Membrane Probe Offers Higher Placement Accuracy, Longer Life

Alexander E. Braun, Associate Editor -- Semiconductor International, 7/1/1999

Typically, most needle cards used in die testing have had some problems associated with them in the manufacturing, mechanical tolerance, alignment and maintenance areas. As long as the pitch remains 80 µm or greater, the number of I/O stays at a few hundred and they do not have to probe more than one device, traditional needles have been a good tool to use for testing.

However, when pitches go below 80 µm and routings must deal with array-type probing of any kind, requiring needles to be brought over one another, it becomes a mechanical assembly nightmare to fit everything into that mechanical layout. There is also a cost issue associated with this technology -- not so much related to material, but to labor.

Cascade Microtech (Beaverton, Ore.) has adapted membrane technology to meet challenges posed by the testing of dies with increasingly smaller architectures.

Although the idea of using flex circuits or membranes to probe ICs dates back to more than three decades (and has been considered an ideal probe technology for equally as long), no one has previously demonstrated low

Fig. 1. The two box plots show the low contact resistance measured during 500,000 cycles with no cleaning on the Pyramid Probe and the higher results with an industry-standard epoxy ring card. (Source: Cascade Microtech)

contact resistance on aluminum bond pads. It has been well understood that probes need to scrub during contact to break through the native aluminum oxide, but previous efforts relied on stretching the membrane or scrubbing the entire membrane relative to the wafer and had marginal success. With this approach, the probe tips in the corners may scrub excessively and the last tip to make contact may not scrub sufficiently.

Cascade's probe architecture makes each probe tip scrub independently. This is accomplished by fabricating a contact structure with the tips on the end of integral supporting beams. The tips on the flexible membrane are pressed against the wafer by a compliant elastomer. The center of effort of the elastomer's pressure is in the beams' center, which is offset from the probe tip on the beam's end. These offset forces create a rotational moment that forces each individual tip of the probe to scrub when it contacts the wafer, causing each tip to consistently scrub 3 to 4 µm and make reliable contact (Fig. 1).

Fig. 2 The micromachined 15 mm diameter probe tips with integral beams buried in the membrane have 3 mm of uniform microscrub on each independent probe tip and provide low contact resistance. (Source: Cascade Microtech)

The probe is fabricated using a photolithographic process that enables the accurate placement of contact points, eliminating tolerance misalignment. The capability to do fine-tip geometries in the plating process results in finer pitches and smaller contact areas for the smaller features that need to be probed. A micromachining process is used to fine-tune the geometries associated with 15 µm diameter probe tips (Fig.2). The probe is self-cleaning and offers placement accuracy within a few microns, better contact resistance and longer probing life.

This use of the membrane configuration allows routing anywhere within the probe area. Because the membrane probe uses two metal layers, it becomes possible to bring in a ground plane close to the device, controlling impedances and minimizing ground inductance to allow on-wafer functional test for clock rates above 500 MHz.