds a number of tiny needles with specific spacings designed to make contact with bond pads on the chips. Test equipment is mostly designed for parallel testing in order to save test time, i.e. at one probe card touch down several dies are contacted and tested in parallel. To this end a multiple site probe card is used. If a needle is damaged or (temporarily) polluted the resistance between probe card and tested die increases. Due to this, chips can fail at the wafer test and a periodic fail pattern can occur in parallel testing. The periodicity corresponds to the design of the multiple site probe card. A graphical display of the pass/fail-sort of each tested chip is called a wafermap. An example of a wafermap is shown in Fig. 1 where green refers to passed chips and all other colors represent different fail classes. Yellow and brown marked fails show a periodic fail pattern. Some touch downs of the four fold probe card are shown with red squares. During testing the bad rows one or more probe card needles seem to be temporarily polluted.

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Monitoring of Probe Card Performance at Wafer Test

Idea: Matthias Ernst, DE-Dresden; Ralph Wichtendahl, DE-Dresden

The wafer test (also called wafer sort, wafer probe) is the test step where each individual die on a wafer is tested and bad dies are marked as bad. Chips which fail the wafer test will not be assembled and decrease the yield of a semiconductor production process. For testing a probe card is used. This card holds a number of tiny needles with specific spacings designed to make contact with bond pads on the chips. Test equipment is mostly designed for parallel testing in order to save test time, i.e. at one probe card touch down several dies are contacted and tested in parallel. To this end a multiple site probe card is used.

If a needle is damaged or (temporarily) polluted the resistance between probe card and tested die increases. Due to this, chips can fail at the wafer test and a periodic fail pattern can occur in parallel testing. The periodicity corresponds to the design of the multiple site probe card. A graphical display of the pass/fail-sort of each tested chip is called a wafermap. An example of a wafermap is shown in Fig. 1 where green refers to passed chips and all other colors represent different fail classes. Yellow and brown marked fails show a periodic fail pattern. Some touch downs of the four fold probe card are shown with red squares. During testing the bad rows one or more probe card needles seem to be temporarily polluted.

At present, the occurrence of probe card signature is not automatically detected by the testing equipment. If this periodic fail pattern is noticed a probe card cleaning and a retest on the affected wafer can be done and the signature will disappear. In case the problem was observed after the signature occurred on several wafers many wafers have to be retested. Otherwise the yield impact will be effective on many wafers. To avoid this, additional tests or enhanced test flows can be installed which check if probe card dependent fails occurred (e.g. probe card contact resistance tests, check for repeatability of a set of calibration test on reference chips). Test equipment weakness can be detected only by means of these special tests. As a consequence the perceptibility of probe card weakness induced fails depends on the frequency of such special tests. But they will cost additional test time. Another disadvantage of these tests is that not all needles can be tested.

The suggested solution is to use an automatic statist...