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

Maybe essentially the most far-reaching technological achievement over the past 50 years has been the regular march towards ever smaller transistors, becoming them extra tightly collectively, and decreasing 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 yr after yr, sensible new improvements proceed to propel the semiconductor business 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 approach to 3D fin-shaped units 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’ve got to look even additional forward as a result of our capacity 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 units that stack atop one another, delivering logic that’s 30 to 50 % smaller. Crucially, the highest and backside units are of the 2 complementary varieties, NMOS and PMOS, which might be the inspiration of all of the logic circuits of the final a number of many years. We imagine this 3D-stacked complementary metal-oxide semiconductor (CMOS), or CFET (complementary field-effect transistor), would be the key to extending Moore’s Regulation into the subsequent decade.

The Evolution of the Transistor

Steady innovation is an important underpinning of Moore’s Regulation, however every enchancment comes with trade-offs. To grasp 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 primary 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 alternative 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 subject 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 enables present to move between the supply and drain.

As we scaled down the traditional planar transistors, what system physicists name short-channel results took heart 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 purported to, as a result of the gate electrode struggled to deplete the channel of cost carriers. To deal with this, the business moved to a wholly completely different transistor structure known 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 system structure has been the workhorse of Moore’s Regulation ever since. With FinFETs, we may function at a decrease voltage and nonetheless have much less leakage, decreasing energy consumption by some 50 % on the similar efficiency degree 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 system can drive extra present by way of a given space of silicon than can a planar system, which solely conducts alongside one floor.

Nonetheless, we did lose one thing in transferring to FinFETs. In planar units, the width of a transistor was outlined by lithography, and due to this fact 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 also known as fin quantization. As versatile because the FinFET could also be, fin quantization stays a major design constraint. The design guidelines round it and the will so as to add extra fins to spice up efficiency enhance 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 properly because the business’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.

Within the RibbonFET, the gate wraps across the transistor channel area to boost management of cost carriers. The brand new construction additionally allows 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 fashioned by nanometer-scale ribbons of silicon. With these nanoribbons (additionally known as
nanosheets), we will once more differ the width of a transistor as wanted utilizing lithography.

With the quantization constraint eliminated, we will produce the appropriately sized width for the appliance. That lets us stability energy, efficiency, and price. What’s extra, with the ribbons stacked and working in parallel, the system can drive extra present, boosting efficiency with out rising the realm of the system.

We see RibbonFETs as the most suitable choice for larger efficiency at affordable energy, and we will probably be introducing them in 2024 together with different improvements, equivalent 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 better working frequencies and better transistor densities.

To this point, all CMOS applied sciences place the usual NMOS and PMOS transistor pair facet by facet. 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 yr, at IEDM 2020, we introduced the design for the primary logic circuit utilizing this 3D method, an inverter. Mixed with acceptable interconnects, the 3D-stacked CMOS method successfully cuts the inverter footprint in half, doubling the realm density and additional pushing the bounds of Moore’s Regulation.

3D-stacked CMOS places a PMOS system on high of an NMOS system in the identical footprint a single RibbonFET would occupy. The NMOS and PMOS gates use completely different metals.Emily Cooper

Benefiting from the potential advantages of 3D stacking means fixing plenty 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, during 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 completely different metallic gates for the 2 transistors. By combining the source-drain and dual-metal-gate processes, we’re in a position to create completely different conductive forms of silicon nanoribbons (p-type and n-type) to make up the stacked CMOS transistor pairs. It additionally permits us to regulate the system’s threshold voltage—the voltage at which a transistor begins to change—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 known as a superlattice. We then use lithographic patterning to chop away components of the superlattice and go away a finlike construction. The superlattice crystal supplies a powerful help construction for what comes later.

Subsequent, we deposit a block of “dummy” polycrystalline silicon atop the a part of the superlattice the place the system gates will go, defending them from the subsequent step within the process. That step, known as the vertically stacked twin supply/drain course of, grows phosphorous-doped silicon on each ends of the highest nanoribbons (the long run NMOS system) whereas additionally selectively rising boron-doped silicon germanium on the underside nanoribbons (the long run PMOS system). 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.

An edge-on view of the 3D stacked inverter exhibits how difficult its connections are. Emily Cooper

By stacking NMOS on high of PMOS transistors, 3D stacking successfully doubles CMOS transistor density per sq. millimeter, although the actual density will depend 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 will’t successfully dope them chemically as we might with a planar transistor. As an alternative, we use a property of the metallic gates known as the work operate to impart the identical impact. We encompass the underside nanoribbons with one metallic 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 advanced, but it surely’s higher than the choice—a know-how known as sequential 3D-stacked CMOS. With that technique, the NMOS units and the PMOS units 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, equivalent to germanium and III-V semiconductor supplies. These approaches and supplies might turn into related as we glance to tightly combine optoelectronics and different features on a single chip.

Making all of the wanted connections to 3D-stacked CMOS is a problem. Energy connections will should be constructed from beneath the system stack. On this design, the NMOS system [top] and PMOS system [bottom] have separate supply/drain contacts, however each units have a gate in frequent.Emily Cooper

The brand new self-aligned CMOS course of, and the 3D-stacked CMOS it creates, work properly 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 system 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 units is barely
55 nm. Whereas the system efficiency outcomes we achieved aren’t information in and of themselves, they do evaluate properly with particular person nonstacked management units constructed on the identical wafer with the identical processing.

In parallel with the method integration and experimental work, we’ve got many ongoing theoretical, simulation, and design research underway trying to present perception into how greatest to make use of 3D CMOS. Via these, we’ve discovered a few of the key concerns 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’s going to enhance parasitic capacitance, and if it’s too lengthy it’s going to enhance the resistance of the interconnects between the 2 units. Both excessive leads to slower circuits that eat extra energy.

Many design research, equivalent to one by
TEL Analysis Heart America introduced at IEDM 2021, deal with offering all the mandatory interconnects within the 3D CMOS’s restricted area and doing so with out considerably rising the realm of the logic cells they make up. The TEL analysis confirmed that there are numerous alternatives for innovation to find the very best interconnect choices. That analysis additionally highlights that 3D-stacked CMOS might want to have interconnects each above and beneath the units. This scheme, known as buried energy rails, takes the interconnects that present energy to logic cells however don’t carry information and removes them to the silicon beneath the transistors. Intel’s PowerVIA know-how, which does simply that and is scheduled for introduction in 2024, will due to this fact play a key function in making 3D-stacked CMOS a business actuality.

The Way forward for Moore’s Regulation

With RibbonFETs and 3D CMOS, we’ve got a transparent path to increase Moore’s Regulation past 2024. In a 2005 interview during which he was requested to replicate on what grew to become his regulation, Gordon Moore admitted to being “periodically amazed at how we’re in a position to make progress. A number of occasions alongside the best way, I believed we reached the top of the road, issues taper off, and our artistic 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 wish to suppose Mr. Moore will probably be amazed but once more.