User:Azeredo1/Metal Assisted Chemical Etching

Definition
Metal Assisted Chemical Etching is a novel wet-etching technique used to prepare porous and crystalline features – e.g. nanowires, micro-nano channels - in bulk semiconductors. It relies on an electrochemical reaction between a semiconductor surface and a solution of hydrofluoric acid and hydrogen peroxide catalyzed by a noble metal architecture. With applications in electronics, optics, plasmonics, energy storage and conversion, this technique is an economical, scalable and, sometimes, self-assembled alternative for the semiconductor industry to manufacture advanced materials for devices such as battery anodes, solar cells, laser cavities, and light emitting diodes.

Overview
Similar to any electrochemical process, the mechanism of Metal Assisted Chemical Etching (MacEtch) encompasses the existence of a local cathode and anode, and its reactions. In such model, the metal catalyst and semiconductor interface represent the cathode and anode, respectively. Through the metal catalyst, charge injection is sustained from solution to substrate and charge is balanced by the cathodic and anodic reactions. The presence of a local site for reaction defines the selectivity of this etching mechanism; bulk Si, for example, etches at a 10nm per hour rate in MacEtch, while the regions at the vicinity of the metal catalyst etch at much faster rates in the order of 1-10nm/min. Several parameters have a significant role during this process. Among which these parameters, researchers have documented the effect of UV illumination, temperature, wafer orientation, catalyst type and shape, and HF to H2O2 molar ratio. It has also been reported the effect of these properties on the morphology of the etched semiconductor surface as well as its resultant crystallographic structure. Most of this characterization work has mostly been done for Silicon.



Theory
At the cathode, it is generally accepted in literature that the reduction of hydrogen peroxide takes place resulting in the injections of holes from solution into the substrate (e.g. Si) through the catalyst. Other cathodic reactions are suggested in literature. At the anode, three mechanisms for Silicon dissolution have been proposed some of which are shown in the Figure 2.

The Effect of Intrinsic Silicon Properties and Etching Parameters on Etch Rate and Side-Wall Characteristics
In light of the proposed mechanisms for MacEtch, it is expected that dopant concentration, temperature, illumination and the molar ratio of HF:H2O2 have a significant effect on the kinetics of the aforementioned mechanisms. For example, it has been observed experimentally that etch rate for Silicon during MacEtch increases linearly with time and non-linearly with temperature. However, the effect of illumination is still not well understood during MacEtch.

The directionality or anisotropy of MacEtch could be explained by a back-bond breaking theory that is based on the crystallographic structure of Silicon and other semiconductor. This theory assigns an energetic cost to break these bonds. On a surface of a wafer, such energy barrier to break a bond is different depending on what facet of the crystal exposed and the number of exposed back bonds. Etching would occur towards the most energy favorable direction. In contradiction with this theory, Zhang, M. et al. have found that etching followed different crystallographic directions for two distinct doping levels of Silicon (p-n type). Since this experiment has been conducted under constant etchant molar ratio (HF:H2O2) and etching time, the back-bond theory would not fully explain the anisotropy of MacEtch.

It has been found that the orientation of a Si wafer plays an important role in creating slanted or straight one dimensional structures (e.g. wires and pores). While researchers have been able to create nanowire arrays onto (100), (110) and (111) Si wafers, they all yield different etching profiles along the direction of the nanowire. At certain etching conditions, the etching direction of a (110) wafer may alternate randomly between the [100] and [010] directions.

Crystallographic Properties of Silicon Treated with MacEtch
For a number of applications of semiconductor based technologies, R&D is often concerned with the crystallographic structure of post-processed materials. Textured semiconductor surfaces are interesting for designing highly absorbing solar cells with increased efficiency. The electrical properties of single crystal, polycrystalline and amorphous semiconductors are orders of magnitude apart. In silicon solar cells, the electron-hole pairs generated by UV exposure are lost in heat dissipation as a results of the lower conductivity of porous Silicon. This fundamental difference in the crystalographic structure translates into whether or not the energy conversion device is feasible. The same line of thought can be applied to thermoelectric energy harvesters and coolers. Optical, thermal and mechanical properties are also significantly different in this case. Thus, researchers have characterized the effect of MacEtch on the crystallographic structure of Silicon. In order to analyse such characteristics of Si, a technique called Transmission Electron Microscopy is commonly used as it can provide atomic resolution. This characterization process is costly and slow, limiting the throughput rate of number of samples that can be analysed. Despite these challenges, initial studies have shown that if the etchant molar ratio ([HF]/[HF]+[H2O2]) is held constant, the MacEtch of higher doped Silicon for both p- and n-type Silicon yields rougher side-walls and sometimes porous structures. Later, Chartier C. et al have established a well-accepted theory on the meso- and macro-porous Si formation during MacEtch. This theory elaborates on the hole injection current density generated with different etchant molar ratios during MacEtch and establishes the regimes of such composition for which different Si crystallographic structures can be obtained. Their main conclusion relative to MacEtch is that with the same doping level (in their experiment, p+ type, boron doped Si), it is possible to obtain meso-, macro- and single-crystal Silicon after Macetch.



Integration with Silicon Manufacturing Methods
Once the anisotropic regime was established, a cascade of researchers have demonstrated the integration of MacEtch with planar lithographical processes – interference lithography, solid-state ionic imprinting, nanoimprint lithography, e-beamlithography – and self-assembled metallic patterning processes (Anodic Alumina Oxide). The latter methods represent ways to define a 2-D metallic architecture on top of a wafer and the former procedure to sink down the metal catalyst and form 3-D cavities in Silicon (Figure 3).

Lithium-ion Battery
Single-crystal Silicon is theoretically the ideal material for lithium-ion battery anodes with charging/discharging rates and storage capacity 4-10 times greater than conventional graphite. However, Silicon undergoes a volumetric expansion of 400% during the battery lithiation cycle, causing the mechanical stresses to destroy the anode's material continuity. Researchers currently propose the use of porous Si and Si nanowires as mechanical architectures that can better accommodate for such observed expansion. It is also proposed the use of MacEtch as an economical alternative process for fabricating such high-performance anodes.

Thermoelectric Energy Harvesters
Through the Seebeck Effect, thermoelectric devices convert low-quality (below 150C Celsius) waste heat into the generation of useful electric work. Such device could recover a percentage of the 15 terawatts of heat waste from thermal sources in the world. Bismuth Telluride - the most commonly used thermoelectric material - is not found in abundance in nature and thus is not suitable for large-applications in energy generation. Researchers are proposing the use of nanostructured Silicon as an efficient thermoelectric material with a thermoelectric figure of merit equivalent to that of Bismuth Telluride. Metal Assisted Chemical Etching of Silicon becomes an important tool to create 1-D, 2-D and 3-D designs from bulk materials through its various integrations with planar lithographically and self-assembled patterning processes discussed previously.

Further Developments
In the Metal Assisted Chemical Etching literature, there is much to be developed. Due to difficulties to perform in situ characterization of chemical reactions, researchers have not proved which mechanisms take place and are dominant in this process. The use of cheaper catalyst materials is another important advance yet to be made. Currently, Au and Pt - both expensive metals - have been extensively researched and the robustness of the process is high with such metals. However, cheaper metals hold the promise of using this technique in industry-level fabrication. Finally, this technique have been extensively applied to Silicon, but an entire set of compound semiconductors has not been demonstrated to be patternable by MacEtch.