BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. 1 is a three-dimensional view of a portion of a probe card employing probes with blunt skates according to the invention.
FIG. 2A is a plan side view of a contacting tip of a single probe from FIG. 1 equipped with a blunt skate.
FIG. 2B is a front cross-sectional view of the contacting tip of the single probe from FIG. 1.
FIG. 3A-D are three-dimensional views of the successive steps in engaging a blunt skate with a low-K conductive pad.
FIG. 4 (prior art) is a graph of contact resistance Rc between a typical flat contacting tip and a conductive pad as a function of touch-down cycles.
FIG. 5 is a graph of contact resistance Rc between a contacting tip equipped with a blunt skate in accordance with the invention and a conductive pad.
FIG. 6 is a diagram comparing the performance of a prior art chisel tip and a tip with a blunt skate in accordance with the invention.
FIG. 7A-D are three-dimensional views of alternative probe tips with one or more blunt skates according to the invention.
FIG. 8A-B are microscope images of a preferred blunt skate prior to use and after one million touch-down cycles.
DETAILLED DESCRIPTION
A portion of a probe card assembly 100 employing probes 102 according to the invention is shown in FIG. 1. Assembly 100 has a block 104 for holding probes 102 by their contact ends 106. A space transformer, electromechanical arrangements as well as a source for providing a test current i to be applied to contact ends 106 are not shown in this drawing for reasons of clarity.
Probes 102 have electrically conductive bodies 108 that end in contacting tips 110 of a tip width 112. Bodies 108 have suitable mechanical properties for engaging with conductive pads or bumps of a device under test (DUT). For example, bodies 108 can be straight, bent or have more complex geometries to ensure sufficient mechanical strength and compliance, as will be appreciated by those skilled in the art. In fact, although probes 102 have bodies 108 that are bent in the present embodiment, the invention can be practiced with probes of any geometry.
Tips 110 terminate in blunt skates 114 that are narrower than tip width 112. In fact, skate width 116 is typically a fraction of tip width 112. For example, tip width 112 can be on the order of 75 μm while skate width 116 is about 12 μm or less. Skates 114 are aligned along a scrub direction 120 indicated by an arrow.
As better shown in the plan side view of FIG. 2A, each blunt skate 114 has a certain curvature along scrub direction 120. In other words, the ridge of skate 114 that is aligned with scrub direction 120 has a certain curvature along that direction. The curvature is defined in such a way as to produce a self-cleaning rotation sometimes also referred to as pivoting or rocking motion of skate 114. In the present embodiment, the curvature has a variable radius of curvature R that decreases toward a front 122 of skate 114. More specifically, the radius of curvature has a small value Rm at front 122 and a larger value Rn near the center of skate 114.
Skate 114 in the present embodiment is symmetric about a center line 124 that passes through a midpoint 126 of skate 114. Therefore, the same variable radius of curvature is found in the back half of skate 114. It is important that the curvature at every point along skate 114 that will engage with a pad is sufficiently large to avoid single point of contact or knife edge effects. These effects cause large amounts of local stress to develop in the pad and in the case of low-K pads can cause damage. Such effects are especially likely to develop along skate 114 at front and back regions, such as region 128 indicated in hatching. To further help avoid these effects, the cross-section of skate 114 has a rounded rather than a flat cross section, as better visualized in the front cross-sectional view of FIG. 2B.
The operation of probes 102 will be explained in reference to the three-dimensional views shown in FIGS. 3A-D. In FIG. 3A contacting tip 110 with blunt skate 114 is positioned above a conductive pad 130 of a device under test (DUT) 132. Only a portion of DUT 132 is shown for clarity. In this position, no test current i is applied (i=O) to probe 102.
It is understood that DUT 132 can be any device that requires electrical testing including, for example, a semiconductor wafer bearing integrated circuits. Also., it is understood that pad 130 can have any geometry and can also be in the form of a solder bump or any other form suitable for establishing electrical contact. In the present embodiment pad 130 is a low-K conductive pad.
In FIG. 3B a contact force Fc is applied between blunt skate 114 and low-K conductive pad 130. This force can be delivered by any suitable mechanism well-known to an artisan skilled in the art. At this time, there is still no test current applied (i=0).
FIG. 3C illustrates how tip 110 pivots and skate 114 performs a scrub motion along scrub direction 120. The scrub motion is caused by a scrub force Fs1 that is due to contact force Fc. The purpose of scrub motion of skate 114 is to clear oxide from pad 130 to establish electrical contact between skate 114 and pad 130. The alignment of skate 114 with scrub direction 120 and the geometry of skate 114, namely its curvature causes the scrub motion to be accompanied by a self-cleaning rotation or pivoting of skate 114.
The self-cleaning rotation removes debris 134 that is accumulated on skate 114 or that is originally located on pad 130 from skate 114. Typically, debris 134 accumulates on skate 114 during previous engagements with or touch-downs on pads. The self-cleaning rotation pushes debris 134 to the back and off the sides of skate 114. Removal of debris 134 from the skate-pad interface enables a low contact resistance Rc to be preserved between skate 114 and pad 130. Once such low contact resistance Rc has been established, a test current i=io is applied to pad 130.
FIG. 3D shows the effects of augmenting contact force Fc to further increase the self-cleaning rotation of skate 114. This can be done whenever excess of debris 134 accumulates on skate 114. In a preferred embodiment of the method of invention, contact force Fc is augmented after a certain number of touch-down cycles or whenever the contact resistance is observed to reach unacceptable levels. This may occur after two or more touch-down cycles or when resuming testing after a long stand-by period. Note that the resultant scrub force Fs2 is larger as a result of the increased contact force Fc and that no test current (i=0) is applied during this procedure.
A graph 140 in FIG. 4 shows the contact resistance Rc between a typical flat prior art contacting tip and a conductive pad as a function of touch-down cycles. Clearly, contact resistance Rc increases from a nominal value Ro of about 1 Ω as a function of cycles n. The slope of the increase grows as a function of n until reaching a maximum resistance Rmax. Testing the pads becomes impossible once contact resistance Rc reaches Rmax. At this point, the prior art tips are sanded down to remove debris and recover nominal contact resistance Ro. This corresponds to the dashed portion 142 of graph 140. Unfortunately, sanding down accelerates the accumulation of debris on the tip. This causes the slope of contact resistance increase to become steeper and reach the unacceptably high value Rmax even sooner. Another sanding denoted by dashed portion 144 is required to again recover nominal resistance Ro.
FIG. 5 shows an exemplary graph 150, of contact resistance Rc between contacting tip 110 with blunt skate 114 in accordance with the invention and a conductive pad. As contact resistance Rc increases from nominal value Ro, the self-cleaning rotation of skate 114 tends to restore it to Ro. In some cases no additional intervention is necessary. If Rc does begin to grow too much and an immediate decrease of contact resistance Rc is desired, then the contact force Fc is augmented to increase the self-cleaning rotation of skate 114. Portions 152 of graph 150 visualize the corresponding reductions of contact resistance Rc to nominal value Ro.
FIG. 6 shows a comparison in the concentration of mechanical stress caused in low-K conductive pad 130 by a prior art chisel probe tip 160 and a blunt skate 162 with a flat cross-section in accordance with the present invention. Pad 130 is made of aluminum and both tip 160 and skate 162 are made of Rhodium. Chisel 160 has a 60 degree taper angle, a 2 mil radius at its contact tip and is 60 μm long. Skate 162 is 10 μm wide, its ends are rounded with a 10 mil radius of curvature and it is also 60 μm long. The contact force Fc applied in each case is 8 g. The stress caused by prior art chisel tip 160 is very large and concentrated in the middle of pad 130. This causes mechanical failure of pad 130 by fracture. In contrast, the stress is well-distributed when blunt skate 114 according to the invention is used to establish electrical contact with pad 130.
Various types of probes can employ blunt skates according to the invention, as illustrated in FIGS. 7A-D. In some embodiments a probe 200 is made of several material layers 202, 204, 206, as illustrated in FIG. 7A. Such layers can be grown, e.g., in a deposition process. In these embodiments a blunt skate 208 can be formed at a tip 210 from an extension of one of the material layers. In the embodiment shown, it is the extension of the central or sandwiched material layer 204 that forms skate 208. The most appropriate material layer for forming a blunt skate from its extension is a hard conductive material such as rhodium or cobalt. In fact, material layer 204 is made of rhodium in the present embodiment. In alternative probes having more layers extensions of other than central layers can be used. In fact, even the outer-most layers may be extended to form blunt skates according to the invention.
FIG. 7B illustrates a probe 220 with a laser machined blunt skate 222. For example, skate 222 has a higher aspect ratio than previous skates and also a single radius of curvature. Such geometry can be employed when relatively short scrub motion is imposed by a higher pitch of conductive pads. In fact, the curvature of skate 222 can be adjusted in concert with the characteristics of the scrub motion as conditioned by the geometry of the probe. These characteristics may include, among other, scrub length, scrub depth and scrub velocity.
In either the layered probe embodiments or still other embodiments it is possible to provide two or more blunt skates, as illustrated by probe 230 of FIG. 7C. Probe 230 is made of three material layers 232, 234, 236 and of those the side layers 232, 236 are extended to form blunt skates 238, 240. Skates 238, 240 are arranged parallel to each other and along the scrub direction. Of course, more than two skates 238, 240 can be accommodated on the tip of a probe when more material layers are available.
Still another alternative embodiment is shown in FIG. 7D. Probe 250 shown here has five material layers 252, 254, 256, 258 and 260 with layers 252, 256 and 260 being extended. Three blunt skates 262, 264, 266 are formed from extensions of layers 252, 256, 260. These skates are also parallel to each other, but in addition they are staggered along the scrub direction.
A person skilled in the art will appreciate that various other combinations of skates are possible. In addition, the blunt skates can be employed at the tips of various types of probes, including probes that are linear or bent. For example, zig-zag probes, S-shaped probes or probes with a knee can employ one or more blunt skates each to improve contact resistance with the pads of the DUTs. Also, when equipped with the blunt skates of the invention, these probes can be used to contact more fragile conductive pads, e.g., very thin pads or pads that use relatively soft metals.
FIGS. 8A-B are microscope images of a preferred embodiment of a blunt skate that has a rounded cross-section, similar to the skate described in FIG. 2. FIG. 8A shows the skate prior to use and FIG. 8B shows it after one million touch-down cycles. The skate has a width of about 10 μm and a length of 200 μm. Note how the skate is free of debris even after the one million touch-down cycles. In fact, the debris has a tendency to be pushed off to the sides of the skate and attach to non-critical portions of the probe tip.
The probe card requires appropriate design and devices, such as a source for delivering the test current i as well as arrangements for providing the overdrive to apply the contact force between the probes and the pads of the DUT. The design of probe cards as well as the aforementioned devices are well-known to those skilled in the art. It will be appreciated by those skilled artisans that probes equipped with blunt skates in according to the invention can be employed in probe cards of various designs, including probe cards with and without space transformers. The probes themselves can be removable in embodiments that use space transformers or they can be permanently attached using soldering techniques or mechanical locking such as press fit into a conductive via.
The probes of invention are thus very versatile and are able to establish reliable electrical contact with even densely spaced fragile conductive pads or low-K pads. The pads can be arranged in accordance with various geometries, including dense arrays. They are able to do that because the combined scrub motion and self-cleaning rotation of the blunt skate does not cause a high stress concentration in the pad. Due to the large number of possible variations and types of probes that employ blunt skates, the scope of the invention should be judged by the appended claims and their legal equivalents.
Patent Application
20080001612
