The work done by JPL focuses on destructive physical analysis for the A1020 (2.0 µm) and 1280 (1.2 µm). Devices from these two FPGA technologies were examined in detail by cross-section analysis of all materials and their respective thickness and interface pattern. The DPA reports for the ACT 1 A1020 and ACT II A1280 are included as JPL PIP report Nos. 304 and 305. In general the die structures and measurements made from SEM photographs were in agreement with information provided by ACTEL.
The only concern is the evidence of metal-2 thinning in a via step to metal-1 and in
metal step coverage in BPSG cuts to poly and silicon contacts. The measured thickness was
25% or less of nominal metal thickness. This violates MIL-STD-883 Method 2018.3 paragrpah
3.7.2. This limited metal step coverage was seen in both the 2.0-µm and 1.2-µm
technology and corroborated by evaluation at TRW. The 1.2-µm technology is more
aggravated because of the scaling effects of metal and subsequent smaller via and contact
sizes.
In order to ascertain the reliability risk created by the step coverage, current density
calculations were done for single contacts. MIL-STD-883 allows a current density of
less than 2 x 105 A/cm2 if the step coverage is 30% minimum for a
geometry less than 1.5 µm. With this current density limit no electromigration
problems are predicted assuming nominal operating conditions. The current density
calculations done for the 2.0-µm technology showed the worst case for a single contact is
1.06 x 105 A/cm2 with a 23.5% step coverage. This does not
meet MIL-STD-883 but can be waived for some noncritical applications if the operating
temperature is less than 90 °C. The assumption being that by ACTEL's design rules a
single contact is limited to 1 mA for the internal transistors which are doing AC logic
switching. Input and output transistors have multiple contacts for current sharing and
were not an issue. The 2.0-µm process electromigration lifetime calculation
approximates 10 years (at 125 °C) and 70 years (at 90 °C) for ttf.01 with a
50% duty cycle. This is acceptable provided the 1-mA current limit assumption is valid.
The 1.2-µm process has a current density of 2.89 x 105 A/cm2
(12.5% metal step coverage). The predicted life is 1.5 years (at 125 °C) and 10 years (at
90 °C). The scaled technology is more at risk unless operating temperatures are kept
below 90 °C. Note that ttf.01 indicates that 1 single contact or via out of
10,000 will fail given a log-normal failure distribution.
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