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3D-Stacked CMOS Takes Moore’s Legislation to New Heights

Maybe essentially the most far-reaching technological achievement during the last 50 years has been the regular march towards ever smaller transistors, becoming them extra tightly collectively, and lowering their energy consumption. And but, ever because the two of us began our careers at Intel greater than 20 years in the past, we’ve been listening to the alarms that the descent into the infinitesimal was about to finish. But 12 months after 12 months, good new improvements proceed to propel the semiconductor trade additional.

Alongside this journey, we engineers needed to change the transistor’s structure as we continued to scale down space and energy consumption whereas boosting efficiency. The “planar” transistor designs that took us by way of the final half of the twentieth century gave solution to 3D fin-shaped gadgets by the primary half of the 2010s. Now, these too have an finish date in sight, with a brand new gate-all-around (GAA) construction rolling into manufacturing quickly. However we now have to look even additional forward as a result of our means to scale down even this new transistor structure, which we name RibbonFET, has its limits.

So the place will we flip for future scaling? We are going to proceed to look to the third dimension. We’ve created experimental gadgets that stack atop one another, delivering logic that’s 30 to 50 % smaller. Crucially, the highest and backside gadgets are of the 2 complementary varieties, NMOS and PMOS, which are the inspiration of all of the logic circuits of the final a number of a long time. We imagine this 3D-stacked complementary metal-oxide semiconductor (CMOS), or CFET (complementary field-effect transistor), would be the key to extending Moore’s Legislation into the subsequent decade.

The Evolution of the Transistor

Steady innovation is a vital underpinning of Moore’s Legislation, however every enchancment comes with trade-offs. To know these trade-offs and the way they’re main us inevitably towards 3D-stacked CMOS, you want a little bit of background on transistor operation.

Each metal-oxide-semiconductor field-effect transistor, or MOSFET, has the identical set of fundamental components: the gate stack, the channel area, the supply, and the drain. The supply and drain are chemically doped to make them each both wealthy in cellular electrons (
n-type) or poor in them (p-type). The channel area has the other doping to the supply and drain.

Within the planar model in use in superior microprocessors as much as 2011, the MOSFET’s gate stack is located simply above the channel area and is designed to venture an electrical area into the channel area. Making use of a big sufficient voltage to the gate (relative to the supply) creates a layer of cellular cost carriers within the channel area that permits present to move between the supply and drain.

As we scaled down the basic planar transistors, what gadget physicists name short-channel results took middle stage. Mainly, the space between the supply and drain grew to become so small that present would leak throughout the channel when it wasn’t alleged to, as a result of the gate electrode struggled to deplete the channel of cost carriers. To deal with this, the trade moved to a completely totally different transistor structure referred to as a
FinFET. It wrapped the gate across the channel on three sides to supply higher electrostatic management.

Intel launched its FinFETs in 2011, on the 22-nanometer node, with the third-generation Core processor, and the gadget structure has been the workhorse of Moore’s Legislation ever since. With FinFETs, we might function at a decrease voltage and nonetheless have much less leakage, lowering energy consumption by some 50 % on the identical efficiency stage because the previous-generation planar structure. FinFETs additionally switched quicker, boosting efficiency by 37 %. And since conduction happens on each vertical sides of the “fin,” the gadget can drive extra present by way of a given space of silicon than can a planar gadget, which solely conducts alongside one floor.

Nevertheless, we did lose one thing in transferring to FinFETs. In planar gadgets, the width of a transistor was outlined by lithography, and subsequently it’s a extremely versatile parameter. However in FinFETs, the transistor width comes within the type of discrete increments—including one fin at a time–a attribute sometimes called fin quantization. As versatile because the FinFET could also be, fin quantization stays a big design constraint. The design guidelines round it and the will so as to add extra fins to spice up efficiency improve the general space of logic cells and complicate the stack of interconnects that flip particular person transistors into full logic circuits. It additionally will increase the transistor’s capacitance, thereby sapping a few of its switching velocity. So, whereas the FinFET has served us effectively because the trade’s workhorse, a brand new, extra refined method is required. And it’s that method that led us to the 3D transistors we’re introducing quickly.

A blue block pierced by three gold-coated ribbons all atop a thicker grey block.Within the RibbonFET, the gate wraps across the transistor channel area to reinforce management of cost carriers. The brand new construction additionally permits higher efficiency and extra refined optimization. Emily Cooper

This advance, the RibbonFET, is our first new transistor structure because the FinFET’s debut 11 years in the past. In it, the gate absolutely surrounds the channel, offering even tighter management of cost carriers inside channels that are actually shaped by nanometer-scale ribbons of silicon. With these nanoribbons (additionally referred to as
nanosheets), we are able to once more fluctuate the width of a transistor as wanted utilizing lithography.

With the quantization constraint eliminated, we are able to produce the appropriately sized width for the applying. That lets us steadiness energy, efficiency, and price. What’s extra, with the ribbons stacked and working in parallel, the gadget can drive extra present, boosting efficiency with out rising the world of the gadget.

We see RibbonFETs as the best choice for larger efficiency at affordable energy, and we will probably be introducing them in 2024 together with different improvements, comparable to PowerVia, our model of
bottom energy supply, with the Intel 20A fabrication course of.

Stacked CMOS

One commonality of planar, FinFET, and RibbonFET transistors is that all of them use CMOS know-how, which, as talked about, consists of n-type (NMOS) and p-type (PMOS) transistors. CMOS logic grew to become mainstream within the Eighties as a result of it attracts considerably much less present than do the choice applied sciences, notably NMOS-only circuits. Much less present additionally led to higher working frequencies and better transistor densities.

Up to now, all CMOS applied sciences place the usual NMOS and PMOS transistor pair aspect by aspect. However in a
keynote on the IEEE Worldwide Electron Gadgets Assembly (IEDM) in 2019, we launched the idea of a 3D-stacked transistor that locations the NMOS transistor on high of the PMOS transistor. The next 12 months, at IEDM 2020, we offered the design for the primary logic circuit utilizing this 3D approach, an inverter. Mixed with applicable interconnects, the 3D-stacked CMOS method successfully cuts the inverter footprint in half, doubling the world density and additional pushing the bounds of Moore’s Legislation.

Two blue blocks stacked atop each other. Each is pierced through by gold coated ribbons.3D-stacked CMOS places a PMOS gadget on high of an NMOS gadget in the identical footprint a single RibbonFET would occupy. The NMOS and PMOS gates use totally different metals.Emily Cooper

Profiting from the potential advantages of 3D stacking means fixing a variety of course of integration challenges, a few of which can stretch the bounds of CMOS fabrication.

We constructed the 3D-stacked CMOS inverter utilizing what is called a self-aligned course of, by which each transistors are constructed in a single manufacturing step. This implies setting up each
n-type and p-type sources and drains by epitaxy—crystal deposition—and including totally different steel gates for the 2 transistors. By combining the source-drain and dual-metal-gate processes, we’re capable of create totally different conductive varieties of silicon nanoribbons (p-type and n-type) to make up the stacked CMOS transistor pairs. It additionally permits us to regulate the gadget’s threshold voltage—the voltage at which a transistor begins to modify—individually for the highest and backside nanoribbons.

How will we do all that? The self-aligned 3D CMOS fabrication begins with a silicon wafer. On this wafer, we deposit repeating layers of silicon and silicon germanium, a construction referred to as a superlattice. We then use lithographic patterning to chop away components of the superlattice and depart a finlike construction. The superlattice crystal offers a powerful assist construction for what comes later.

Subsequent, we deposit a block of “dummy” polycrystalline silicon atop the a part of the superlattice the place the gadget gates will go, defending them from the subsequent step within the process. That step, referred to as the vertically stacked twin supply/drain course of, grows phosphorous-doped silicon on each ends of the highest nanoribbons (the long run NMOS gadget) whereas additionally selectively rising boron-doped silicon germanium on the underside nanoribbons (the long run PMOS gadget). After this, we deposit dielectric across the sources and drains to electrically isolate them from each other. The latter step requires that we then polish the wafer right down to good flatness.

Gold columns are bridged by a purple polygon and a green one. A rectangle bisects the polygon. It's pink on top and yellow on the bottom.An edge-on view of the 3D stacked inverter reveals how difficult its connections are. Emily Cooper

Blue, pink and green rectangles representing different parts of transistors are arranged in a larger circuit on the left and one half the size on the right.By stacking NMOS on high of PMOS transistors, 3D stacking successfully doubles CMOS transistor density per sq. millimeter, although the true density relies on the complexity of the logic cell concerned. The inverter cells are proven from above indicating supply and drain interconnects [red], gate interconnects [blue], and vertical connections [green].

Lastly, we assemble the gate. First, we take away that dummy gate we’d put in place earlier, exposing the silicon nanoribbons. We subsequent etch away solely the silicon germanium, releasing a stack of parallel silicon nanoribbons, which would be the channel areas of the transistors. We then coat the nanoribbons on all sides with a vanishingly skinny layer of an insulator that has a excessive dielectric fixed. The nanoribbon channels are so small and positioned in such a means that we are able to’t successfully dope them chemically as we’d with a planar transistor. As a substitute, we use a property of the steel gates referred to as the work operate to impart the identical impact. We encompass the underside nanoribbons with one steel to make a
p-doped channel and the highest ones with one other to kind an n-doped channel. Thus, the gate stacks are completed off and the 2 transistors are full.

The method might sound complicated, however it’s higher than the choice—a know-how referred to as sequential 3D-stacked CMOS. With that technique, the NMOS gadgets and the PMOS gadgets are constructed on separate wafers, the 2 are bonded, and the PMOS layer is transferred to the NMOS wafer. Compared, the self-aligned 3D course of takes fewer manufacturing steps and retains a tighter rein on manufacturing value, one thing we demonstrated in analysis and reported at IEDM 2019.

Importantly, the self-aligned technique additionally circumvents the issue of misalignment that may happen when bonding two wafers. Nonetheless, sequential 3D stacking is being explored to facilitate integration of silicon with nonsilicon channel supplies, comparable to germanium and III-V semiconductor supplies. These approaches and supplies could change into related as we glance to tightly combine optoelectronics and different capabilities on a single chip.

Orange elongated blocks connect to several narrower blocks of a variety of colors. Making all of the wanted connections to 3D-stacked CMOS is a problem. Energy connections will should be created from under the gadget stack. On this design, the NMOS gadget [top] and PMOS gadget [bottom] have separate supply/drain contacts, however each gadgets have a gate in widespread.Emily Cooper

The brand new self-aligned CMOS course of, and the 3D-stacked CMOS it creates, work effectively and seem to have substantial room for additional miniaturization. At this early stage, that’s extremely encouraging. Gadgets having a gate size of 75 nm demonstrated each the low leakage that comes with wonderful gadget scalability and a excessive on-state present. One other promising signal: We’ve made wafers the place the smallest distance between two units of stacked gadgets is just
55 nm. Whereas the gadget efficiency outcomes we achieved usually are not data in and of themselves, they do examine effectively with particular person nonstacked management gadgets constructed on the identical wafer with the identical processing.

In parallel with the method integration and experimental work, we now have many ongoing theoretical, simulation, and design research underway seeking to present perception into how greatest to make use of 3D CMOS. By these, we’ve discovered a number of the key issues within the design of our transistors. Notably, we now know that we have to optimize the vertical spacing between the NMOS and PMOS—if it’s too brief it should improve parasitic capacitance, and if it’s too lengthy it should improve the resistance of the interconnects between the 2 gadgets. Both excessive ends in slower circuits that devour extra energy.

Many design research, comparable to one by
TEL Analysis Middle America offered at IEDM 2021, deal with offering all the required interconnects within the 3D CMOS’s restricted house and doing so with out considerably rising the world of the logic cells they make up. The TEL analysis confirmed that there are a lot of alternatives for innovation to find the perfect interconnect choices. That analysis additionally highlights that 3D-stacked CMOS might want to have interconnects each above and under the gadgets. This scheme, referred to as buried energy rails, takes the interconnects that present energy to logic cells however don’t carry knowledge and removes them to the silicon under the transistors. Intel’s PowerVIA know-how, which does simply that and is scheduled for introduction in 2024, will subsequently play a key function in making 3D-stacked CMOS a business actuality.

The Way forward for Moore’s Legislation

With RibbonFETs and 3D CMOS, we now have a transparent path to increase Moore’s Legislation past 2024. In a 2005 interview by which he was requested to replicate on what grew to become his regulation, Gordon Moore admitted to being “periodically amazed at how we’re capable of make progress. A number of instances alongside the way in which, I believed we reached the tip of the road, issues taper off, and our inventive engineers provide you with methods round them.”

With the transfer to FinFETs, the following optimizations, and now the event of RibbonFETs and ultimately 3D-stacked CMOS, supported by the myriad packaging enhancements round them, we’d prefer to assume Mr. Moore will probably be amazed but once more.

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