Draft:Direct Atomic Layer Processing

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Direct Atomic layer processing(DALP), is a subset technique of Atomic Layer Deposition(ALD) and Atomic Layer Etching(ALE), utilizing the same surface chemical principles as these methods. Processing is used to indicate that both ALD and ALE are possible, making both deposition and etching possible. More specifically it is a subset of Spatial Atomic Layer Deposition, where DALP employs micro-nozzles to precisely control localized processing n in three-dimensional space (XYZ), enabling micro-spot deposition/etching, as illustrated in Figure 1.

Fig1. a) DALP micro gas 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.

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/substractive mode. This work via the spatial ALD/ALE 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 micro-nozzles and machines to drive them in the gallery below.

Gallery1-1. First ever working DALP machine

Gallery1-2. First ever working DALP nozzle clamped in its print head

Gallery1-3. The first industrial prototype of a DALP machine, constructed under the public funded H2020 EU project ATOPLOT

Gallery1-4. The Mesomorph machine, at SUPSI university, constructed under a public H2020 EU project of the same name. The right most position is occupied by a DALP module.

Gallery1-5. 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: The reactant, R does not stick to the surface
Rule 3: The reaction is only possible one way. P+surface>P*>P*+R>F , with F representing the film that is created or etched, and also
represents a new surface upon which P can stick again. Then the model is further governed by the size of the precursor zone, also called the "Line Width" (LW), and the distance to the reactant zone D. With these rules in mind, one can imagine the concentric model moving from point A to point B.

• The reactant moving in front of the precursor does nothing, since there is no P* on the surface for it to react. The precursor sticks to the surface as P* making a line of P* as the nozzle moves along.
• Lastly the reactant behind reacts with P* that's 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 non-reacted 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 the reactant and gets reacted. This does indeed happen and produces what we call "half moon" patterns, Figure 4.

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. Green color represent the deposited material. 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.

Examples supporting the simplified model explanation

Following are examples of measurements, showing the effects explained with the simplified model, and demonstrating that the growth regime is indeed ALD via growth curves and growth characteristics. Keep in mind with DALP we are talking about "Growth Per Pass"(GPP) while in normal ALD we talk about "Growth Per Cycle"(GPC). Each example has the relevant information in the figure description.

Fig3. 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

Fig4. Stitched optical contrast image of DALP deposited half moons, using a nozzle with LW of 400μm. Such half moons can also be overlapped in any position, creating more complex patterns.

Fig5. a)high resolution(in XY) thickness map of a TiO2 line edge(imaging ellipsometry) b) cross section of the demonstrating the various ALD precursor concentration zones that lead to the edge creation

Fig6. a) SEM top view of a 36 nm thick Pt line, showing the line edge . b) Angled view of the line edge of the same line, demonstrating the effect of the deposition characteristics on the edge length

Fig7. Characterization of self-limiting surface chemistry and digital growth in DALP. Thicknesses are determined for two distinct DALP materials by ellipsometry. a) Increase of deposit thickness with number of passes. b) Deposit line height profiles after various pass numbers, demonstrating maintained line width and edge sharpness. c) Demonstration of the self-limiting characteristic of the surface reactions by varying precursor dosage in the form of either gas flow rate or sample motion speed. d) Dependence of the growth rate (determined from a linear growth curve) on the substrate temperature, demonstrating the presence of an “ALD window”.

Fig8. Materials characterization of DALP deposited platinum. a) Scanning electron micro-graph of two Pt lines. b) Grazing-incidence X-ray diffraction exhibiting the peaks expected of micro-crystalline, metallic Pt. c) X-ray photo-electron spectrum recorded near the Pt 4f edge displaying a perfect spin-orbit split 4f7/2/4f5/2 doublet, demonstrating chemical purity. d–f) Transmission electron micro-graphs of a Pt line investigated in cross-section at three different magnification levels. The high-resolution data allow one to observe the presence of large (>100 nm) individual crystals oriented with the [112] direction perpendicular to the substrate surface. The inset shows the selected-area electron diffraction pattern obtained from one single crystal.

Rastering

More complex patterns are created by a combination of multiple lines, usually multiple hundreds to thousands of lines. While the overall model is governed by relatively simple rules, their interaction over multiple hundreds iterations increase the complexity. For example, when one rasters a square, the strategy applied to it can create large differences in the surface morphology. See the gallery below for examples.

Gallery2-1. a) thickness map of 2 overlapping Pt lines(imaging ellipsometry) b) optical image of the same c) cross section of the thickness map, as indicated by the arrow

Gallery2-2. a) thickness map from a section of a TiO2 rastered gradient (imaging ellipsometry) b) cross section of the map at a certain thickness

Gallery2-3. a) thickness map from a section of a TiO2 rastered gradient (imaging ellipsometry) b) cross section of the map showing desired micro steps in thickness

Gallery2-4. a) thickness map of a TiO2 rastered gradient (imaging ellipsometry) b) cross section along the gradient showing its thickness profile

Gallery2-5. On the right are thickness maps(imaging ellipsometry) of 2 TiO2 squares, of roughly the same thickness 30±5 nm. On the left of them are the statistical distributions of thickness from the maps. This demonstrates the difference a rastering approach can make.

Gallery2-6. A photo of a DALP deposited wafer. The material is the same, color is coming from the interference thickness. This wafer has been made for public marketing purposes.

Unique use cases

While DALP has many potential uses, we would like to highlight some unique ones for those potentially interested in the technology.

• Local deposition/etching on substrates that cannot handle lithography. If a substrate cannot handle standard lithography for some reason, such as polystyrene foils or sensitive metals films, then DALP is one of the few ways to create films on such substrates. Provided the selected ALD recipe is chemically compatible with the substrate.
• Gradient depositions/etching. Since DALP deposits/etches by programmed motion in XY, usually of the substrate, any large enough gradient shape is possible. Large enough shapes are those, where movement produces a full line, approximately LW+2*D. Gradients can also be exploited for device design, such as for optical elements.
• Multi Thickness deposition/etching. With DALP, it is possible to program which area on a wafer have what thickness. Since there are many ALD recipe compatible with lithography masks, it is possible to combine the 2 approaches, and produces a wafer with multitude of thicknesses, in a single lithography step. Traditionally, this would take as many lithography steps as the number of thickness points, potentially hundreds. Such an approach can be either used for prototyping of devices, where instead of a single thickness point for a wafer, now we have hundreds. Or it can be used to exploit the thickness effects themselves, such as hard programmed resistor arrays on wafers.
• DALP can produce 3D structures, either on its own via combining ALD and ALE in an appropriate way, or via post-process etching. Essentially using the same technique as other 3D printing techniques, of sacrificial layer printing. This sacrificial layer is used as a scaffold to built further layers upon itself, and then is removed. And example of a sacrificial layer would be zinc oxide.
• Fast experiments design/re-design, and ease of parameter variation for development of devices.

Disclosure

The author of this article, is one of the originators of the DALP technology, therefore there is a clear conflict of interest. The author however, wished to write an article to bring information about DALP to the public, and hopes that the article is informative only. Any of the non cited figures, have been made by the author or co-made by the Author in employment to ATLANT 3D Nanosystems. The company has agreed to to publicly share these images.