IEDM 2011: IBM displays via-middle TSV process for die stacking

A few days after IBM and Micron publicized their hybrid memory cube, IBM gave their TSV paper at IEDM on the Monday afternoon (paper 7.1).

Entitled "3D Copper TSV Integration, Testing and Reliability," they described a node-agnostic through-silicon via (TSV) technology which takes a via-middle configuration, making contact to the upper metal (fat-wire) layers in the device structure. By "node-agnostic" they mean that they proved the concept in devices fabbed on processes ranging from 90nm down to the 32nm HKMG process. In doing so, the TSVs have anything from three to nine metal layers below the contact level, and have to cope with dielectric k-values from 4.1 down to 2.4, and bulk and SOI wafers.

The paper doesn't specifically say so, but it appears that the TSVs are annular. Once the lower metal/dielectric stack is formed (including the via dielectric for the metal layer that contacts the TSVs), the TSVs are drilled through to the silicon, and then a Bosch etch is used to drill the vias about 100μm into the substrate, with a minimum pitch of 50μm.

After drilling, a conformal oxide is deposited, the barrier and seed layers are sputtered in, the copper fill is plated in, and any excess copper is CMP'd off. The dielectric for the contact level metal is put down, and then the top fat-wire metal levels are conventionally defined.

Fig.1: SEM cross-section image of annular TSV integrated into M10 level in 45nm technology.


Fig. 1 above shows the TSV contacting the metal 10 level in a 12-metal part. If we guesstimate that M10 is ~1μm thick, that gives us a TSV diameter of about 15μm (which agrees with a verbal comment at the presentation), and the annular copper ring is 4-5μm thick.

Fig.2: Cross-section of TSV bottom, showing Bosch-etch striations and fully-filled via.


It appears by the time M3 is complete, there's about 250μm of bow in the wafer, which continues and will likely get worse by the end of wafer fab. That makes it difficult to bond the wafer flat for thinning, so at the via-2 and via-3 levels compressive oxide is used which pulls the wafer flat again -- see Fig. 3 below.

Fig.3: Wafer bow vs. metal step and change with high-stress oxide at via-2 and via-3 levels.


That will mean that if TSVs are to be used there will have to be a discrete process module within the BEOL, and also the M2/M3 levels will have to be laid out to compensate for dense oxide rather than low-k in processes at 90nm nodes and below. Another cost adder for TSVs!

The completed wafers were bonded to glass handle wafers and thinned to expose the copper at the bottom of the TSVs, after which a protective oxide/nitride was deposited and patterned before forming and defining a copper redistribution layer (RDL). Lead-free C4 solder balls were then put on the the RDL, and the thinned TSV wafer was ready for joining to another die or substrate. After dicing the dies can be bonded with the RDL side on a package substrate with the device side face-to-face with another die, or face down with another die flip-chipped on to the RDL.

Fig. 4 shows a module in which a thin TSV wafer has been packaged RDL-down and another full-thickness die face-to-face with the thinned die.

Fig.4: Module containing thinned TSV die stacked face-to-face with full thickness die.


Test devices were subjected to considerable thermal and reliability testing without adverse effects. The TSV etch process was shown to affect nearby PFETs under certain conditions, but was optimized to solve the problem. The stress associated with the vias was not significant and not expected to create any mobility effects in nearby transistors.

As another test of the TSV process, a 32nm SOI 3D embedded memory module was fabricated with a 128Mb DRAM stacked on top of a 96Mb general purpose DRAM; both with 0.039m² eDRAM cells in high-k/metal gate technology. The memories were tested with no performance or retention degradations observed.

I've no idea when we'll see a real memory cube in production; no dates were given in the announcement, but hopefully sometime next year. Whether we'll be able to get hold of one is another matter!

Don't forget I'm a Twit these days -- follow chipworksdick for new blogs and industry news that catches my attention.

IEDM 2011 Preview

Next week the researchers and practitioners of the electron device world will be gathering in Washington D.C. for the 2011 IEEE International Electron Devices Meeting. To quote the conference web front page, “IEDM is the world’s pre-eminent forum for reporting technological breakthroughs in the areas of semiconductor and electronic device technology, design, manufacturing, physics, and modeling. IEDM is the flagship conference for nanometer-scale CMOS transistor technology, advanced memory, displays, sensors, MEMS devices, novel quantum and nano-scale devices and phenomenology, optoelectronics, devices for power and energy harvesting, high-speed devices, as well as process technology and device modeling and simulation. The conference scope not only encompasses devices in silicon, compound and organic semiconductors, but also in emerging material systems.”

From my perspective at Chipworks, focused on chips that have made it to production, it’s the conference where companies strut their technology, and post some of the research that may make it into real product in the next few years.

In the last few days I’ve gone through the advance program, and here’s my pick of what I want to try and get to, in more or less chronological order. As usual there are overlapping sessions with interesting papers in parallel slots, but we’ll take the decision as to which to attend on the conference floor.

For the first time the conference starts on the Saturday afternoon, with a set of six 90-minute tutorials on a range of leading-edge topics:

• Microresonator Filters and Oscillators: Technology and Applications, Roy H. Olsson III, Sandia
• Graphene Nanoelectronics, Walter De Heer, Georgia Tech
• Modeling and Characterization of Noise in Advanced CMOS, Andries Scholten, NXP
• Technology CAD for Modeling and Design of Bio-Devices, Yang Liu and Robert Dutton, Stanford University
• Kinetic Energy Harvesting - Technologies and Applications, Tomasz Zawada, Meggitt
• IGBT and Superjunction - Leading Power Device Technologies, Florin Udrea, University of Cambridge  

The first three are from 2.45 – 4.15, and the remainder from 4.30 – 6.00. I won’t make it to any of them; dedicated nerd I may be, but I want at least some of my weekend!

On Sunday December 4th, we start with the short courses, “VLSI Technology Beyond 14nm Node” and “Advanced Memory Technology”. Philip Wong of Stanford of has organised the former, and we have some impressive speakers – Jeff Sleight, (IBM – Nanowires), Shinichi Takagi, (U Tokyo – High Mobility Materials), Alan Seabaugh, (U Notre Dame – Tunnel FETs), Ian Young, (Intel – MOSFET extrinsic R-C parasitics), and long-time attendee Bill Arnold (ASML – Lithography).

As I said last year, having started in the business on 10-micron geometries, 14-nm devices seem crazy to me, but on the Intel clock it’s only two - three years away! I’m starting to tell folks to think about the end of silicon, at least as we know it, since my brain will not wrap around the idea of 11- and 8-nm gates, and 11-nm is only five years away (and 30 – 40 atoms across, depending on orientation!). The guys in the R&D labs have been thinking about that for the last decade or more (as we’ve seen at IEDM), so this should be an interesting day to see what they’ve come up with and how we get there.

Roberto Bez of Micron has set up the other short course; now that we are getting into the 1x-nm flash (literally counting electrons) and 2x-nm DRAM eras, conventional scaling has probably reached its limits, and other storage mechanisms have to come into play. S.Y. Cha of Hynix kicks off the sessions with a review of DRAM technology, followed by Y.J. Choi (Adata) on NAND flash. The we get into the alternative mechanisms with Rainer Waser (RWTH-Aachen) on redox-based ReRAM, Janice Nickel (HP – Memristors), Andrea Lacaita (Politecnico Milano – Phase Change Memory), and Bill Gallagher from IBM finishing up on magnetic memories.

So some good solid stuff – although the courses make a long Sunday, from 9 a.m. to 5.30 p.m., but it’s worth sticking around to the end.

Monday morning we have the plenary session, with three pertinent talks on the challenges of contemporary electronics:

• Approach Towards Achieving Sustainable Mobility by Takumi Matsumoto, Toyota Inc. – given the trend towards electric vehicles, and Toyota’s lead in that arena, this should be illuminating;
   
• Perspective on The Past, Present and Future of Transistors by Mark Bohr, Intel – the big question here, of course, is will Mark give us any clues on the upcoming 22-nm trigate product? I’m inclined to doubt it – we’ll have to wait and see!
   
• Various Technologies of MRAM and Logic-in-Memory Architecture Based on Hybrid CMOS/Magnetic Technology by Bernard Dieny, CEA – another perspective on where memory is heading. 

After lunch we have seven parallel sessions coming up! Session 2 gets straight into the way-ahead material with papers on graphene and nano-optical devices, although we seem to be moving away from R towards D in the R&D spectrum; for example, paper 2.2 from IBM builds graphene-based RFICs on 200 –mm wafers. Jim Montgomery at ElectroIQ has posted more details here.

Session 3 details phase-change and resistive memories, with papers on a 20-nm PRAM by Samsung (paper 3.1), and a 4F-sq cell 1-Gb PCRAM by Hynix (3.3). Session 4 provides the first of the inter-session clashes, with a paper (4.1) on 14-nm finFET circuit/device interactions from GLOBALFOUNDRIES.

In session 5, Asen Asenov’s device modelling group at the University of Glasgow has a paper on variability and reliability in finFETS (5.4), and IBM (5.5) discusses transistor matching and SOI thickness variation in extra-thin SOI (ETSOI).

There are a couple of interesting analytical papers in session 6; scanning spreading resistance measurement on a finFET by IMEC (6.1), and electron holography mapping of an under-30-nm MOS transistor. IBM starts session 7 with TSVs integrated with what looks like a 32-nm HKMG/e-DRAM process (7.1), and Renesas reports BEOL transistors using an InGaZnO channel integrated into the copper interconnect (7.4) – a different form of 3D!

Session 8 focuses on image sensors, and seeing that we do a lot of CMOS image sensor (CIS) reports, we won’t want to miss those – in particular papers from Sony (8.1), TSMC/Omnivision (8.2) and Panasonic (8.3). Sony discusses a flat (no STI in the array), 1.12-µm pixel CIS, possibly the one we’ve seen in the Apple iPhone 4S. Omnivision, who have had a lot of wins with Apple (but not the 4S!), talk about a 0.9-µm pixel back-illuminated CIS, and Panasonic’s paper is on a front-illuminated sensor fabbed in a 45-nm process using light pipes, with claimed performance better than back-illuminated sensors.

Tuesday morning we start with session 9 on flash memories: Hynix (9.1) discusses their “middle 1x-nm” (15 nm?) flash, now with air gaps similar to Micron's paper last year; Macronix studies a thin (less than10 nm thick) floating gate at less than 20 nm (9.2), and Infineon reviews the use of embedded flash in uses such as automotive microcontrollers and smart cards (9.4).

Energy harvesting and ultra-low power is the topic of session 10, with a set of invited papers on thermoelectric, MEMS, and RF energy harvesting , and the types of device that could use such low energy supplies.

The other four morning sessions are predominantly academic; graphene and ultimate device modelling (S11), memory reliability (S12), InGaAs FETs (S13), and thin film technology (S14).

The speaker at the conference lunch will be Masaaki Tsuruta of Sony Entertainment, so we will likely see what’s coming up in the gaming experience - whole body sensors?

Session 15 in the afternoon is nominally about circuit/device interaction, but there are a couple of Samsung papers describing processes; paper 15.1 discusses their 20LP process, but the abstract does not say whether it’s gate-first or gate-last – it should be gate-last, but we’ll see! 15.6 talks about process improvements that enhance scalability of the 28LP gate-first HKMG process. There are two invited papers, by ARM (15.4) and STMicroelectronics (15.7), that get more into circuit/device interaction, ARM discussing scaling problems from the design perspective, and ST presenting process-design co-optimization at the 28-nm node, including TSV integration.

Sessions 16 – 19 are also at the academic end of the spectrum, with papers on Simulation of Memory Devices (S17), HKMG reliability (S18), and GaN devices (S19).

Session 20 gets into RF MEMS and resonators, starting with an invited paper from IDT (20.1) on MEMS oscillators (looks like they’re making MEMS as well, now). Nitronex, known for their GaN-on-Si RF transistors, are detailing a ~800 MHz MEMS resonator (20.3).

At the end of the afternoon Applied Materials is hosting a panel on “How will RAM Change for the Mobile Computing Era?” for a couple of hours. The panelists will be:

          Dr. Narbeh Derhacobian - president and CEO, Adesto Technologies, Corp.
          Dr. Gyoyoung Jin - senior vice president, Samsung Microelectronics, Ltd.
          Dr. Jae-Sung Roh - research fellow, Hynix Semiconductor, Inc.
          Dr. Gurtej Sandhu - senior fellow, Micron Technology, Inc.
          Dr. Klaus Schuegraf - chief technology officer, Applied Materials, Inc.
          Dr. Geoffrey Yeap - vice president of technology, Qualcomm, Inc.

Get registered here.

In the evening we have the conference panel sessions, “Is 3 Dimensional Integration at Best a Niche Play?”, and "Will SiC or GaN Replace Si as the Semiconductor for Power Devices?" – do panels always have to ask questions to get us interested?

In a way the three panels reflects the diversity of the business now – we tend to forget the huge range of applications these days, from the latest logic and memory nanotechnology in our smartphones, to the demands of bringing high-voltage DC power in from offshore wind-farms. I shall probably bounce back and forth between the two conference panels, they again have good speakers, and the 3D one has Jan Vardaman to keep them grounded – I don’t think she’s seen real 3D yet any more than we have at Chipworks.

Wednesday morning has sessions 23 – 29; S23 on carbon nano-tubes and Si-nanowire devices, which inevitably takes the academic track; S24 covers RAM and specialty memories, which is also a touch academic, but Samsung (24.1) discusses vertical spin-transfer torque MRAM, something we probably have to watch for in the not-too-distant future. IBM has two consecutive papers on their “racetrack memory” (24.2 and 24.3), and TSMC finishes up the session with a HKMG embedded DRAM in 28-nm technology using MIM capacitors (24.7).

Low voltage design and device variability is covered off in S25 – the “Albany consortium” (GLOBALFOUNDRIES, IBM, Infineon, Renesas, Samsung, STMicroelectrics, Toshiba, in alphabetical order) has a potentially interesting paper (25.6) on a layout dependency effect in HKMG which could read on the AMD Llano chip analysis we did a few weeks ago.

Now we get into scheduling clashes with S26 – TI has a stacked-die NexFET power module (26.1) which should be interesting, and Panasonic has two papers, the first (26.2) on a low-leakage GaN-based multi-junction diode, and the second (26.6) describes a SiC MOSFET which somehow uses the channel as a diode current path. Rohm also has an invited paper on SiC trench transistors (26.5).

Session 27 deals with MOSFET reliability, specifically bias-temperature instability, mobility and noise, and is a bit beyond me; however, S28 has some meat in it for advanced CMOS geeks.

In paper 28.1 the GLOBALFOUNDRIES/IBM, Infineon/Samsung/Toshiba group discusses their dual-channel (Si/SiGe) HKMG gate-first technology that we saw a version of in the AMD Llano. Later on IBM/GloFo describe an atomic layer oxidation technique for a gate-last process (28.5). Sony, Panasonic and Fujitsu in conjunction with IMEC also have a dual-channel HKMG paper (28.6), but this time in both gate-first and gate-last versions. There is also a review paper by UTokyo on germanium CMOS (28.4).

I shall be torn between paper 28.1 above, and Benedetto Vigna’s invited paper on MEMS sensors (29.1) at the same time. Benedetto has shepherded STMicroelectronics into the leading position in consumer MEMS in the last five years or so, so it’s likely that whatever phone or game system you have will have an ST motion sensor in it.

Lunchtime Wednesday, ASM will be holding a seminar across the road at the Churchill Hotel, with Ivo Raaijmakers hosting; and I have the privilege of speaking on “High-k/Metal Gate in Leading-Edge Silicon Devices”. To register, email Roseanne de Vries at rosanne.de.vries@asm.com.

The pace continues in the afternoon with session 30 on nano device technology, session 31 on resistive RAM, S32 on advanced SRAM, S33 on III – V FETs, S34 on simulation, S35 on high mobility, and S36 on biosensing and solar conversion.

Papers of interest for me in the RRAM session are 31.2 on HfO-based RRAM, bipolar ReRAM by Panasonic (31.4), a WOx RRAM by Macronix (31.5), and a vertical RRAM by Samsung (31.8).

Session 32 has one of the seven papers by Intel at this IEDM (32.1), on low-voltage SRAM operation (but no paper on 22-nm trigate transistors!). Suvolta and Fujitsu may come out of stealth mode and show details of their channel engineering and associated VDD reduction in paper 32.3.

Another Intel paper co-authored with substrate maker IQE (33.1) looks farther ahead at hi-k tri-gate InGaAs quantum-well FETs, and the same groups discuss a tunneling FET (33.6) in the last paper of the session.

In session 34, IMEC and Panasonic have a SiGe channel paper (34.3), and, as noted in my last blog, Intel sheds some light on their NMOS stress mechanism seen in their 32-nm process (34.4) – or at least, that’s my speculation!

Session 35 starts with another Intel/IQE paper, this time comparing MOVPE and MBE III-V quantum-well FETs on Si substrates (35.1). The Sony/Panasonic/Fujitsu/IMEC group have another paper (35.4), detailing a Ge-Sb-Te liner for putting compressive stress on PMOS finFETS. SEMATECH and CNSE discuss an ultra-shallow doping technique for finFETs (35.5), and IMEC/Panasonic/Ultimate Junction Tech. also talk finFET doping (35.6), this time specific to NMOS.

The last paper of the session (35.7) has NU Singapore and Varian describing what looks like a fairly complex way of tuning nickel silicide contact resistance by implanting aluminum, and locking into place with carbon. They claim an 18% drive current improvement for nFETs; so complex it may be, but it’s another way of pushing up performance, and processes are plenty complex already!

The conference finishes about 5 pm – by then a lot of attendees will be heading for home, and I’m usually thankful when the last paper’s done. As always, no peace for the curious!

Last year the SOI Industry Consortium held a workshop starting at 5 p.m. after the conference proper was finished, with some notable speakers from academe and industry. I haven’t heard anything of a similar event this year, but I will post an update to the blog with the details if I do.

On a completely different note, I have grudgingly joined the Twitterverse, so any Twitterati can follow me at @chipworksdick, I will be trying to post interesting industry tidbits. I can’t say I’ve ever been particularly good at one-liners, but we’ll see!

Intel clarifies 32nm NMOS stress mechanism at IEDM 2011

I was browsing through the advance program for the upcoming IEDM conference when, almost at the end, I came across paper number 34.4, "Modeling of NMOS Performance Gains from Edge Dislocation Stress," by Weber et al. of Intel. According to the abstract: "Simulations show stress from edge dislocations introduced by solid phase epitaxial regrowth increases as gate pitch is scaled, reaching over 1GPa. This makes edge dislocations attractive, as stress from epitaxial and deposited film stressors reduces as pitch is scaled. We show dislocation stress varies with layout and topography."

The abstract doesn't have much detail, but it does re-enforce the teachings from a Samsung paper at last year's IEDM conference, paper 10.1, "Novel Stress-Memorization-Technology (SMT) for High Electron Mobility Enhancement of Gate Last High-k/Metal Gate Devices" (Lim et al.).

The essence of this paper is that if you give the source/drains a deep amorphization implant, and then anneal to create solid-phase epitaxial re-growth with a tensile stress liner in place, then crystalline dislocations are formed adjacent to the gate edge, which apply tensile stress to the channel.

Source: IEDM/Samsung

Like the embedded SiGe stress for PMOS, this works better with a gate-last process, since the surface is not locked by a polysilicon gate. Samsung claimed ~1% lattice distortion, verified by nano-beam diffraction measurements. A vertical slice was taken below the center of the gate and the color coding shows strain of ~1%:

Source: IEDM/Samsung

When I saw this paper it made me wonder if this mechanism was what we had been seeing in the Intel 32nm parts; none of the earlier stress mechanisms seemed to being used. Intel were the first to apply stress to transistor channels at the 90nm node, using (for NMOS) the contact etch-stop layer (CESL) silicon nitride; and then at the 45nm node they evolved to using the contact plug itself and the gate-fill metal, since the CESL is almost gone.

But in the 32nm process the contact plugs have been polished away, and there is less gate metal (since it's a smaller gate) -- so what is supplying Intel's fourth-generation strain?

In the light of these two papers we can now take a good guess when we see what Intel's 32nm NMOS transistor looks like:

And as we can see, there are stacking faults on both sides of the gate, and they look similar to the ones in the image from the Samsung paper:

Source: IEDM/Samsung

Samsung claimed an increase in electron mobility of 40%-60% and drive current improvement of over 10%.

Stacking faults are not normally what we want to see in transistors, because they can be leaky if they go through a junction, but as long as they are contained within the source/drain diffusions, they should not be a problem. They are certainly in every NMOS transistor that we imaged (though given the billions of transistors in the millions of processors shipped, we cannot exactly claim a large sample).

At an intuitive level it makes sense that this mechanism should work -- a stacking fault is a missing layer of atoms within the crystalline lattice, and we are now working with channel lengths of a hundred atomic spacings or less. So if a couple of atomic layers are missing at opposing ends of the channel, it seems logical that tensile stress would be induced in the channel.

Like the other stress techniques, this only works now that we are down in the nanometer range, but the good thing about this one is that the applied strain should increase as the channel length gets shorter.

So it seems that we have finally deduced at least some of what Intel are doing in their 32nm NMOS transistors. Now, of course, the question will be -- can it be transferred to the trigate structures we're looking forward to in the 22nm process?

Looking forward to IEDM, in addition to the conference program, ASM will be holding a lunchtime seminar on the Wednesday, Dec. 7th, at 12 noon, with Ivo Raaijmakers hosting; and I have the privilege of speaking on "High-k/Metal Gate in Leading-Edge Silicon Devices." To register, email Roseanne de Vries at rosanne.de.vries@asm.com. We hope to see you there!

GlobalFoundries takes a different approach to HKMG in AMD’s Llano CPU/GPU

After much anticipation, and with quite a few design wins, AMD's Llano CPU/GPU chip arrived on the scene a couple of months ago. Fabricated by GlobalFoundries (more easily known as GloFo) in their 32nm SHP process, it was the first foundry-based gate-first HKMG product to come on the market.

As a processor, it garnered pretty favorable reviews, but of course we were keen to get it into the lab and see how it had been put together. When we did, it became a bit of a mystery -- we couldn't see any significant differences in gate stack between NMOS and PMOS! It's common wisdom that you need different work function materials in the NMOS and PMOS gates to differentiate them and make up the CMOS circuitry.

For example, Panasonic uses lanthanum to tweak the work function of their NMOS transistor and distinguish it from the PMOS stack in their HKMG Uniphier chip that we looked at back in the spring.

Fig. 1  Panasonic 32-nm HKMG Transistor

As we can see in Fig. 1 above, the gate metal is titanium nitride under the polysilicon, and the hafnium-based high-k layer is below that, over the interface oxide. There was no apparent physical difference between NMOS and PMOS until we start looking in detail, and then we found just a tickle of lanthanum in the NMOS stack, but presumably enough to move the work function into the NMOS regime.

When we look at the Llano, it also uses a gate-first transistor style, with TiN as the gate metal, but there the resemblance stops. Below (Fig. 2) is a composite image of the Llano NMOS/PMOS transistors, and you can see that they are more complex.

Fig. 2  AMD/GloFo 32-nm HKMG NMOS and PMOS Transistors

Dual-stress liners are used to add tensile and compressive stress; we can see from the above that the PMOS (compressive) nitride is twice as thick as the NMOS (tensile) layer. The PMOS device also has embedded SiGe in the source/drains to add more compressive stress, whilst there is possible evidence of stress memorization (SMT) for NMOS. And if we look carefully, the PMOS SOI layer is also a little thicker than the NMOS SOI.

The NMOS and PMOS gate stacks shown in Fig. 3 appear to be the same -- highly silicided poly on a thin AlO barrier layer, on TiN gate metal, which is on the Hf-based hi-k layer with a SiO interfacial layer on the substrate. The AlO layer in the PMOS stack is more diffuse, and some of the aluminum has migrated into the TiN, and arsenic is present as expected in the NMOS, but essentially they are the same.

Fig. 3  AMD/GloFo Transistor Gate Stacks

So now we have a bit of a mystery; how are the NMOS and PMOS transistors differentiated? We looked long and hard in both NMOS and PMOS for a dopant such as the lanthanum used by Panasonic, something other than hafnium, silicon, or titanium, but if it's there's, it's below the detection limits. Aluminum is known as a dopant for PMOS, but to be effective it has to be present at the Hf/SiO interface to create Vt-shifting electrical dipoles, and we see no evidence of migration that far.

The extra thickness in the SOI is the clue to what we think is going on in this part. The extra thickness is actually a layer of epitaxial SiGe, which changes the relationship with the gate metal and shifts the Vt, instead of using a dopant in the hi-k. Some work was done on this topic at SEMATECH a few years ago [1], and of course AMD and IBM were members and would have received the results.

The schematic in Fig 4 shows conceptually what happens; the valence band of the substrate is shifted because of the Ge, and also due to the compressive strain applied by the embedded SiGe source/drain and the nitride stress layer.

Fig. 4  Schematic of Band Diagram for Transistor with SiGe Channel [1]

Fig. 5 illustrates the drive current improvement for a >10% SiGe channel in the SEMATECH device, which will also include the effect of the inherent improved hole mobility in the SiGe.

Fig. 5  Drive Current Improvement in SiGe-Channel Device

That accounts for the PMOS; the NMOS was still a bit of a mystery, since one would still expect a dopant at the hi-k/oxide interface, and we see none. All we see is TiN, and Intel uses that as their PMOS work-function metal, which which on the face of it  doesn't make sense. However, more SEMATECH [2, 3] work indicates that the work function of TiN can be manipulated by adjusting the growth conditions and thickness, enough to shift it from the NMOS to the PMOS regime.

In fact, SEMATECH's ESSDERC paper from 2005 [3] agrees nicely with what we see in the AMD and Intel parts. The Llano has a ~2nm TiN layer in the NMOS, whereas Intel uses ~2nm layer plus a 1nm Ta-based cap and another ~4nm TiN on top of that in their PMOS. Fig. 6 indicates that this extra material could be enough to move the work function in Intel's transistor from NMOS to PMOS.

Fig. 6   Effective Work Function of TiN electrode when 10-nm thick ALD TiN and TaN Films are Used as Overlayers on ~3.6 nm TiN Layer [3]

We actually had a clue a couple of years ago, if we had known what we are looking at. In a CICC paper [4] GlobalFoundries showed an image (Fig. 7) of a transistor that looks as though it had a SiGe channel -- but of course they didn't say so!

Fig. 7  Experimental  GLOBALFOUNDRIES Transistors [4]

Of course all of the above is pure speculation, but if the literature is correct it, does hang together and account for the difference between this latest HKMG product and the others we have seen. Now, will IBM, Samsung, and the other alliance members do the same thing?

References


1. H.R. Harris et al., Band-Engineered Low PMOS VT with High-K-Metal Gates Featured in a Dual Channel CMOS Integration Scheme, Symp. VLSI Technology 2007, pp 154-155

2. K Choi et al., Growth Mechanism of ALD-TiN and the Thickness Dependence of Work Function, Symp. VLSI Technology 2005, pp 103-104

3. K. Choi et al., The Effect of Metal Thickness, Overlayer and High-k Surface Treatment on the Effective Work Function of Metal Electrode, ESSDERC 2005, pp 101-104

4. S. Krishnan et al., Advanced SOI CMOS Transistor Technologies for High-Performance Microprocessor Applications, CICC 2009

Intel Enlarges Process Lead over Their Competition

22-nm Trigate Transistors Discussed

At a morning session at the Intel Developer Forum Tuesday, Mark Bohr tooted the Intel trumpet and put a slide up to emphasise their lead over the other leading semiconductor companies:

Intel Process Evolution Since 90-nm

One can quibble a bit about the odd month here or there for the dates, but essentially things have been as they say -- they were the first with embedded SiGe for PMOS strain, they were a node ahead of everyone else at HKMG, and if the trigate launch comes to pass as planned at the end of this year, they will be years ahead with their version of the FinFET.

The main focus of the talk was Intel's upcoming 22-nm trigate transistor technology to be used for the Ivy Bridge processors due out in the New Year. Essentially it was a re-run of the May announcement, with a little more about the SoC version and a look forward to 14-nm in (presumably) 2013.

Intel Schematic of Trigate Transistor in Inversion

Transistor Delay vs Voltage (pale grey line is planar 22-nm)
Source: Intel

Mark said that they made the choice for trigate back in 2008, when it became clear that the performance benefit from the fully depleted triple-gate structure (compared to 22-nm planar) was significant enough to justify the additional effort and cost of another step-function change in process architecture.

Compared with the 32-nm equivalent, the trigate gives a 37% performance increase at a lower voltage or a 50% power reduction at constant performance. Somehow Intel does this with no extra mask levels and only 2-3% additional cost (although extra litho steps are used, because of the need for double patterning).

Of course, I was keen to hear when we'll be able to get hold of some of these chips, after all they're going to be fascinating to take apart!. According to Mark, they are "just about ready to start production" in Q4, with public availability in the first half of next year. They are definitely sampling, since Ivy Bridge Ultrabooks are on show here. The strict two-year clock appears to have slipped slightly, since previous launches have been in November; but we quibble, since Intel has a clock -- their competitors make an announcement, and then we wait!

Which brings us to the roadmap; as you can see in the first graphic above, 14 nm is predicted in 4Q13 (which is itself a subtle change, since it was 15-nm a couple of years ago -- Intel seems to be aligning itself with the other companies which have gone the 28 -- 20 -- 14 nm route).

Intel is also continuing the parallel development of SoC processes down to 14 nm:

New Process Roadmap  Source: Intel

Talking to the guys on the floor here, Cedar Trail (32-nm SoC) netbooks and mini-desktops will be out for the Christmas market, and I gather the intent is to reduce the gap between the CPU and SoC processes to a year or so from the current two -- three.

Given the extension to 14 nm, Intel must have already verified that the transistor-related SoC features (low leakage and high-voltage transistors, and the different varieties of SRAM) work with trigates, the rest are all back-end related so should just suffer the normal scaling problems.

Unfortunately it appears that there will not be a paper on the 22-nm process at IEDM this year, so we will have to wait for Ivy Bridge chips to come on to the shelves to get a few more clues -- it should be an interesting spring!

A SEMICON West snippet: AMAT launches new products, prepares for 450mm

SEMICON West is usually taken as a barometer for the industry, and my subjective impression is steaming along nicely, but no record breaking years coming up! According to Tom Morrow of SEMI, this year's preregistrations were flat, but there about 10% more booths than last year.

I kicked off the show by sitting in at the Applied Materials (AMAT) press and analysts breakfast. As usual AMAT had a flurry of press releases preceding the show, and eight new products and product updates are being launched. A couple of years ago AMAT was putting more emphasis on their solar and display divisions, but this year silicon processing is again getting a high profile.

We had a series of presentations from Mike Splinter, Randhir Thakur, Steve Ghanayem, and Bill McClintock, and then Q-and-A from the analysts present.

Mike S. did the corporate overview: he saw the industry outlook as soft in the short term, but was basically upbeat since the industry drivers are still there -- Moore's law scaling, 3D transistors (in logic, flash and DRAM), and pushing them all, the mobile revolution. On the solar side, he predicted that solar modules will cross the $1/Watt threshold sometime this year, and hit $0.80/W next year, so cost reductions will help drive that end of the business.

Randhir Thakur then reviewed the product launches at the show, putting them into the context of the recent and upcoming changes in chip processing. Rather than list the new products, here's the slide:

Steve Ghanayem focused on the Centura gate stack tool -- essentially an ALD chamber has been added into the Centura system to give it high-k capability, all within vacuum:

He put a lot of emphasis on the cluster nature of the tool, so that the wafers only see vacuum between the process steps, claiming that exposure to atmosphere reduces mobility and increases threshold voltage spread.

The last technical presentation (Bill McClintock) covered off the new Black Diamond 3 (BD3) and Nanocure 3 extreme low-k dielectric and curing combination, giving a dielectric constant (k) of 2.2, down from k=2.5 in the previous generation. One of the things he pointed out (that I hadn't thought about) was that the pre-metal dielectric layer at the bottom of the metal stack has to survive more than 150 process steps before wafer out in today's 10-12 metal-layer processes, never mind the stresses of the packaging and assembly sequence.

So the challenges are formidable as the k-value is pushed down, to get both physical and material integrity; AMAT claims that by going to a closed-pore structure, with tighter pore size distribution, they can achieve k=2.2.

According to Bill, we can expect to see BD3 at the 22/15 nm nodes, so a couple of years yet before we see it in high-volume products.

Then we got to the Q-and-A session. Ironically, the first question was not about any of the product launches -- it was about the spend on 450mm next year! Mike Splinter was reluctant to give a specific number, but he did say it would be "well over $100 million," mostly on early test systems in-house. Not exactly small change, all the same. A later question prompted the statements that "450 is going to happen," and that they are closely linked to the leading customers that will drive the move there. They are clearly now viewing 450mm as a strategic way of gaining market share when it does come.

Other questions covered off potential product expansion, and of course the future demand from foundries in what seems to be a softening market.

Randhir Thakur identified AMAT's flowable CVD, Siconi clean and the Raider copper deposition tools as having found more applications than originally intended. The flowable CVD was targeted on one application, but ended up replacing CVD fill for STI, and other CVD steps with high conformality requirements. Siconi clean has evolved from a PVD clean, but has now moved into CVD and epi areas, any area where interfaces are critical. The Raider copper tool was developed from a Semitool product for packaging, but now has potential for damascene copper on die.

When it comes to the foundries, it appears that the fab shells are ready, and the message for the equipment companies is to be ready -- things may be soft at the moment, but they could come back very quickly. Demand is controlled by the consumer market, and that has proved remarkably resilient considering some of the economic challenges in the last year or so.

All in all, an interesting session, both in the industry and technical senses. AMAT has the webcasts and presentations up on their investor website for until August 12, 2011.

TSMC HKMG is Out There!

I have to apologise for a hiatus in posting due to pressure from the day job, but this week is Semicon West week, so it seems appropriate to announce that we've started analysing TSMC's 28-nm gate-last HKMG product, in this case a Xilinx Kintex-7 FPGA, fabbed in TSMC's HPL process.

Having seen two generations of Intel's HKMG parts (the 45-nm Xeon and 32-nm Westmere) using gate-last technology, it's inevitable that we'll compare those with the TSMC process.

The Kintex family is the mid-range group in the latest 28-nm generation 7-series of FPGAs from the company. These are optimised for the highest price/performance benefit, giving the performance of the previous Virtex-6 parts at half the price.

The Kintex-7 has eleven layers of metal (Fig. 1); the 1x layers run from metals 1-4, with a pitch of ~96 nm, the smallest we have ever seen.

Fig. 1 General Structure of Xilinx Kintex-7

Contacted gate pitch is ~118 nm in our initial analysis, with minimum gate length of ~33 nm, though since this is replacement gate there is no way of knowing absolutely the original poly gate width, which defines the source/drain engineering.

Plan-view imaging (Fig. 2) indicates that TSMC has implemented the restricted design rules that have been much discussed in the gate-first/gate-last debate. Regular, uni-directional patterning of functional gate and dummy gate lines helps out the lithography, but inevitably reduces packing density compared with Manhattan layout schemes.

Fig.2 Plan-View Image of Gates and Active Silicon

By the look of it, double patterning with a gate plus a cut mask has been used. FPGAs are usually laid out in a more relaxed manner than dense logic, so here we can see lots of dummy gates, and also dummy active regions.

The gate structure itself definitely has some similarities with Intel's 45-nm, as we can see from figures 3 and 4.

Fig.3 Intel 45-nm (left) and TSMC/Xilinx 28-nm NMOS Transistors

Fig.4  Intel 45-nm (left) and TSMC/Xilinx 28-nm PMOS Transistors

In both it appears that the buffer oxide, the high-k layer and a common work-function material are put down before the sacrificial polysilicon gate. Then the source/drain engineering is performed, and dielectric stack deposited and planarized back to the polysilicon; and the sacrificial gate is removed, and the NMOS/PMOS gate stacks are put in and planarized.

Of course there are also differences – TSMC is not using embedded SiGe for PMOS strain, and there is an additional high-density metal layer in the PMOS gate. There is also no distinct dielectric capping layer in the TSMC structure, and there is an extra sidewall spacer (likely part of the source/drain tuning). The wafers are also rotated to give a <100> channel direction.

Intel stated that they applied stress to NMOS devices using the gate metal stack and the contacts; TSMC could be doing the same, although the contacts are spaced further from the gate edge. If there is PMOS stress, the mechanism is unclear, though it is possible that the extra high-density layer in the gate could be for that purpose. However, this part is fabbed in the HPL low-power process, and typically we do not see e-SiGe in such processes.

Analysis is ongoing – more details to come, and possibly a comparison with the AMD Llano gate-first HKMG part, that's in our labs at the moment.

N.B. We are at Semicon West, at Booth 2337 - drop by and get a coupon for a free die photo!

Intel Goes Tri-Gate at 22-nm!

In a pair of press and analyst briefings this morning, Mark Bohr and Steve Smith announced that Intel will indeed be using a 3D transistor structure for their 22-nm product, settling one of the big questions about Intel's process development over the last few years - do they stay planar or not? (And, incidentally, settling a bet between me and Scott Thompson - Scott wins!)

The big debate at IEDM last year about advanced CMOS was whether transistor structures would move to a 3D structure (finFET, tri-gate, whatever label you choose), or use ultra-thin SOI layers to attain fully depleted operation. The debate was not resolved - I was definitely left with the impression that the adherents in both camps held to their opinions, which probably means we will have two process groupings, much as we have with the gate-first/gate-last high-k/metal gate (HKMG) structures.

Intel have come down on the side of tri-gate - apparently the decision was taken in 2008, after their researchers had showed that the gate-last HKMG gate structure would work in 3D, and that the planar version could not give enough of a performance boost. So for the last three years they've been developing the process and getting it manufacturable for the production of the Ivybridge product line later this year.



Intel's Research and Development Sequence to Reach the Tri-Gate 22-nm Node


I may have lost the bet about planar, but my gut feel that their HKMG process could be extended to 22-nm seemed to be right, since Mark confirmed that they are using gate-last (replacement gate) technology, with evolutions of existing NMOS and PMOS strain technology. Immersion lithography and double patterning will be used where necessary, and no extra mask layers are needed so the additional cost is only 2 - 3%. And apparently it's scalable to 14 nm!

The schematic below shows a gate formed on three sides of three fins, to give more drive strength than available from one fin:

Schematic of Tri-Gate Across Three Fins (Source - Intel)

When translated to gate-last HKMG, it looks like this in this Intel image from 2007 (the section is through three gates, with three fins buried under oxide running across the field of view):

Gate-Last HKMG Tri-Gate Transistors (Source - Intel)

And now a new image from today's briefing, showing an array of transistors with six fins in the centre, and some with two fins at the top right and bottom left:

Intel Tri-Gate Transistors (with STI and gate mold oxide removed) (Source - Intel)

Clearly this means a whole new set of design and layout paradigms, and we can see evidence here of double patterning using fin and gate masks, with cut masks to define the individual fins and gates.

During the briefings, Mark also scotched the rumour that appeared a few weeks ago about a hybrid process, where the SRAM is tri-gate and other areas are planar - all of the chip area will be tri-gate. In addition a parallel SoC process is being developed so that the Atom line of products can be extended to 22-nm.

For us commentators, going tri-gate was always a possibility for Intel; they have been publishing papers on the topic for almost ten years, with a flurry of them five years ago - here's an image from a press briefing in 2006:

TEM Image of HKMG Tri-Gate Transistor, Sectioned Through the Fin(Source - Intel)

Their R-D-M (Research-Development-Manufacturing) methodology has been well established for quite a while now, and enabled them to keep to their schedule of a new process generation every two years. Based on comments today, we can expect to see 22-nm production in the second half of this year, and product on the shelves in the New Year.

Then we'll see what it really looks like!

A Shameless Plug for ASMC

Winter is finally starting to fade in Ottawa, and the early signs of spring are showing. The maple sap is running, the first migrant birds have arrived, the frogs are peeping, and we have evening daylight. On the conference calendar, spring means that ASMC (IEEE/SEMI Advanced Semiconductor Manufacturing Conference) is on the horizon, this year in Saratoga Springs, New York on May 16 -18. There, spring should be well advanced, and it will be a great time of year to visit the Empire State.

As the name says, ASMC is an annual conference focused on the manufacturing of semiconductor devices - in this it differs from other conferences, since the emphasis is on what goes on in the wafer fab, not the R&D labs, and the papers are not exclusively research papers.

I’m plugging ASMC because it seems to be one of the more under-rated conferences, unlike IEDM and the VLSI symposia, which get the media attention for leading-edge R&D and processes. However, it’s the nitty-gritty of manufacturing in the fab that gets the chips out of the door, and this meeting discusses the work that pushes the yield and volumes up and keeps them there.

I always come away impressed by the quality of the engineering involved; not being a fab person myself any more, it’s easy to get disconnected from the density of effort required to equip a fab, keep it running and bring new products/processes into production. Usually the guys in the fab only get publicity if something goes wrong!

This year, in addition to the 50-plus papers, there are keynotes from Norm Armour of GLOBALFOUNDRIES (GloFo), Gary Patton of IBM, and Peter Wright of Tradition Equities, as well as a panel discussion on partnerships in semiconductor manufacturing, moderated by Dave Lammers. There are also tutorials, on 3D (by James Lu of Rensselaer Poly), and EUV (by Obert Wood of GloFo), and an invited session of ISMI papers.

The technical sessions include:

• Factory Optimization
• Advanced Metrology
• Advanced Equipment, Materials and Processes
• Advanced Process Development and Control
• Advanced Lithography
• Defect Inspection and Yield Optimization
• Data Management

Of course, I’m biased to some extent because we’ll be giving a paper there again. I can't make it this year, but a colleague of mine, Ray Fontaine, is presenting on "Recent Innovations in CMOS Image Sensors". This will be the seventh year running we’ve given a paper, the manufacturing and equipment engineers that attend seem to like seeing what their competitors are doing. In this case Ray will run through some of the changes in the camera chips that we all take for granted in our phones these days.

Other papers that caught my eye may give us some clues as to what to expect in the lithographic field; the IBM/Glofo/Toshiba alliance has one on contact patterning strategies (paper 6.3), and another cooperative paper by IBM/JSR/KLA Tencor/Tokyo Electron on double patterning (6.1), and an IBM/ASML contribution on advanced overlay control (2.5). And on the materials processing side, there are three papers on low-k dielectrics from GloFo/KLA Tencor (2.4), UAlbany/Air Liquide (3.5), and Novellus (poster in session 4); and a couple on nickel silicide by GloFo (5.3) and Ultratech (poster in session 4); and a clue to the mysteries of high-k dielectrics from UMC/National Cheng Kung U (3.4).

More stategically aimed discussions are by Infineon (1.1) on the challenges in having a global supply chain, Sumita Bas of Intel will be speaking on sustainable/green in the chip business (1.3), and two talks by SEMATECH, one on 450 mm manufacturing (ISMI session), and the other on 3D/TSV manufacturing (3.1).

Out of the conference room, there's a poster session and reception on the Monday evening, and on the Tuesday, Dave Lammers' panel session, "Models for Successful Partnerships in Semiconductor Manufacturing". Partnership is one of the buzzwords in chipmaking these days, and the panelists we have should know it well; Ari Komeran from the industry development side of Intel, Michael Fancher from Albany, Olivier Demolliens, head of LETI-NANOTEC in France; and Dr Walid Ali, from ATIC in Abu Dhabi.

After the panel session, what could be a highlight of the conference, a tour of the Luther Forest Technology Campus, including a look at GLOBALFOUNDRIES (Norm Armour's) new Fab 8, followed by a reception at the Canfield Casino.

Register soon - rates go up on May 8th!

Panasonic Gate-First HKMG also First Out of the Gate

As I suggested a few months ago, we put some credence in Panasonic’s press release last September that they would be shipping their first 32-nm HKMG parts last October. Samsung had announced their Saratoga chip, and both Altera and Xilinx have displayed silicon from TSMC, but until last Friday (18 March), none have said that they were shipping product. As of Friday Xilinx announced that they were shipping their Kintex-7 product, the first of their 7-series of FPGAs.

Earlier this month our faith in Panasonic was rewarded, and we found the chip! It took a few false starts buying Panasonic products that we tore down and threw away, but now we have a verified 32-nm, gate-first, high-k metal-gate (HKMG) product. The supply chain was a bit longer than we had hoped, but as promised the chip was shipped with a week 41 date code, in October.

So, for the curious, this is what a transistor looks like:

Panasonic's 32-nm HKMG NMOS Transistor

We can see the TiN metal gate at the base of the polysilicon, and the thin line of high-k at the base of the TiN. Also noticeable are a dual-spacer technology (sometimes referred to as differential offset spacers), and a thin line of nitride over the source/drain extension regions (possibly indicating a nitrided oxide under the high-k). The salicide is the usual platinum-doped nickel silicide. Less visible are mechanisms of applying strain, other than the nitride layer over the gate; embedded SiGe and dual-stress liners are not used.

All of which is typical for Panasonic – their 45-nm product did not appear to use any enhanced strain techniques, and the only concession to PMOS enhancement was wafer rotation to give a 1-0-0 channel direction. The emphasis is different from Intel; rather than raw performance, the targets are increased integration, die size reduction/reduced cost, and now we have high-k, reduced leakage/lower power. The September press release does say that transistor performance is improved by 40%, but it also claims 40% power reduction and a 30% smaller footprint.

Here’s a 45-nm transistor for comparison:

Panasonic's 45-nm Generation Transistor

And, for good measure, Intel's 32-nm device:

Intel 32-nm NMOS Transistor

The part itself uses a nine-metal (eight Cu, one Al) stack with a hybrid low-k/extra-low-k stack. Die size is ~45 mm² in a conventional FC-BGA package. Minimum metal pitch is specified as 120 nm [1], and we have found 125 nm in our early investigations.

Panasonic 32 nm General Structure

Analysis is ongoing – stay tuned for more details, and of course we’ll be doing reports!

[1]S. Matsumoto et al., “Highly Manufacturable ELK Integration Technology with Metal Hard Mask Process for High Performance 32nm-node Interconnect and Beyond”, IITC 2010