Back-Etching

The final step of fabricating membranes is back-etching of the original substrate after bonding of the HBR structure to a new substrate. This is done in a three-step wet chemical etching process. Such procedures of wet-etching GaAs include the use of an oxidizing agent and an acid or base used as an dissolution agent of that formed oxide. Wet chemical etching is a rather complicated process, since various effects are happening simultaneously, all depending on different conditions, lowering the overall reproducability. A profound overview of etching of GaAs devices is given in the book of Baca and Ashby [56].

In a simplified model, an oxidizer such as H₂O₂ lowers the electron content in the surface of the solid, making it hole-rich and ready to react and form the Ga oxide and As oxide, as in

GaAs+(oxidizer)→GaO_x/AsO_y (3.1)

with x, y being stoichiometric parameters, depending on the oxidizer. The rate is described by

rate=kox[oxidizer]^m{GaAs}exp

−E_ox^∗ k_BT

, (3.2)

where k_ox is a reaction prefactor, [oxidizer] is the concentration of the oxidizer, m is the number of oxidizing agent molecules involved in oxidizing single Ga and As atoms,{GaAs}is the effective concentration of GaAs available for reaction at the surface,E_ox^∗ is the activation energy of an oxidizing event, while k_B andT are the Boltzmann’s constant and the tempera-ture in Kelvin, respectively. Although oxidation can in principle limit the whole etching rate, this is not the case for most etching systems. An acid or a base can dissolve these oxides which results in ions consistent of Ga and As in the solution, as in

GaOx/AsOy+(acid or base)→Ga³⁺+AsO²₄⁻. (3.3) The etch rate is given by

rate=k_diss[acid or base]ⁿ{GaO_x/AsO_y}exp

−E_diss^∗ k_BT

, (3.4)

with all terms defined equivalently to Equation 3.2. Equation 3.4 allows one to classify a re-action asdiffusion controlled, meaning the supply of acid or base, i.e., the term[acid or base], is limited locally, or as reaction-rate limited, which corresponds to the exponential term with E_diss^∗ to be dominating. The relative magnitude of these terms in the equation is of the only importance to distinguish among the two regimes. They are in principle realizable for any reaction, since concentrations, solution mixing and ambient temperature can shift those magnitudes. These two types of etching show vastly different features that are especially ap-parent when etching micro- and nanostructures. Diffusion controlled reactions etch trenches and bulges near non-etching features like masks, due to a higher local availability of reactants, while reaction-rate limited reactions show anisotropic, so-called crystallographic etching pro-files, emerging through different lattice planes of GaAs being occupied by a distinct number of Ga or As atoms, and therefore, differing in their activation energy, decreasing the reaction rate of Ga rich faces up to the factor of 5. A crucial point on these properties is that, when trying to lower etch rates by diluting solutions, one might unwantingly change the complete type of reaction, resulting in undesired etch profiles.

A general remark about wet etching includes that many etching systems yield very smooth surfaces, when the pH value is kept below 7.5 and H2O2 concentrations <15 %, otherwise, the oxide dissolution reactions are carried out too slowly, resulting in thick oxide layers that flake off as a whole, increasing surface roughness. Another remark is that an aged solution

might not have a constant etch rate, due to decomposition of reactants. Furthermore, solutions should be given time when mixing supplies, since this resembles an either endo-or exothermic reaction, altering etching rates, especially fendo-or reaction-rate limited processes.

Evaporating by-products might cool the solution more than expected when operated under a fume hood, therefore a cover of the beaker with a watch glass or petri dish is advisable.

Slow agitation with a magnetic stirrer can help to better reproduce etch rates and uniform thinning in diffusion controlled reactions, fast agitation has the possibility to alter undercut aspect ratios, when using masks to wet etch micro- and nanostructures. Agitation can also help prevent bubbles, that might form in solutions with a high concentration of H2O2, since they locally mask the surface from the etchant. Another suggestion for good reproducability is to choose etch systems with slow rates, as uncertainties are minimized, especially for short etching times. To stop the reactions on time, the etching agent is washed off in a continuous flow of DI water or dips in several beakers with fresh DI water, since reactants left on the sample surface can continue etching also in air.

Preparation for Back-Etching Membrane Samples To remove the original substrate of the sample, the new substrate, as well as the edges of the sample, need to be covered for not being attacked by the etching solution. For this purpose, some photoresist like MicroChem S1805 or S1818 can be used as an adhesive and acid repellent. A drop of it is put on a chip-carrier, e.g., a piece of Si, and the walls are covered carefully using a syringe. When the surface of the original substrate is free of drops, the photoresist can be cured, otherwise the sample should be put in acetone to repeat the process. Curing is done in a convection oven for 15 min at 90 °C. The original substrate should be kept free of any contamination that could slow down the etching process. Pre-cleaning by a dip in diluted ammonium hydroxide is recommended for better reproducability by Baca and Ashby [56] but was not tested, since results were already satisfactory.

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Figure 3.4–(a)The prepared sample is put in a H3PO4solution for fast etching and should be stopped in time before reaching the sacrificial layer. (b)The process is continued with the selective solution with C6H8O7. (c)The etch-stop layer is removed in a short dip in diluted

HF.(d)Dissolving the photoresist releases the sample from the Si carrier.

Phosphoric Acid The first of three etching steps used employs the H₃PO₄/H₂O₂ system.

The volumetric mixing ratio is 3H3PO4:7H2O2 using 85 % orthophosphoric acid and 30 % hydrogen peroxide, resulting in anisotropic and very smooth etching profiles. In Ref. [57], an etch rate of 4 µm min⁻¹ is reported without agitation, however, experience shows an etching rate of approximately 5µm min⁻¹ when agitated with a magnetic stirrer at 150 rpm. Mixing the solution is performed at 300 rpm for at least 15 min, as this is an exothermic reaction, boosting the etching rate undeterministically, if samples are put into the solution too soon.

Experience shows that it is necessary to turn samples 90° every 5 min during etching to avoid trench patterns due to laminar flow of the agitation. The process needs to be stopped in time, before the sacrificial layer is reached. For that purpose, the sample can be taken out of the solution, rinsed with DI water to stop further etching and examined under the microscope. Thanks to a scale on the focus screw of the microscope, the substrate thickness that is left can be estimated. Etching sould be continued until the substrate thickness is well below 100 µm. Since the etch rate is also dependent on factors like sample size, it is hard to provide general information about etching time but 3×3 mm² on 9 ×9mm² Si carriers can be etched for 60-65min. However, decomposing supplies can change this expectation.

Citric Acid Following the fast etch with phosphoric acid, the C₆H₈O₇/H₂O₂ system is used for selective but slower etching of the GaAs substrate. An aqueous solution is obtained by mixing citric acid monohydrate powder with the same weight of DI water. Since this mixing resembles an endothermic reaction, a mixing time of a day is recommended in [56]. However, since the etch rate is not crucial within this approach, the recommendation is neglected, since etching will uniformly stop at the sacrificial layer due to the solution’s high selectivity between GaAs and Al_0.7Ga_0.3As. Mixing the solution under the same circumstances like the phosphoric acid system, but using a volumetric ratio of 4C₆H₈O₇:1H₂O₂, results in anisotropic and smooth etching with a rate of approximately 0.5µm min⁻¹ [58], significantly lower than etching with the phosphoric acid system. Reaching the sacrificial layer is clearly visible, since it emerges as a very shiny and flat surface, showing a bright color.

Hydrofluoric Acid The final step is the removal of the sacrificial layer, by dipping the sample into 10 % diluted HF.Caution! HF is not only caustic but also toxic and must not be handled without proper training! The use of appropriate protective equipment is mandatory!

Since the acid is able to etch glass, etching is performed in PTFE-coated beakers. The high selectivity of the acid between GaAs and AlxGa1−xAs forx >0.45arises from the lack of an oxidizing agent in the solution. The removal of the sacrificial layer can be followed by eyes, since its degradation shows changing colors due to thin-film interference. A dip for 15 s has shown to be enough for a 400 nmthick layer of AlGaAs.