What Growth Rate Variations Occur During Selective Area Cvd Of Diamond Thin Films On 3D Microstructures?

When diamond grows selectively on 3D surfaces, the film thickness is not uniform. Flat top surfaces in patterned microstructures deposit diamond at 8.3 to 9.7 nm per minute, while inclined sidewalls grow more slowly at 5.8 to 6.7 nm per minute. This variation is driven by the geometrical exposure of each facet to the plasma‑borne growth species, not by a lack of reactants reaching the deep features.

The roughly 30–40% slower growth on angled facets is a predictable, geometry‑driven consequence of how plasma species strike the surface. In selective area CVD of diamond on microstructures like inverted pyramids or trenches, mastering this rate differential is the key to fabricating dimensionally accurate, high‑yield diamond MEMS and cantilevers.

Why Growth Rates Differ on 3D Microstructures

The Observed Growth Rate Variation

Selective area diamond CVD uses lithographically defined oxide masks to confine growth precisely. On silicon inverted pyramids or trenches, the flat top surfaces consistently show a higher deposition rate—between 8.3 and 9.7 nm/min. The sloped sidewalls, however, deposit diamond at only 5.8 to 6.7 nm/min.

This delta is not a process instability. It repeats across multiple samples and reactor configurations when the feature depth is on the order of tens of microns.

Geometrical Influence vs. Reactant Depletion

A common assumption would be that sidewalls receive fewer reactive species because the gas is depleted inside narrow cavities. That is not the case here.

The mean free path of film‑forming radicals in a standard microwave plasma CVD reactor is approximately 50 µm. This distance equals or exceeds the depth of typical microstructures used for diamond MEMS. So the gas‑phase species can easily reach the bottom and sidewalls without being consumed along the way. The slower sidewall growth is therefore a purely geometrical effect, not a transport limitation.

The Physics of Flux and Feature Geometry

Understanding Solid Angle Exposure

In plasma CVD, growth species arrive from all directions in the glow discharge. What matters is the solid angle subtended by the plasma as “seen” from each point on the substrate.

A flat horizontal surface is exposed to a full hemisphere of incoming flux. An inclined sidewall, however, presents a smaller effective projection toward the bulk plasma. The cosine of the incidence angle reduces the number of species that actually strike and stick. In simpler terms, the sidewall simply “sees” less plasma, so it grows more slowly.

Implications for Patterned Diamond Devices

For engineers designing diamond cantilevers, membranes, or MEMS resonators, this gradient forces a design choice. If the sidewall must reach a minimum thickness for mechanical integrity, the flat top will inevitably overgrow. A 50–70% thicker top film becomes a built‑in consequence of the geometry.

Post‑processing steps such as selective etching or polishing may be required to level the coating, but those add cost and complexity. Predicting the growth profile from the outset—using the known flux scaling—is far more efficient.

Navigating the Pitfalls of Geometry‑Dependent Growth

Overgrowth on Flat Regions

Achieving a required sidewall thickness often means the top surface receives nearly double the intended deposition. This extra material can introduce:

Higher residual stress, risking delamination.
Increased mask erosion from the longer plasma exposure.
Longer overall cycle times, eating into throughput.

In MEMS cantilevers, asymmetric stress from a thicker top layer can warp the beam, degrading performance.

Mask Erosion and Selectivity

The oxide mask used for selective area deposition itself erodes gradually in the plasma. Because the top surface sees a higher flux, the mask there may degrade faster than on the sidewalls. This can lead to slightly broader pattern openings on the top, subtly altering the final shape of the grown diamond.

Secondary Influences: Surface Finish and Confinement

While the dominant factor is the solid‑angle exposure, local surface roughness and extreme confinement can add minor variation. A rough sidewall may have peaks coated preferentially, and very narrow, high‑aspect‑ratio features may begin to experience subtle diffusion limitations. However, in the feature sizes typical of diamond MEMS (tens of microns), these effects are secondary to the geometric flux argument.

How to Apply These Insights to Your Process

The 30–40% rate drop on inclined surfaces is predictable and manageable. Your process design should explicitly account for it from the first mask layout.

If your primary focus is achieving uniform sidewall thickness on a diamond cantilever: Plan for a thicker top coat, then incorporate a controlled etch‑back or overgrowth‑and‑polish step to level the structure after deposition.
If your primary focus is minimizing overall process time and cost: Accept a moderate thickness variation. Use the known rate ratio to grow the sidewall to the minimum viable dimension while allowing the top to run thicker, and verify that the extra top‑side mass does not compromise device function.
If your primary focus is producing precisely patterned diamond in inverted pyramids or trenches: Leverage the mean‑free‑path argument to rule out gas‑phase starvation. Model the solid‑angle exposure for each facet and adjust the mask opening or process pressure to fine‑tune the flux distribution.

Master the geometry, and you make selective area diamond CVD a repeatable, precision‑patterning tool rather than a source of uncontrolled film thickness variation.

Summary Table:

Substructure Feature	Diamond Growth Rate	Underlying Mechanism	Design & Process Impact
Flat Top Surfaces	8.3 – 9.7 nm/min	Full solid angle exposure to plasma flux	Overgrowth risk, higher stress, potential mask erosion
Inclined Sidewalls	5.8 – 6.7 nm/min	Reduced effective flux (cosine law projection)	~30–40% thinner film; requires thickness compensation
Deep Cavities / Trenches	Rate matches sidewall angle	Unaffected by gas depletion (mean free path ~50 µm)	Uniform radical transport; rate driven purely by geometry

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