One key scaling factor is narrowing the space or pitch between fins to reduce the overall device footprint. It is very challenging to pattern narrow fins with narrow pitch, so the industry standard approach is to pattern the fins to a manageable width and pitch, and then trim them afterwards. This is traditionally achieved by oxidizing the top few surface nanometers of the fin, then using a wet chemistry to remove the thin oxide layer in a single step. This "global fin trim” approach etches all fins at the same rate for maximum controllability, but also creates serious challenges for advanced devices.

The first issue with global fin trim involves the mechanical strength of the fins after trimming. In the flow shown in Figure 2, there are many thermal processes after fin trim, creating stresses that affect the inner and outer fins differently. This can cause the fins to fan out like peacock feathers, and that distortion creates differences in fin channel stress and therefore channel mobility.

The second challenge occurs because global fin trimming trims the channel and the source and drain (S/D) regions at the same time. If the S/D are narrowed, the access resistance will go up and negate any benefit of the fin trim in the first place.

Resolving these issues requires an innovative way to trim only the channel region while leaving the S/D areas untouched. In a paper we presented at the recent IEDM conference^[4], we showed the capability of using a selective etch technology to trim silicon fins after dummy gate removal.

In this "local silicon fin trim” scheme, shown in Figure 3, the untrimmed fins retain the maximum mechanical strength during the steps between fin formation and metal gate. As a result, they are less susceptible to deformation during subsequent steps. An added benefit of this new process is that the S/D are not exposed, so the trimming is limited to the channel. This gives us the ability to create the exact profile that we need: narrow channel, wide source and drain, with minimal fin deformation.

The key to unlocking this performance boost is the ability to controllably etch only two nanometers of silicon using the Producer® Selectra™ Etch system. This is the equivalent of removing a single atomic layer of silicon from the surface of the wafer. To make things even more challenging, we need to have extreme selectivity to oxide and nitride to prevent damage to spacer or other materials (low k films). In our recent paper we demonstrated the feasibility of this scheme with precise etch control and even improved line edge roughness of the silicon fin (LER 2.1nm pre, 1.8nm post). These physical results were validated to achieve a ~10 percent improvement in Ion at a fixed Ioff (or a ~50 percent Ioff reduction at a fixed Ion) compared to the control device (no local fin trimming).

This result is a great example of Applied’s “New Playbook” for semiconductor design and manufacturing using new materials-based approaches to enable PPAC improvements for advanced devices. 

^[1] C. Auth et al., “A 22nm high performance and low-power CMOS technology featuring fully-depleted Tri-Gate transistors, self-aligned contacts and high density MIM capacitors,” 2012 Symp. VLSI Tech. Dig., pp. 131-132
^[2] S. Natarajan et al., “A 14nm logic technology featuring 2nd-generation FinFET transistors, air-gapped interconnects, self-aligned double patterning and a 0.0588um2 SRAM cell size,” 2014 IEDM Tech. Dig., pp. 71-73
^[3] C. Auth et al., “A 10nm high performance and low-power CMOS technology featuring 3rd generation FinFET transistors, self-aligned quad patterning, contact over active gate and cobalt local interconnects,” 2017 IEDM Tech. Dig., pp. 673-676
^[4] T. Miyashita et al., “Selective fin trimming after dummy gate removal as the local fin width scaling approach for N5 and beyond,” 2018 IEDM Tech. Dig., pp. 827-830

Tags: FinFET, transistors, Moore’s Law, etch, selective removal