Atomic-scale films critical to transistors

As we move past the already-problematic 28nm node, films with thicknesses measured in Angstroms are increasingly vital to the functioning of transistors, and even to the ability to create patters on the wafer. A science-fiction-like ability to deposit a uniform, pure layer of material only Angstroms thick has become a gating skill in several areas of advanced IC manufacturing.

The most obvious critical application for film deposition is the high-k/metal-gate (HKMG) gate stack with which foundries are already struggling at 28nm. Applied Materials executive vice president and general manager for silicon systems Randhir Thakur, speaking at Semicon West earlier this month, sketched the seriousness of the problem.

“The industry is actually at a very early stage with HKMG,” Thakur said. “I don’t expect mass production before 2014. In comparison, we will see the first 3D transistors in 2013, reaching mass production in 2016.”

Applied vice president and general manager for metal deposition and front-end processing Steve Ghanayem explained further. “In HKMG processing, we are replacing one oxide film with four or five complete processes, including two layers of oxide, atomic-layer deposition of hafnium-oxide, annealing, and nitridization. The layers we are forming are as thin as 3 Angstroms. By the 22nm node, literally half the atoms in the gate stack are in the interfaces between these films. Very small changes in oxide growth, or tiny numbers of trapped impurity atoms, show up in model-level variations in mobility and threshold voltage for the transistors.”

Once 3D transistors—finFETs—reach production, an entirely new issue becomes critical: conformality. “We require 15-20 layers to form these 3D transistor structures,” Thakur said. Worse than the sheer number of layers, though, is the shape. In planar processes, all of the films in the gate stack are deposited on a flat surface or, at worst, onto the flat bottoms of shallow holes. But for finFETs, the gate stack must lie across the fin like a saddle blanket, conforming to the right-angle corners at the edges of the fin and going straight down the sides. Any variation in the thickness of these films, or any rounding or cracking at the corners, will show up as variations in transistor parameters. Thus processes must deposit atomic-layer films that conform almost perfectly to a surface comprising row upon row of tall rectangular ridges.

On a finFET, the gate stack films are in the nearly-invisible layer between the fin and the arched gate electrode that crosses over the fin.

Nor are gate stacks the only issue. Novellus vice president for process applications Girish Dixit pointed out that conformal film deposition is increasingly vital in lithography as well. Today each mask exposure requires deposition of several films: a hardmask, an antireflection layer that may itself include two or more films, and a resist layer. “Each of these films is usually conformal,” Dixit explained. In double- or quad-patterning scenarios, at least the hard mask has to lie over a surface that already has considerable topology, so the film must conform to complex ups and downs. In the case of a finFET process, the surface may have very high aspect-ratio features on it during the second phase of patterning. “But because the films are conformal,” Dixit continued, “any roughness accumulates. And roughness will distort the pattern.” Hence these films must not only conform to the topology, but must be smoothable to tolerance much finer than the feature size.

In many different steps, then, the ability to deposit unimaginably thin films, often over complex topology, has become necessary to creating transistors. Each variation in conformality, thickness, purity, or smoothness can appear to chip designers as a variation in transistor-model parameters. It has become a game of Angstroms.