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|Ultra-Fine Pitch Wedge bonding for Device Reliability Characterization|
|Keywords: wedge bonding, ultra fine pitch, Device Reliability|
|The rapid node scaling afforded by Moore’s law has resulted in smaller and smaller device features. This potentially allows for higher densities of test structures per test chip or fewer required number of test rows to fully characterize the devices and process technologies. However, pad sizes and pitches do not scale at the same rate as the technology nodes due to limitations in the feature sizes and pitches of the probe cards. Furthermore, the use of smaller pad sizes and pitches pose a challenge when wire bonding is required to characterize the devices. This is particularly true in the reliability testing of packaged devices wherein the devices are wire bonded to the frame of the carrier inline package and subjected to extreme stress conditions such as high temperature electromigration (EM) testing, temperature cycling and bake. It has been demonstrated that wedge bonding can be used down to 30um pitch using a 0.7mil Al wire wherein the bonded wires are parallel to each other . In reliability testing and packaging, the wires may have to be angled with respect to each other due to the difference between the pad pitch of the test row and the lead-frame pads of the carrier inline package. Heel shorting to adjacent pads or features due to wire angling is a concern as well as possible wire droop shorting during stress characterization . A wider pitch may be necessary to accommodate the different wire angles required for packaging and to avoid shorting. Ultra-fine pitch bonding therefore poses a significant challenge for device reliability testing. In this paper, we present a 43um pitch wedge bonding infrastructure being used at Intel’s EM/Device Stress Labs. With a pad size of 30um x 37um, the resulting increase in the number of test structures is approximately 2.6x that of the 86um pitch test row (with a pad size of 53um x 60um). At 43um pitch, we are able to maintain a heel-to-heel distance of 10um even at a wire-to-wire angle of 45°. The primary concern in developing a robust ultra-fine pitch bonding process is the shorting margin behind the foot of the destination bond. We discuss how contact angle can be adjusted to prevent electrical shorting to the structures behind the foot. Key parameters that must be evaluated and tested are looping parameters, bond angles, and repeatable placement of assembled parts. A shorting margin monitor was used to evaluate the robustness of the process. The challenge with increasing or maximizing the contact angle is the resulting increase in loop height necessary to mitigate shorting effects, which, in turn, brings with it packaging concerns and potential rework to ensure the loop height is recessed inside the cavity of the package. The 43um pitch wedge bonding process yielded above 98%, comparable to the 86um pitch wedge bonding process previously used. The wire pull result of the 0.7mil Al wire on the 43um pitch is ~1.5gF. This is lower than the >2.0gF achieved from the 1mil Al wire bonded on the 86um pitch. This is likely due to the difference in the area of the alloyed interface between the wire and the pad. When the pull strength of the 0.7mil Al wire was evaluated on both the 43um and 86um pitch structures, the bond pull strength was similar when the bond position is centered on the pad. When the bond is off-centered, the 43um pitch pads produced decreased bond pull strength compared to the 86um pitch pads. This is expected as an off-centered wire on a smaller pad will have a smaller effective alloyed interface area compared to an off-centered wire on a larger pad. The tighter pitch infrastructure was also tested to see if it can withstand the extreme stress conditions used in EM testing. Similar test structures, one set with anode and cathode connected to the 86um pitch test row while the other set connected to the new 43um pitch test row were packaged and stressed at 300C and at 5x the rated maximum current for the test structures under normal use conditions. Failure times were shown to be statistically matched between the 43um and the 86um pitch test rows in all the test structures. Samples from the two test rows were also subjected to high temperature, high humidity conditions  prior to EM testing. Results show no significant difference in the result between the 43um pitch and the 86um pitch. In conclusion, the 43um pitch wedge bonding infrastructure and process has been demonstrated to show very similar results to that of the 86um pitch wedge bonding infrastructure and process in terms of bond quality (wire pull), yield and reliability characterization. The challenges of heel shorting have been addressed using loop height optimization, bond rotation, and tighter assembly controls. The resulting impact of this 2x decrease in pitch is a 2.6x increased efficiency in the use of Si real estate.|
|Lacey Badger, Corporate Quality Network Stress Engineer