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Examining metal eFuses

Posted: 16 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:eFuses? Intel? processor? Westmere/Clarkerdale? TSMC?

We have replicated Intel's fuse blowing process with one of our Westmere/Clarkerdale processors and we show a TEM cross section of a blown fusible links in figure 2. This particular sample was delayered to expose the metal 2 traces that contact the metal 1 fusible link. A four-point probe connection was then made to these traces and a 0 to 2 V bias pulse applied with a 13 mA peak current. The void in the metal 1 trace indicates a successfully blown fuse.

Intel is not alone with eFuses as we find them in a few products fabbed using TSMC's 20 nm planar high-k metal gate (HKMG) process. The image below shows four TSMC fuses formed at metal 2, two of which are blown. The fuses come as pairs with one element being the fusible link, the other is likely a reference element for use in a differential sense circuit.

Figure 2: This particular sample was delayered to expose the metal 2 traces that contact the metal 1 fusible link. A four-point probe connection was then made to these traces and a 0 to 2 V bias pulse applied with a 13 mA peak current. The void in the metal 1 trace indicates a successfully blown fuse.

Source: Custom Analysis of the eFUSE Structures used in the Intel Westmere Clarkdale 32nm Processor, TechInsights

Figure 3: TSMC eFuse 20 nm planar HKMG process

Source: Qualcomm Gobi MDM9235 Modem 20 nm HKMG Logic Detailed Structural Analysis, TechInsights

The fuses shown in figure 4 have a bow-tie shape with a narrow fusible link connected to wider transition regions and large end-pieces. Six vias are seen contacting the end-pieces and this via redundancy provides a low-Ohmic connection to the fusible link.

Figure 4: Enlarged view of TSMC eFuse Source: Qualcomm Gobi MDM9235 Modem 20 nm HKMG Logic Detailed Structural Analysis, TechInsights

The large ends also serve as heat sinks to keep the ends of the fuses cooler than the central regions during the fusing process. The middle portion of the fusible link should be the hottest as it is furthest from the cold ends. This might explain why we see the blown portions of the fuses consistently in the middle and not near an end.

We can estimate how hot the middle portion of the TSMC eFuse gets. We begin by assuming the end pieces do not heat up during the fusing process and the heat from the fusible link flows into the end pieces and not into the surrounding dielectrics. This is not unreasonable, as the thermal conductivity of copper is about a 1,000 times greater than the low-k dielectrics surrounding the fusible link (Cu 385 w/mK, SiOC ~0.4W/mK). We also assume for this calculation that the resistivity of copper does not change with temperature. This assumption is wrong but greatly simplifies our calculation. We estimate the temperature rise of the middle portion of the fusible link to be about

Where p is the electrical resistivity and k is the thermal conductivity of Cu, respectively. L is fuse length and A is the cross-sectional area of the fuse. I is the drive current that due to the bias circuitry, is taken to be a constant.

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