By Dick James

At IEDM last month, there was much ado about the adjacent 7-nm late-news papers from TSMC and the GLOBALFOUNDRIES/IBM/Samsung group consortium from the Albany Nanotechnology Center, and with less ado, Samsung gave a 5-nm presentation later in the conference. Here we discuss all three talks, and try and make some comparisons.

TSMC 7 nm

In paper 2.6 [1], TSMC announced the “world’s first 7nm CMOS platform technology for mobile system-on-a-chip (SoC) applications, featuring FinFET transistors”. They claimed the world’s smallest-ever SRAM cell at 0.27 µm², and 3x the gate density of the 16-nm (16 FF+) process, together with a 35 – 40% speed gain or over 65% power reduction. In addition, the process uses 193 nm immersion lithography, dual raised source/drain epi, a novel contact technique, and a 12-layer copper/low-k interconnect stack.

The fourth-generation fin profile and width are “carefully optimized” for the fifth-generation HKMG gate-last, dual gate oxide process, with an effective gate length (l_eff) centered around 16.5 nm. Sub-threshold swing has been pushed down to ~65mV/decade, and DIBL is ~40 mV/V.

There are four device Vt options with a range of ~200 mV.

The contacted poly pitch (CPP) is not stated (Scotten Jones speculates that it is 54 nm, the same as Intel’s 10-nm process), enabled by a novel contact process, and we also have “novel strain engineering and new process knobs” which boost mobility and reduce parasitic resistance to give increased drive current (at least in arbitrary units).

The 1x metal pitch is 40 nm for M0 to M4, and M5 – M9 are 1.9x (76 nm). The paper states that “single patterning is adopted for metal layers with 2X minimum metal pitch and above” – which make me wonder if they’ve managed to push single patterning to the 76-nm pitch, or whether they are going with double patterning for the first nine levels.

An earlier SRAM paper was given in June at the VLSI meeting [2], as a sub-0.03 µm² bitcell, aimed at a “beyond 10-nm node”, so likely the same SRAM. It also has an l_eff centered on ~16.5 nm, and claims similar performance figures. Some details of the inter-well spacing are also included [1]:

Which allows us to speculate about device sizings, at least in the SRAM cell itself. Typically, a 6-transistor (6T) SRAM unit cell is 2 x CPP high, so if we take the guesstimate of 54 nm for CPP, the 0.27 µm² cell should have a height of 108 nm. Dividing that into 27,000 nm², we get a cell width of 250 nm. The 16FF cell was 0.70 µm² [3], 2.6 x the area of the 7-nm cell, confirming the claim of a 2.6 x array density increase in the paper.

I don’t have a plan-view image of the TSMC 16-nm cell, which I assume is a 1:1:1 PU:PG:PD cell (i.e. one fin for each of the pull-up/pass gate/pull-down transistors), but Intel kindly provided one of their 14-nm cell in a JSSC paper [4]:

The Intel 14-nm cell size is 140 x 360 nm, to give a cell size of ~0.050 µm², considerably smaller than the 16FF cell; we can see that each transistor uses one fin, and there are four fins in the cell. In this case the fin pitch is ~80 nm, instead of the nominal 42 nm, but we have to allow space between the fins and the edge of the N-well that the PMOS pull-up transistors sit in. Theoretically the two pull-up transistor fins could use the minimum pitch, but Intel have chosen not to do that here.

Applying these considerations to TSMC’s cell, if we use the maximum fin-well edge spacing of 23 nm shown above, plus (say) 8 nm for the fin width, then we get a PU – PD/PG spacing of 2 x 23 = 46, + 8 = 54 nm; if we assume a SADP (self-aligned dual patterning) minimum fin pitch of 40 nm between the PU transistors, then we get a total of 148 nm for the center of fin 1 – fin 4, which leaves us 52 nm at each end for the PD/PG – PG contact spacing. If the PU/PU pitch is also 54 nm, that only leaves 45 nm at the end of the cell, which is pushing the limits for double-patterned contact spacing. Which gives us something like this – just guessing!

GLOBALFOUNDRIES/IBM/Samsung 7 nm

The other 7-nm paper [5] from Albany was clearly a research paper, but illuminating in that it shows other possible directions, not the least being the use of EUV lithography, SiGe channels for PMOS, and stress applied to the channels using a strain-relaxed buffer (SRB) substrate.

The application of a SRB substrate to generate channel stress takes me back 15 – 20 years, to the late 90’s and the turn of the millennium, when a lot of work was published on the topic by Stanford, MIT, and IBM. If a silicon epitaxial layer is grown on a SiGe substrate, then the lattice mismatch creates biaxial tensile stress in the layer, and the greater the Ge content, the greater the stress. The earliest reports I can find date back to 1992/4/5 [6, 7, 8,], but the effect is nicely summarized in this plot from IEDM 2003 [9]:

As we can see, low Ge concentration gives a large increase in electron mobility, but a high Ge content is required to enhance hole mobility.

In this paper, we have the following structure:

This gets around the weak PMOS improvement in silicon from the SRB by using 25% Ge in the SRB and growing a 50% Ge fin; if silicon is tensile-stressed, then a layer with more Ge than the SRB will be compressively stressed; and as we know, compressive stress is a big lever for PMOS performance. The authors claim that this combination gives ~1.6 GPa enhancement stress in both NMOS and PMOS devices. SiGe also has a higher hole mobility, compounding the performance gain.

As I remember it, SRB stress never made it into production, likely for two reasons – it was difficult to get rid of the dislocations formed in the SRB, and they propagated through into the sSi; and more production-friendly sources of uniaxial stress could be supplied by tensile nitride and embedded SiGe source/drains.

Now that we are in the finFET era, and twenty years on, we have the advantage of better process control, (so likely lower defect density), and any defects that are formed cannot propagate up the fin because of its narrow aspect ratio. In addition, fins formed in the correct orientation on a SRB use only one axis of the biaxial stress, giving the uniaxial stress that we are used to; so maybe this technique can become the stress mechanism for the 7/5 nm nodes.

If I read the paper correctly, the SSRW is grown epitaxially as part of the SRB (“An epi based SSRW technique is utilized to improve sub-fin isolation” [5]), before the strained silicon (sSi) epi is grown; the sSi is then etched back and the 50% SiGe layer is formed and (presumably) polished back to separate the sSi and SiGe regions before fin etch [11].

Self-aligned quadruple patterning (SAQP) was used for the fins (my notes say the fin pitch was 27 nm), and SADP for the gates with a CPP of 44/48 nm. EUV was reserved for the middle-of-line (MOL) and lower metal levels, with a minimum metal pitch of 36 nm.

The EUV process was presented at last year’s IITC/AMC conference [12]; a metal hard mask was used to pattern lines and self-aligned vias into an ultra-low-k dielectric (k~2.45), and a TaN/Ru liner stack was filled conventionally with a CVD Cu seed and plating. A Co cap and SiCN/SiNO layer sealed the interconnect, giving acceptable TDDB (time-dependent dielectric breakdown) and electro-migration results.

Contacts are self-aligned, with the use of a M0 level, CA/CB contacts, and a sub-contact (TS) for source/drains. The CA/CB/M0 metallization is dual-damascene cobalt, lowering line resistance, while the TS sub-contacts appear to be tungsten. Before the TS contacts are filled, Si:P and SiGe:B epi is grown in the contact trenches, and then implanted and annealed, to give improved contact resistance.

The gate profile was modified by etching back the high-k layer before depositing the work-function metal (WFM), which helps isolate the high-k from the self-aligned contact process reactants, and improves control of the WFM recess before metal fill and dielectric deposition.

As you can see from the above, there was quite a bit of detail in this presentation, which can be summarized in the process sequence shown [5]:

Strangely, despite the tighter pitches when compared with the TSMC SRAM bitcell, the size is the same, ~0.27 µm², though we don’t know if this is a 1:1:1 cell or not – if (say) a 1:2:1 configuration is used, that would add at least ~0.052 µm² to the cell size, assuming the 48 nm CPP. (The paper does not state bitcell size, but in the Q & A’s, we were told that it was 50% of the 10-nm bitcell, which was quoted at ~0.53 µm².) The Q & A’s also mentioned that there were three flavours of Vt, and high-Vt I/O transistors were not studied, reinforcing the research nature of the paper.

Samsung 5 nm

Later in the conference (paper 28.1), Samsung presented a “co-integration scheme for 5nm logic” [13] which clearly drew on the 7-nm work from Albany detailed above, and illuminating some more development problems that that must have been seen in Albany.

A SRB substrate is used with a SiGe fin for PMOS, and a common interlayer, high-k, and work-function materials. The SiGe fin, combined with e-SiGe source/drains, gives an estimated 1 GPa compressive stress, and the SRB applies similar tensile stress to the NMOS channel. As with the Albany process, the Ge concentration increases as we go from SRB to fin to e-SiGe source/drains.

Defect density from the SRB was definitely a concern, and was reduced to 5e4/cm², and then demonstrated by SRAM that leakage levels are comparable with those of a reference SRAM structure on bulk Si. My notes say that a thicker SRB was used, but no actual thicknesses were mentioned.

Another problem was migration of the Ge to the surface of the SiGe fin (shifting Vt and degrading interface state density), because of later thermal processing, as shown in this LEAP (laser enhanced atom probe) image:

Careful optimization of the thermal sequencing reduced this to about a 4% variation. Since the STI penetrates into the buffer layer, and we have a SiGe fin, a new STI formation process had to be developed, to reduce any side-effects from oxidation.

More details were given of the stress development; the presenter showed that the strain was uniaxially transferred to the fin, and also that the source/drain recess etch relaxed the channel stress – in the PMOS device the e-SiGe epi restored the stress, but for NMOS a non-recessed S/D was used.

When it comes to the electrical results, long- and short-channel plots were shown, but with no numbers for either device size or measurements, so we have to trust that they actually fit 5-nm node dimensions, or at least are for smaller pitches than the 7-nm papers detailed. However, as an integration scheme it is interesting, as gives us some clues as to what we might see from Samsung and GLOBALFOUNDRIES as we go from 10 – 7 – 5 nm.

Given the lack of detail in TSMC’s presentation, we don’t know what their novel contact and strain engineering and process knobs are – could they be contact epi and SRB strain? I guess we’ll see in a couple of years or so.

References

• S-Y Wu, et al., “A 7 nm CMOS Platform Technology Featuring 4th Generation FinFET Transistors with a 0.027 um² High Density 6-T SRAM cell for Mobile SoC Applications”, IEDM 2016, pp. 43 – 46
• S-Y Wu, et al., “Demonstration of a sub-0.03 um2 High Density 6-T SRAM with Scaled Bulk FinFETs for Mobile SOC Applications Beyond 10nm Node”, VLSI 2016, pp 92 – 93
• S-Y Wu, et al., “An Enhanced 16nm CMOS Technology Featuring 2nd Generation FinFET Transistors and Advanced Cu/low-k Interconnect for Low Power and High Performance Applications”, IEDM 2014, pp. 48 – 51
• Karl, et al., “A 0.6 V, 1.5 GHz 84 Mb SRAM in 14 nm FinFET CMOS Technology With Capacitive Charge-Sharing Write Assist Circuitry”, IEEE JSSC, VOL. 51, NO. 1, (Jan 2016), pp. 222 – 228
• Xie et al., “A 7nm FinFET Technology Featuring EUV Patterning and Dual Strained High Mobility Channels”, IEDM2016, pp. 47 – 50
• Welser et al., “NMOS and PMOS Transistors Fabricated in Strained Silicon-Relaxed Silicon-Germanium Structures”, IEDM 1992, pp. 1000 – 1002
• Welser et al., “Strain Dependence of the Performance Enhancement in Strained-Si n-MOSFETs”, IEDM 1994, pp. 373 – 376
• Rim, et al., “Enhanced Hole Mobilities in Surface-channel Strained-Si p-MOSFETs” IEDM 1995, pp. 517 – 520
• Rim, et al., “Fabrication and Mobility Characteristics of Ultra-thin Strained Si Directly on Insulator (SSDOI) MOSFETs”, IEDM 2003
• IEDM 2016 Short Course, “Technology Options at the 5 Nanometer Node”, session 3, N. Collaert, “Novel channel materials for high-performance and low-power CMOS”, sl. 17
• Guo et al., “FINFET Technology Featuring High Mobility SiGe Channel for 10nm and Beyond”, VLSI 2016, pp. 14 – 15
• Standaert, et. al., “BEOL Process Integration for the 7 nm Technology Node”, IITC/AMC, 2016, pp. 2 – 4
• D, Bae et al., “A novel tensile Si (n) and compressive SiGe (p) dual-channel CMOS FinFET co-integration scheme for 5nm logic applications and beyond”, IEDM 2016, pp. 683 – 686