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July, 2017


www.us-tech.com Shadow Moiré Measurement Techniques Continued from page 55


view and is more sensitive to less optimal surfaces. The technique is accurate to 5 µm and easily effective in deter- mining an initial height estimate. In this particular case, the estimated step height


for the die was 470 µm. Without the step height predic- tion, the shadow moiré unwrapping across the sudden height change is likely to produce an incorrect result. However, this error is always a discrete error — the fringe value multiplied by an integer.


Die Height and Tilt Angle Results The results from the case study concerning predic-


tive die height of 60 samples are as follows: the die height prediction was 470 µm, the minimum die height found was 439.1 µm, the maximum die height found was 481.8 µm, the average die height was 460.3 µm, and with a die height standard deviation of 12 µm. Variation in die height is hypothesized to be more


a result of overall package warpage difference between samples. The con- sistency and range of the results sug- gests that both the die height predic- tion and die height measurements are within expected tolerance, as die height never varied outside of 31 µm from the predicted nominal. For the purpose of comparing


shadow moiré with the point-to-point based technologies used to measure die angle the die tilt angle (DTA) is processed in one of two ways. The first is to process the die corners as measured, as would be done in a point-to-point system. However, one could argue that with full-field data a more realistic value for die tilt angle can be generated between the die and substrate by fitting both data sets to a plane instead of single points. For the purpose of comparison


die tilt angle was calculated based on both warped dice and plane-fit dice. The 60 samples of warped dice saw a minimum DTA of 0.006, a max of 0.096, an average of 0.040, and with a standard deviation of 0.019. The 60 samples of dice fit to plane had a min- imum of 0.003, max of 0.075, an aver- age of 0.040, with a standard devia- tion of 0.018.


Throughput Timing for measurement of die


height and die tilt angle were also evaluated. A primary reason to use shadow moiré to measure these gauges is purely for throughput. Because shadow moiré is a full-field technology, once the test plan is established for a single sample type and layout, which is also a quick process, running high volumes of samples scales well. This throughput is complemented by software to qual- ify pass/fail limitations for good or bad samples based on gauge informa- tion. Therefore, for high-volume, pro- cessing time is essentially negligible and the bottleneck becomes equip- ment time. Measurement time for a batch


or tray of samples with current solu- tions is mainly taken up by load and unload time, as the actual measure- ment takes only two seconds. Also, a fixture or jig should be used to ensure repeatable placement of the sample for each load step. With a manual loading solution the samples used in this case study had an approximate total cycle time of 20 seconds, taking care with the sam- ples and moving somewhat carefully. The case study in question had 44 samples per tray, thus a throughput of 7,920 samples/hour. Throughput time could be fur-


ther improved by automated han- dling. The throughput of 7,920 sam-


ples/hour does not include any sorting of good and bad samples, which could also be done downstream. The measurement methodologies here were devel-


oped purely through software and process improve- ments, thus extending useful applications without adding costly hardware. Highly specular surfaces, such as die surfaces,


remain challenging for optical metrologies that rely on diffuse light reflectance, such as shadow moiré and dig- ital fringe projection; though software development has narrowed the gap of sample surfaces that cannot be measured with shadow moiré. Shadow moiré technology now has extended


usability and measurement application value outside of its most common use cases. Contact: Akrometrix LLC, 2700 N.E. Expressway,


Calculating the average step height between substrate and die areas.


Building B, Suite 500, Atlanta, GA 30345 % 404-486-0880 E-mail: emoen@akrometrix.com Web: www.akrometrix.com r


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