Project Black Silicon (2012-2015)        

"Black Silicon optical properties and growth mechanisms"

Objectives    
Results
Publications

Objectives

Black Silison (BSi) are silicon micro and nanostructured surfaces typically fabricated with cryogenic deep reactive ion etching (cryo-DRIE). These self-generated surfaces can be fabricated in a single step procedure and provide large surfaces with reduced reflectance over a broad range of wavelengths and angles of incidence. BSi has been used in a wide range of applications, like high-efficiency absorbers for solar cells, infrared-optimized photodetectors, or superhydrophobic surfaces that can find applications in microelectromechanical devices such as sensors

We have developed in this project three main state-of-the art contributions: 

Results

Three-dimensional BSi reconstruction

We developed 2 reconstruction techniques to obtain a three-dimensional model of the BSi topography based on:
The nanotomography provides resolution below 10 nm with accurate informations on the depth of the nanostructures. The grey-level conversion to height of a top-view SEM micrograph has the advantage to be non-destructive and is easy to implement, however the accuracy in the deep areas  is limited by the SEM dynamical range. Therefore, it has to be limited to samples with small or medium aspect ratios.

Pictures of BSi 3D reconstruction
SEM close-up view of a BSi sample (left) and its 3D reconstruction with the FIB nanotomorgaphy (right).

Static and dynamic aspects of black-silicon formation

While the effects of each cryo-DRIE parameter on the formation of BSi have been previously described, including the molecules interactions between the plasmas and the substrate that allow the formation of microstructures, the mechanism by which a complex topography such as BSi can be generated from a flat surface is not currently well understood.

We developed a model that is capable of simulating the entire evolution of a surface from a planar substrate to a fully-developed BSi topography. We showed that the observed evolution of BSi from a planar substrate can be accurately modelled by including the long-range effects of geometric occlusion (or self-shadowing), and no role for defects or micromasking. This is the first successful attempt to model such high aspect ratio structure formation. The dynamics of BSi formation, including the early-stage development of new etch fronts at topological saddle points (reported for the first time), as well as the late-stage evolution of aspect ratio with process time, are correctly predicted by the model.

Example of modeling the BSi formation
SEM image (left) and model result (right) after a 2 min time-lapse experiment. 

The resulting simulation model corroborated with experimental observations allowed us to create a phase diagram that indicates the combination of control parameters giving rise to BSi formation, and their effect on the final BSi aspect. The complex geometry of BSi, consisting of needles and holes of various heights and depths, is captured by the model and explains the exceptionally low reflectivity of BSi.
Phase diagram of BSi aspect ration
Phase diagram of BSi aspect ratio dependence on passivation layer formation rate (vpf) and passivation layer etching rate (vpe).

BSi optical modeling

Optical modelling of subwavelength structures is a crucial step to find the optimal topographic parameters that provide reduced reflectance, increased absorbance and that meet additional constrains imposed by the application. In order to simulate micro/nano-textured surface such as BSi using FTDT, FEM or RCWA technics, it requires:
  • to model a unit cell that represents a typical structure representative of the surface. Then boundaries are imposed in this unit cell in order to represent an infinite array of structures, 
  • to provide the material bulk complex relative permittivity and permeability in function of frequency.
Getting good concordance between measurements and simulations using a unit cell based on actual BSi topography (obtained by nanotomography) is very challenging: because of complexity of the BSi structure, the model size is generaly very limited (it depends on the computer capabilities). Using simple model like single cones or inverted cones whose dimensions are based on statistical analysis from the 3D model leads also to strong discrepancies with experiments. So we proposed a more efficient equivalent multi-structure unit cell consisting of 4 inverted cones with variable dimensions based on large scale standard deviations of the structure spacing distribution calculated from a large BSi sample.


FEM simulation of BSi reflectance
Comparison of reflectance simulation with single and multiple equivalent structures based on dimensions obtained from BSi sample nanotomography with their respective measured reflectance.

Related publications 

Journals

Conferences

 

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