Leti Looks at Using Strain with FD-SOI for High-Perf Apps
Posted by Adele Hars, Editor-in-Chief, Advanced Substrate News
The researchers at Leti working on FD-SOI have extremely deep expertise in it. One of the areas they’ve looked at is performance boosters. With the interest in FD-SOI rapidly increasing on the heels of the recent ST-GF announcement, their work becomes even more timely.
A key Leti team wrote a summary of some recent strain work, which first appeared as part of the Advanced Substrate News special edition on FD-SOI industrialization. In case you missed it there, here it is again.
Leti: Adding Strain to FD-SOI for 20nm and Beyond
The outstanding electrostatic performance already reported for planar FD-SOI technology can be improved by the use of ION boosters in order to target-high performance applications, as already demonstrated in the past.
As illustrated in Figure 1, strain can be incorporated at various places in the transistor:
- In the channel through the use of c-SiGe for PMOS devices and strained SOI (sSOI) material for NMOS.
- In the source and drain region with the use of SiGe or SiC for P and NMOS respectively.
- In the Middle-of-Line process with the deposition of tensile or compressive Contact Etch Stop Layers (t- or c-CESL).
First, it is worth noting that local stressors are often more effective on FD-SOI than on bulk at a given geometry because of the mechanical properties of the buried SiO2, which is less stiff than Si.
We have assessed different boosters on the FD-SOI architecture. The results are summarized in Figure 2.
For NMOS, one can see that sSOI is the more promising stressor with an ION improvement of 20-35 % for wide devices; and, it can increase up to 50 % for W = 50 nm narrow transistors . Our preliminary results let us predict a better scalability for sSOI than for t-CESL or SMT. Moreover, the compatibility of sSOI was already proved (even if the ION-boosts are not always totally additive) with t-CESL for NMOS and with rotated substrates, e-SiGe, SiGe channels and (110) substrates for pMOS.
For pMOSFETs, there are several options to enhance the ION, the simpler being the 45° rotated substrates with a 8 % boost and r-SiGe with a 18 % improvement by an access resistance reduction (37 % if a strain can also be generated into the channel). Once again, the scalability of the global boosters is certainly better than for the local ones (c-CESL and e-SiGe).
In conclusion, thanks to all the experiments already run, we are confident in the fact that strain can be incorporated in the planar FDSOI architecture, thus boosting performance even further at 20 nm and beyond.
NOTE: This article was adapted from the Leti presentation, “FD-SOI strain options for 20 nm and below”, given at the SOI Consortium’s 6th FD-SOI Workshop. The complete presentation is available at www.soiconsortium.org.
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