Draft:Direct Atomic Layer Processing

Direct Atomic layer processing(DALP), is a subset technique of Atomic Layer Deposition and Atomic Layer Etching, using exactly the same chemical processes. More specifically it is a subset of Spatial Atomic Layer deposition, where DALP is using micro-nozzles to have a fully constrained system in XYZ, essentially allowing for deposition with a micro-spot as seen on figure 1.

Fig1. a) DALP Microfluidic nozzle concept: Schematic view of the nozzle in frontal view (top) and in cross-section (lower panel). b) Demonstration of direct pattern generated by a DALP deposition. Top is a thickness map by Low Energy Ion Spectroscopy, bottom is a Pt concentration map (log scale) by the same technique. From ref. [1]

Introduction

Development

DALP has via the development of micro-nozzles and appropriate driving gas systems achieved direct processing, essentially allowing ALD and ALE to be used in an additive manufacturing mode. This work via the spatial ALD route, where the precursor and reactant combination of ALD/ALE is separated in space via gas dynamics as seen on figure 1. Currently DALP is being developed by the company ATLANT 3D Nanosystems and an FAU university group Chemistry of Thin Film Materials. See examples of micronozzles and machines to drive them on figures 2 to X.

First ever working DALP machine

First ever working DALP nozzle clamped in its print head

The first industrial prototype of a DALP machine, constructed under the public funded H2020 EU project ATOPLOT

The Mesomorph machine, at SUPSI university, constructed under a public H2020 EU project of the same name. The right most possition is occupied by a DALP module.

The second industrial prototype of a DALP machine

Simplified model explanation

A simplified model can be made to explain the basic nature of the DALP process, which consist of a circular precursor zone, with a concentration gradient from 0 to 1 in the center. This is surrounded by a reactant zone, with a concentration gradient from sides to the middle. Between these 2 zones there is a distance D with 0 concentration of either.
The system is intentionally concentric, since then all movements in XY are equal due to circular symmetry. Such a model is what a substrate sees in terms of chemical concentration, and this is achieved by the gas dynamics from the micro nozzle shown in Figure 1.
If we wish to add layers of reality to the model, we would start by adjusting the shape from a perfect circle, to some less ideal shape.

We can represent the nature of ALD, with a few set rules to explain how the deposition/etching is created.
Rule 1: P , the precursors, sticks and saturates on the substrate, and remains until it is reacted by R, the reactant
Rule 2: R does not stick to the surface
Rule 3: The reaction is only possible one way. P+surface>P*>P*+R>F , with F being the film that is created or etched, and also
represents a new surface upon which P can stick again. With these rules in mind, one can imagine the concentric model moving from point A to point B. The R moving in front of P does nothing, since there is no P* on the surface for it to react. The P sticks to the surface as P* making a line of P* as the nozzle moves along. Lastly the R behind reacts with P* thats on the surface, but does not reach all of it at point B. At point B a line the length of D of P* is left. Same thing happens when moving back, with the only addition that the unreacted P* from previous movement reacts. After 2 passes we are left with a line, that has a deposition/etching of N=2 passes in the middle, and N=1 pass on the sides. This is what we call "edge effect". One can also imagine the movement between A and B being smaller than the distance D, when nothing happens until a bigger movement is done. Also, one can imagine a small movement of D+Δ, where only a small part of P* sees R and gets reacted. This does indeed happen and produces what we call "half moon" patterns.

Fig2. a) Simple model of DALP deposition, showing a precursor zone in the middle, surrounded by a reactant zone with a distance of D b) "depositing" a line with the simplified model between point A and B, showing what happens in 2 passes. Most importantly, the demonstration naturally provides that the line will have edges where the deposition has N/2 passes and a middle where there are N passes

The general situation is of course more complex for more complex shapes, and a secondary deposition mode from parasitic Chemical Vapour Deposition(CVD) also happens. This mode is created when diffused P and R react in the gas phase, and C is produced in the gas phase. C condenses from the gas phase onto the surface, making the CVD contribution time dependent.

The edge of the lines and patterns in general are created due to concentration gradients. On the substrate where nothing is deposited the concentration must have been 0, of either P or R or both. In the deposition regions, it should reach 1 if the system is driven properly. Therefore there needs to be an increasing gradient between 0 and 1, represented on the picture as a color gradient.

A simulator based on this model is developed, however not fully released to the public as of this moment. The gradient information is taken from calibration measurements. It might be possible to reach out to ATLANT 3D for a test version.

An example deposition if TiO₂ a) optical snapshot of sample of TiO2 print. ​b) thickness map (imaging ellipsometry). c) cross section of the thickness map From ref.[2]