Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

Summary

A method of uniform thickness solution-derived chalcogenide glass film deposition is demonstrated using computer numerical controlled motion of a single-nozzle electrospray.

Abstract

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge₂₃Sb₇S₇₀ and As₄₀S₆₀, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 °C and 200 °C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As₄₀S₆₀ film as compared to Ge₂₃Sb₇S₇₀. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

Introduction

Chalcogenide glasses (ChGs) are well-known for their broad infrared transmission and amenability to uniform thickness, blanket film deposition ^1-3. On-chip waveguides, resonators, and other optical components can then be formed from this film by lithography techniques, and then subsequent polymer coating to fabricate microphotonic devices ^4-5. One key application that we seek to develop is small, inexpensive, highly sensitive chemical sensing devices operating in the mid-IR, where many organic species have optical signatures ⁶. Microphotonic chemical sensors can be deployed in harsh environments, such as near nuclear reactors, where ex....

Protocol

Caution: Please consult Material Safety Data Sheets (MSDS) when working with these chemicals, and be aware of the other hazards such as high voltage, mechanical motion of the deposition system, and high temperatures of the hotplate and furnaces utilized.

Note: Begin this protocol with bulk chalcogenide glass, which is prepared by well-known melt-quench techniques ².

1. Preparation of ChG Solutions

Note: Two solutions are utilized in this study, Ge₂₃Sb₇S₇₀ and As₄₀S₆₀, both dissolved in ethanolamine at a concentration of 0.05 g/ml. The preparat....

Results

A schematic representation of the serpentine path utilized to obtain uniform thickness films with single nozzle electrospray is shown in Figure 2. Figure 3 shows an example transmission FTIR spectrum of a partially-cured As₄₀S₆₀ film made with serpentine motion of the spray, as well as the spectrum of pure ethanolamine solvent. From the information that can be obtained from the FTIR spectra such as shown in Figure 3,.......

Discussion

At the beginning of a uniform thickness film deposited with serpentine motion of the spray relative to the substrate, the film thickness profile is increasing. Once the distance travelled in the y-direction exceeds the diameter of the spray (upon arrival at the substrate), the flow rate becomes approximately equivalent for every point on the substrate, and thickness uniformity is achieved. To determine the appropriate deposition parameters of a uniform thickness electrosprayed film, theoretical film thickness, T, is util.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ethanolamine	Sigma-Aldrich	411000-100ML	99.5% purity
Si wafer	University Wafer	1708	Double side polished, undoped
Syringe	Sigma-Aldrich	20788	Hamilton 700 series, 50 microliter volume
Syringe pump	Chemyx	Nanojet
CNC milling machine	MIB instruments	CNC 3020
Power supply	Acopian	P015HP4	AC-DC power supply, 15 kV, 4 mA