BY DEBRA VOGLER, SEMI, Milpitas, CA
With chipmakers looking toward 5nm manufacturing, it’s clear that traditional scaling is not dead but continuing in combination with other technologies. The industry sees scaling enabled by 3D architectures such as die stacking and the stacking of very small geometry wafers. Interconnect scaling also comes into play. This year’s Scaling Technologies TechXPOT at SEMICON West (Scaling Every Which Way! – Thursday, July 12, 2:00PM-4:00PM) will provide an update on the evolution of scaling and describe how the various players (foundry, IDM, fabless, and application developers) are jockeying for innovation leadership. As a prelude to the event, SEMI asked speakers to provide insights on important scaling trends. For a full list of speakers and program agenda, visit http://www.semiconwest.org/programs-catalog/scaling-every-which-way.
Challenges for gate-all-around (GAA) and FinFET devices
Common performance boosters for gate-all-around (GAA) FETs and FinFETs include lower access resistance, lower parasitic capacitance, and stress. “However, one specific performance booster that only applies to GAA is the reduction of the spacing between the vertical wires or sheets,” says Diederik Verkest, imec distinguished member of technical staff, Semiconductor Technology and Systems.
“This reduces parasitic capacitance without affecting drive current and hence benefits both performance and power.” He further notes that imec demonstrated the first stacked gate-all-around (GAA) devices in scaled nodes. “In fact, we are the only ones that published working circuits – ring oscillators in a scaled node using industry-standard processes – in our case replacement metal gate (RMG), and embedded in situ doped source/drain (S/D) epitaxy.”
“There are two elements of the stacked GAA architecture that need to be addressed,” says Verkest. “The first is that this architecture uses epitaxially-grown layers of Si and SiGe to define the device channel. The use of grown materials for the channel and the lattice mismatch between the two materials represent a departure from the traditional fabrication of CMOS devices, so the industry needs to develop and gain confidence in novel metrology that allows for good control of the layers and also proves their low defectivity.” The second aspect is the three-dimensional nature of the GAA devices. “During the processing of these devices, we have ‘non-line-of-sight’ hidden features that are difficult to control and characterize and may also lead to new defect mechanisms that would impact yield, and possibly product reliability.”
Huiming Bu, director, Advanced Logic/Memory Research – Integration and Device, IBM Research, Semiconductor Group, says that naming of technology nodes has been used extensively for marketing strategies in “foundry land,” but the designations have lost much of their meaning as technology scaling differentiators. “That said, when it comes to technology innovation and value proposition, IBM, in conjunction with Samsung and GLOBALFOUNDRIES, has developed the GAA NanoSheet transistor for 5nm to provide a full technology node scaling benefit in density, power and performance,” says Bu (FIGURE 1). The key parameters for intrinsic device optimization when scaling to the 3nm node, explains Bu, are the NanoSheet width for better electrostatic characteristics, and the number of sheets for increased current density. Also necessary are strain engineering for carrier transport enhancement, and interconnect innovations for parasitic RC reduction.
“Beyond that, the industry needs to look into something different, something more disruptive.”
Materials challenges
Materials challenges are also a concern as the industry moves to 5nm and below. “We see increasing complexity in the material systems that are being used,” explains Verkest. One example he cites in scaled FinFET or GAA technologies is the use of two to three layers of different materials–typically metals such as TiN – to which small amounts of other elements are added to set device characteristics such as the threshold voltage. “At the same time, the requirements for the thickness of these materials, driven by gate dimensions for example, or the distance between the wires, are increasingly challenging.” Other examples of materials challenges are the use of two to three different types of insulators in the middle-of-the- line, each with different etch contrasts. “We use novel materials such as carbon containing oxides or oxynitrides that have lower dielectric constants in order to boost the performance of circuits,” he says, noting that the materials list “is quite long.”
Several critical dimensions in transistors at advanced technology nodes have already reached a few monolayers of atoms, fueling expectations for innovation at the material level for transistor scaling, Bu notes. “The other argument is that there is a growing gap between computing demand and the slowdown of technology advancement driven by conventional scaling,” says Bu. One trend that addresses this gap is integrating more computing functions that make the technology solution more modular, which naturally leads to the incorporation of more materials for more applications. Bu cautions, however, that intro- ducing new materials in semiconductor technology has never been easy. “It takes many years of R&D to reach this implementation point, if it ever happens. So, do we need new materials when the industry moves to 5nm and 3nm? Yes, though I expect new material implementation to be a lot faster in interconnect and packaging at these nodes rather than intrinsic to the transistor.”
Challenges in developing atomic-level processes
There will be challenges in developing atomic-level processes used in scaling, such as atomic layer depositions (ALD) and atomic layer etches, notes Verkest. “These classes of processes are both required to handle the scaled dimensions at the 5nm and 3nm nodes, and also the 3D nature of the scaled technologies – and here we are talking about logic and memories,” Verkest says. “With respect to depositions, we would need to develop thermal ALD processes (not plasma-based) that enable accurate and conformal depositions in non-line-of-sight structures.”
Adhesion and wetting, smoothness, and throughput would also need to be addressed. “Longer term, these processes need to facilitate selectivity and self-alignment to address gap-fill challenges in highly scaled structures,” he says. Other concerns he notes with respect to atomic layer etches are selectivity to various materials, and fidelity requirements that increase the requirements for metrology accuracy. “Throughput is also a concern.”
Bu believes that a new device architecture beyond FinFET is required to provide a full technology node scaling benefit (i.e., density, power and performance) at 5nm and 3nm.
“Beyond 3nm, we may need to continue the transistor scaling in the vertical direction and start to stack them together,” Bu says. He also cites the need for parasitic R/C reduction in the interconnect to take advantage of the intrinsic transistor benefit at the circuit and chip levels. “We see a lot of opportunity in atomic-level processes, especially in atomic layer etch and selective material deposition, to address these challenges in the transistor and the interconnect.”