Nanostructured anti-reflective coatings for substrates

#The present disclosure concerns a process for depositing materials, generally as thin films, on substrates, particularly a chemical bath deposition process that uses a microchemical system comprising a micromixer, a microchannel fluid applicator, or both, and articles made according to the process, including nanostructured, anti-reflective coatings for transparent and absorbing surfaces.

#Chemical bath deposition (CBD), the aqueous analog of chemical vapor deposition with the constituent ions dissolved in a water solution, can be used to deposit epitaxial layers at low temperatures. For example, CBD has been successfully used as a batch process to deposit many semiconductor compounds, including Ag2S, CdS, CdSe, CoS, Cu2S, PbS, SnS, SnO, MoS2, SbS, TiO2, ZnO, SnSe, CdSnS, CuInSe2, and PbHgS. Batch CBD processes also have been used to make thin film transistors. Gan et al., U.S. Pat. No. 6,225,149, for example, entitled “Method to Fabricate Thin Film Transistors and Circuits,” describes forming thin film transistors by preparing a chemical deposition bath and depositing a semiconductor layer using the chemical bath. Other methods for depositing thin films are disclosed in U.S. Publication No. 2007/0184576, which is incorporated herein by reference.

#Though CBD has many advantages, it also has drawbacks. One major drawback is the formation of particles, as well as the unwanted deposition that occurs with such processes, which generates waste and causes device defects. For batch CBD processes, the heat needed for chemical reaction may be supplied from the solution bath to the sample surface. With reference to CdS as an example, this results in both heterogeneous CdS nucleation at the surface as well as homogeneous CdS formation in the bath. Hence, with thermal jacket baths (glass beaker etc.) or water baths, significant CdS deposition also occurs on the walls of the vessels. The bath must be stirred continuously to ensure uniform thermal and chemical mixing and to minimize adhesion of homogeneously nucleated CdS particles to the growing film surface. Moreover, the unequal bath volumes used to form desired CdS films generates substantial waste and also creates device defects. Batch CBD processes also typically require that the deposited material be subjected to relatively high-temperature annealing processes. These high temperatures generally are not compatible with substrates, such as polymeric materials, that are used to make flexible electronic devices, such as flexible transistors, or plastic (e.g., polycarbonate) eyeglass lenses.

#FIG. 1 is a schematic diagram illustrating CBD-CdS growth mechanisms.

#FIG. 2 is a schematic diagram of one embodiment of a continuous flow microreactor system.

#FIG. 3 is a schematic diagram of one embodiment of an impinging flow bath deposition reactor, with the inset illustrating one embodiment of an interdigital micromixer.

#Unless otherwise indicated, all numbers expressing quantities of ingredients, molecular weights, sizes, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Unless otherwise indicated, non-numerical properties such as amorphous, crystalline, homogeneous, and so forth as used in the specification or claims are to be understood as being modified by the term “substantially,” meaning to a great extent or degree. Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters and/or non-numerical properties set forth are approximations that may depend on the desired properties sought, limits of detection under standard test conditions/methods, limitations of the processing method, and/or the nature of the parameter or property. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word “about” is recited.

#The fundamental aspects of CBD are similar to that of a CVD process. Without being limited to a theory of operation, CBD generally involves mass transport of reactants, adsorption, surface diffusion, reaction, desorption, nucleation, and growth. Earlier studies suggested a colloidal-by-colloidal growth model. I. Kaur, D. K. Pandya, K. L. Chopra, J. Electrochem. Soc., 1980, 140, 943. However, more recent investigations by Ortega-Borges & Lincot, based on initial rate studies using a Quartz Crystal Microbalance (QCM), suggested that different growth kinetics are involved. R. Ortega-Borges, D. Loncot, J. Electrochem. Soc., 1993, 140, 3464. Ortega-Borges et al. identified three growth regimes: an induction period with no growth observed; a linear growth period; and finally a colloidal growth period, followed by the depletion of reactants. Ortega-Borges et al. proposed a molecular level heterogeneous reaction mechanism for CdS deposition according to the equations provided below.

#Cd ⁡ ( NH 3 ) 4 2 + + 2 ⁢ OH - + Site ⁢ ← → k 1 k - 1 ⁢ Cd ⁡ ( OH ) 2 ⁢ ⁢ ads + 4 ⁢ NH 3 [ 1 ] Cd ( OH ) 2 ⁢ ⁢ ads + SC ⁡ ( NH 2 ) 2 ⁢ ⟶ k 2 ⁢ [ Cd ( SC ⁡ ( NH 2 ) 2 ⁢ ( OH ) 2 ] ads ⁢ [ Cd ( SC ⁡ ( NH 2 ) 2 ⁢ ( OH ) 2 ] ads ⁢ ⟶ k 3 ⁢ CdS + CN 2 ⁢ H 2 + 2 ⁢ H 2 ⁢ O + Site [ 2 ]