Tuesday, April 7, 2015

A spatial ALD oxide passivation module in an all-spatial etch-passivation cluster concept!

Prof. Fred Roozeboom and co-workers F. van den Bruele, Y. Creyghton, P. Poodt, and Prof. W.M.M. Kessels (all from Eindhoven University of Technology and TNO, as driving forces behind Spatial ALD and ALE technology), have just published a fantastic open access publication in ECS Journal of Solid State Science and Technology. Just taste the title of this blog text for a moment and then continue reading or down load the article - it´s free, it´s OPEN ACCESS.

Cyclic etch /passivation-deposition as an all-spatial concept towards high-rate room temperature Atomic Layer Etching [OPEN ACCESS]
F. Roozeboom, F. van den Bruele, Y. Creyghton, P. Poodt, and W.M.M. Kessels
ECS Journal of Solid State Science and Technology, 4 (6) pp. N5067-N5076 (2015). doi:10.1149/2.0111506jss

Conventional (3D) etching in silicon is often based on the ‘Bosch’ plasma etch with alternating half-cycles of a directional Si-etch and a fluorocarbon polymer passivation. Also shallow feature etching is often based on cycled processing. Likewise, ALD is time-multiplexed, with the extra benefit of half-reactions being self-limiting, thus enabling layer-by-layer growth in a cyclic process. To speed up growth rate, spatial ALD has been successfully commercialized for large-scale and high-rate deposition at atmospheric pressure. We conceived a similar spatially-divided etch concept for (high-rate) Atomic Layer Etching (ALEt). The process is converted from time-divided into spatially-divided by inserting inert gas-bearing ‘curtains’ that confine the reactive gases to individual injection slots in a gas injector head. By reciprocating substrates back and forth under such head one can realize the alternate etching/passivation-deposition cycles at optimized local pressures, without idle times needed for switching pressure or purging. Another improvement toward an all-spatial approach is the use of ALD-based oxide (Al2O3, SiO2, etc.) as passivation during, or gap-fill after etching. This approach, called spatial ALD-enabled RIE, has industrial potential in cost-effective back-end-of-line and front-end-of-line processing, especially in patterning structures requiring minimum interface, line edge and fin sidewall roughness (i.e., atomic-scale fidelity with selective removal of atoms and retention of sharp corners). 

The publication starts with a History of 3D etching and a description of how and why plasma etching is a key enabling technology and then it gets down to business to introduce the concept behind layer-by layer growth (ALD) or etch (ALE) and more importantly the concept behind spatial layer-by-layer processing. Then via the cyclic Bosch process, and Spatial RIE with Spatial passivation we land at the Grand Finale - Spatial RIE process mode with Spatial ALD passivation!  Or even more beautifully formulated by Prof. Roozeboom himself a spatial ALD oxide passivation module in an all-spatial etch-passivation cluster concept!

Layer-by layer-processing
Figure 3.

Schematic of conventional CVD and plasma etching and their layer-by-layer counterparts, ALD and ALEt. ALEt is cycled between modification by chemisorption of a reactant at the surface and, subsequent volatilization of, ideally, one (sub)monolayer by irradiation with an energetic beam or reaction with a co-reactant. For simplicity reasons the etch processes (bottom pictures) are cartooned in plasma-assisted mode, and the deposition processes (top pictures) in thermal mode. The latter two could be plasma-assisted as well. In the conventional processes (CVD and Plasma etch) the chemical reactants are supplied simultaneously and non-interrupted, and in the layer-by-layer processes (ALD and ALEt) they are alternated. (picture used with permission)

Spatial ALD

Figure 4. 

Schematic representation of spatial ALD: a wafer moves horizontally back and forth under spatially divided and confined reaction zones. Arrows pointing upwards indicate exhaust lines. Notice the difference in height of the gas bearing compartments (typically ∼20 to 100 μm) and the deposition compartments (typical height a few mm, and lengths and widths of order ∼1-10 mm).(picture used with permission)

Convential Bosch etching by cyclic surface passivation half-cycles
Figure 6. 

Conventional Bosch etch process scheme for etching silicon with a pre-patterned hard mask atop, using alternating etch and passivation half-cycles. (picture used with permission)

Spatial RIE process mode with C4F8 passivation

Figure 7.

Schematic of spatial RIE process mode with C4F8 passivation of a wafer that reciprocates under spatially divided reaction zones. Arrows pointing upwards indicate exhaust lines. Notice the difference in height of the gas bearing compartments (typically ∼20 to 100 μm) and the plasma compartments (typical height ∼10 mm, and length of several 10 mm's and width of order ∼1 mm). The compartments are connected through a gas bearing envelope. Not to scale; wafers will pass the entire zones before shuttling back.  (picture used with permission)

Spatial ALD oxide passivation module in an all-spatial etch-passivation cluster concept

Figure 8. 

Schematic of alternative all-spatial RIE process mode with spatial ALD oxide passivation (e.g., SiO2, Al2O3, ..). ‘Si’ denotes a Si-precursor, TMA is trimethyl aluminum. Note, that for deep etching and for shallow (‘layer-by-layer’) etching the wafer exposure times in the respective zones will differ, which will imply different residence times, or different numbers of unit cells in the two main compartments.  (picture used with permission)

At the end after showing a number of case studies, Prof. Roozeboom et al summarizes - and we all believers will agree on these conclusions - namely that:
  • The potential of ALD-assisted nanomanufacturing technologies like Atomic Layer Etching (ALEt) concepts derived from etch-purge-passivation/deposition-purge subroutines in (D)RIE and ALD is now clearly being recognized and promoted.
  • The ongoing scaling of Moore's Law will soon require the implemention of these complementary technologies to meet the 10-nm challenges in surface and sidewall passivation of resist and feature patterns that is required to minimize interface, line edge and fin wall roughness.
  • For cost reasons and flexibility in local pressure, i.e. (an)isotropy control, in the spatial etch and purge compartments one can envisage a gradual shift to the adoption of ALD-enabled RIE (we abbreviate it as ALDeRIE) in the spatial domain as well. 
  • Obviously, the spatially divided version is not commercially available yet and not straightforward, but – once realized for dedicated materials and topographies – it will certainly lead to far improved price-performance ratios in Atomic Layer Etching.  


Fred Roozeboom appointed as ECS Fellow, The Electrochemical Society appointed Prof. dr. Fred Roozeboom as Fellow of the Electrochemical Society  for his Scientific contributions to Solid-State Science & Technology and its impact on the society. He has been awarded especially because of his contributions on the areas of rapid thermal processing, passive 3D and heterogeneous integration, reactive ion etching and atomic layer deposition (ALD). He received his award at the Plenary Session of the 226th ECS meeting. October 5, 2014, Cancun, Mexico.

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