From vertical probes to MEMS probe, a change of paradigm in the test industry?

From vertical probes to MEMS probe, a change of paradigm in the test industry?

Author: Samuel Benketaf, Ph.D.

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Trend and convergences in the semi-conductor industry

Compelled by Moor’s scaling lawⁱ, the semi-conductor industry shifted toward 3D multi-layer circuits for building system-on-chip. Despite the high degree of sophistication of the latest lithography machinery, fabrication isn’t yet without defects. Probing is an essential and crucial step in value generation. The challenge there is three-fold: achieving smaller pads, higher density, and longer lifetime of a test system. The wafer probing industry also is mutating to keep up with the growth of the volume and with the changing requirements. This article explains wafer-probing techniques (vertical and MEMs probes) and compares their fabrication processes.

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What is wafer probing

Wafer probing consists of touching contact points with tiny needles (probes) and reading electrical signals with a circuit analyser (ATE). A steady force is maintained on the probes onto the substrate. A fixtureless, flying probe robotic station can do low volume. For greater parallelism, a probe card is deployed. At the core of it is an array of probes. The pad layout is inherent to the design and mode of action of probes (Figure 1). Cantilevers flex under load: the tip “scrubbing” the surface. Vertical probes buckle; therefore, their lateral scrub is shorter, which makes them compatible with array probing contrarily to cantilever probes which are limited to peripheral rows. Despite its merit, size is limited due to mechanical considerations. An assembly of precision plates (usually ceramics) ensures guiding and accurate positioning. The probe card is built by inserting the needles into the tiny holes; a task still often executed by skilled hands. Hence, probes count in the range of“many thousand”.

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Traditional probes can be cut directly from precious metal alloys by Laser. For that purpose, 5-axis galvo-scanners are fast (several mm per second) and the precise control of the Laser angle of incidenceⁱⁱⁱ yield straight edges and 2.5 D features (beveled tips). The same can also drill ceramic guide plates at the rate of one second per hole (Figure 2). This versatile tool is a reliable work horse for probe card makers ^iv. The state of this art attains over 150k hole at a pitch of 50 µm or less with process accuracy of ± 2 µm.

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Overcoming the challenge of probing fine-pitch grid arrays.

The “MEMs” leverage the lithography process, which offers significant gain in parallelism while making pitch < 30 µm attainable. The wafer-layer depositions and etching generate large batches in a highly reproducible manner. The 3D geometries are written with a resolution of 1µm by maskless raster-scanning exposure^vi. Moreover, the iterative process and the multimaterial capability facilitate the optimization of mechanical and electrical characteristics^vii.

Another key advantage of MEMs process is the batch-transfer of the probes to a multilayer substrate (MLS) by the flip-chip method (thermal reflow, followed by the etching-release). Also called space transformer, this miniature circuit interfaces the tiny probes (on one side) with larger bump connections on the other side. In figure 3, a typical probe-card is shown, where precision interposers and guide-holes finalize the assembly.

Conclusion

The market for wafer-test is likely to continue to grow along with the semi-conductor industry. Traditional vertical probe is a mature art of engineering, mostly sustained by chemical process (etching) or laser cutting machines. MEMs technology enables the mass-replication of probes and quick assembly onto multi-layer carriers by flip-chip bonding. Although the infrastructure is heavier, it is a cost-effective solution for the most demanding test requirements. The next evolution is toward probing of photonics chips^ix, for which the MEMs process is likely to enable innovative solutions.

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ⁱ International Technology Roadmap for Semiconductors - Wikipedia

ⁱⁱ Tunaboylu, B., & Soydan, A.M. (2018). MEMS Technologies Enabling the Future Wafer Test Systems. In MEMS Sensors - Design and Application. InTech. MEMS Technologies Enabling the Future Wafer Test Systems | IntechOpen

ⁱⁱⁱ precSYS Micromachining System | Scanlab

^iv Probe card guide plates micro-drilling for semiconductors wafer testing industry

^v Precision Micro Machining Solutions | Posalux Swiss Tools

^vi µMLA Tabletop Maskless Aligner ǀ Heidelberg Instruments

^vii Micro-Electro-Mechanical Systems (MEMS) Probe Testing | FormFactor

^viii Kim, B., Kim, J., & Kim, J. (2009). A Highly Manufacturable Large Area Array MEMS Probe Card Using Electroplating and Flipchip Bonding. IEEE Transactions on Industrial Electronics, 56, 1079-1085 A Highly Manufacturable Large Area Array MEMS Probe Card Using Electroplating and Flipchip Bonding | IEEE Journals & Magazine | IEEE Xplore

^ix Q. Yuan et al. "ully Automated WaferLevel Edge Coupling Measurement System for Silicon Photonics Integrated Circuits", in IEEE Transaction on Semiconductor Manufacturing, vol.38, no.2,pp. 168-177, May 2025, IEEE Xplore Full-Text PDF: