Probe Cards Simplify Testing and Replacement Processes

The relentless downward price pressure on memory devices has driven down the cost of test. Many companies are increasing throughput by testing more devices simultaneously. Over the past few years, the development of test probe cards has allowed more devices to be tested in parallel—the number of devices under test (DUTs) that can be tested simultaneously has increased from 32 to 64 to 128—reducing the number of test platforms. This has increased test throughput by completing a single test on a 300 mm wafer, rather than requiring four touchdown (TD) tests.

It typically takes 10 minutes to test a flash chip. That’s the amount of time before the wafer is changed, and another two minutes to change the wafer. Another three minutes are needed to change the wafer boat; thus, only about 80% of the time is the tester actually testing the wafer. About 60% of the idle time when the tester is not testing wafers is related to the status of the probes or tester; 40% is related to the probe cards. Because probe cards are very expensive, vendors are very cautious when purchasing additional probe cards; thus, a systematic approach that allows for good reusability and easy maintenance becomes very important.

To solve this problem, FormFactor has developed the Harmony probe card, which consists of four identical probe arrays that are plug-and-play (see figure). If a part is damaged, only the relevant quarter array needs to be replaced without replacing the entire probe head. There is a spare array that can be replaced at any time. Since the overall undulation is limited by the surface pattern, each quarter array allows the position of the probe foot plane to be set to match the position of the local ceramic ring without matching the entire pattern.

The bridge structure derived from the Harmony probe card provides a rigid reference surface, shortens immersion time and provides automatic tilt correction. The structure can also be used on the tester or probe, reducing the number of components on the probe card and reducing the cost of the system.

There can be thousands of resistors and capacitors on the probe card. Additional components are necessary for a single TD. The probe head occupies almost the entire surface of the probe card facing the wafer, leaving very little space for other components. The Harmony system approach considers the tester, probe and probe card, creating more space by moving the components to the back of the probe card.

Most manufacturers provide test equipment for specific purposes, and the electronic components that match the pins must be integrated into the test head. The electrical connection between the probe card and the wafer and the tester must be good. NAND flash testing has relatively simple system requirements due to the large size of each chip and the need for fewer test probes (about 11,000 pins). DRAM chips are smaller in size and will require up to 50,000 pins in the future, and the power supply of the system must be considered. As the force increases, the possibility of damaging the wire bonding pads also increases.

Bond pad damage is also related to coplanarity. Poor coplanarity will result in greater overtravel of the probe card, resulting in higher forces and larger scratches at the device bond pads. Coplanarity is determined by two components: the natural coplanarity of the spring probes (affected by distance from the wafer) and the dependency of the probe card on the customer’s system. Tilt creates slope X, distance Y. When testing 300 mm wafers, the same amount of tilt is applied but the distance is doubled, resulting in coplanarity issues. The portion of the probe card that is closer to the wafer due to tilt will have greater overtravel and larger scratches as each spring introduces more force into the system, while the portion that is further away due to tilt will have poor contact and leave a barely visible scratch. Given the large area of ​​300 mm wafers and the need to make repeatable contacts, it is attractive to tilt the probe card towards the system. An

automatically adjusted coplanar system reduces the potential for damage from higher forces and allows compatibility between test equipment and different systems. The Harmony bridge structure allows the probe to collect information and then adjust the tilt of the probe card to achieve the optimal position in less than 3 minutes. The entire test system works simultaneously, reducing ineffective time, dynamic processes and manual adjustments, and can achieve better contact resistance and greater throughput - all of which improve the primary TD value.