Micro Loading and its Impact on Device Performance: A Wiggling Active Area Case in an Advanced DRAM Process

In a DRAM structure, the charging and discharging process of capacitor-based memory cells is directly controlled by the transistor [1].With transistor sizes approaching the lower limits of physical achievability, manufacturing variability and micro loading effects are becoming increasingly critical DRAM performance (and yield) limiters. The transistor’s AA (active area) dimension and profile are important factors that influence advanced DRAM yield and performance. In this study, we will demonstrate how SEMulator3D can be used to study micro loading and manufacturing variability in an advanced DRAM process that exhibits a wiggling AA profile [2].

A. Wiggling AA and its mechanism

Wiggling AA profiles can be found in almost all commercialized DRAM products produced by leading DRAM manufacturers. The wiggling AA profiles exhibit not only wiggling center lines, but also CD differences in the regions neighboring the cut areas (see Fig. 1) [3,4,5].

Fig.1. Planar view of the AA profiles of 1x DRAM devices from three different manufacturers [4,5,6] (Courtesy: TechInsights).

Fig. 2 Illustration of the wiggling AA profile in the Fin etch process (a) hard mask top view before etch, (b) pattern dependence etch comparison between site A and B, (c) top view after fin etch.

B. Wiggling AA modeling

SEMulator3D provides a novel pseudo-3D approach to pattern dependence modeling based on 2D proximity functions.   By using this pattern dependence modeling technique, we can create a 3D model of a DRAM device and mimic a wiggling AA profile.  Figure 3 presents the layout design, pattern-dependent mask, 3D structure and planar view of a DRAM simulation undertaken in SEMulator3D. Comparing 3(d) with Fig. 1(c), a similar wiggling AA profile is observed, demonstrating that the model properly reflects real Si manufacturing results. Figure 4 displays the AA profile at different fin heights, highlighting that the wiggling is much more severe at the bottom of the structure than at the top of the device.

Fig. 3: (a) Layout design, (b) PDE mask generated from hard mask, (c) 3D structure after fin etch, (d) AA profile seen using a planar cut on the middle of the fin.

Fig. 4: AA profile at different fin heights (a) 3d view cut on wordline, (b) cross-sectional view cut on wordline, (c) 3d view cut at fin top, (d) 3d view cut at fin middle, (e) 3d view cut at fin bottom

C. Device simulation and analysis

In a DRAM cell with a buried word line, the transistor channel is located near the middle of the fin, where the wiggling profile is more severe than at the fin top (see Fig. 4 (c) and (d)).   Consequently, the final fin CD under the channel will be much larger due to sidewall passivation.

Fig. 5: DRAM structure after capacitance contact formation, (a) 3D view, (b) single device chopped, (c) Along fin cut and port define

To evaluate the impact of the wiggling AA profile on device performance, sidewall angle splits of 0.1, 2.5, and 5 degrees were modeled in SEMulator3D to mimic varying levels of AA wiggling. A single device was cropped out from a full loop DRAM structure to perform electrical analysis (see Fig. 5(b)). Electrical ports (source, drain, gate, and sub) were assigned in SEMulator3D to collect electrical measurements (see Fig. 5(c)). A built-in drift-diffusion solver in SEMulator3D was then used to calculate the electrical performance changes that could be expected due to different wiggling AA profiles.

Fig. 6 displays off-state leakage current distribution calculated in the fin at various sidewall angles.  Most leakage current was concentrated in the center of the fin (for all sidewall angles), far from the gate metal and not strongly controlled by the gate electrical field. Because of the diminished gate controllability in a fatter fin (at a higher sidewall angle), the leakage current density is much higher in a fat fin than in a thinner fin.

Fig. 6: Channel leakage profile from the fin surface to the fin center at different sidewall angle splits.

Conclusions

In this study, transistor micro loading effects in an advanced DRAM process were analyzed and modeled using SEMulator3D. The analysis demonstrated that micro loading induced during pattern-dependent etch can cause a wiggling AA profile. This micro loading has a large influence on the device’s electrical performance, especially the off-state leakage which is a key factor in determining the data retention capabilities of the DRAM cells.

References

1. Lee D, Kim Y, Pekhimenko G, Khan S, Seshadri V, Chang K, Mutlu O. Adaptive-latency DRAM: Optimizing DRAM timing for the common-case. In 2015 IEEE 21st International Symposium on High Performance Computer Architecture (HPCA) 2015 Feb 7 (pp. 489-501). IEEE.
2. http://www.coventor.com/products/semulator3d
3. DDR4 SDRAM 1x nm tear down report from TechInsights
4. LPDDR4X SDRAM 1y nm ear down report from TechInsights
5. LPDDR4 SDRAM 1x nm tear down report from TechInsights
6. Fried D, Greiner K, Faken D, Kamon M, Pap A, Patz R, Stock M, Lehto J, Breit S. Predictive modeling of pattern-dependent etch effects in large-area fully integrated 3D virtual fabrication. In 2014 International Conference on Simulation of Semiconductor Processes and Devices (SISPAD) 2014 Sep 9 (pp. 209-212). IEEE.