The overall goal of this investigation is to develop electrospray film deposition of chalcogenide glasses for mid-infrared photonic applications, and to explore the unique flexibility of this deposition method. The main advantages of this technique are that it allows localized deposition of material with the ability to fabricate an engineered film thickness profile. This makes it possible to create a spatially-varying compositional gradient.
Due to these advantages electrospray can advance the field of chip-based mid-infrared chemical sensing devices by allowing more flexible device design. To set up the deposition process, first place the end of the needle into the chalcogenide glass. Draw the solution into the syringe by setting the syringe pump to Extract Mode at a slow rate, such as 150 microliters per hour to prevent the formation of bubbles.
Set the working distance between the end of the nozzle and the top of the silicon substrate by using the computer numerical control in Manual Movement Mode. Place the silicon substrate on an aluminum plate connected to the power supply ground return. Then, dispense some liquid from the syringe, utilizing the syringe pump, to allow a small volume of liquid to coat the outside surface of the nozzle.
Turn the hot plate on at a surface temperature of about 75 to 100 degrees Celsius. Wait for approximately two hours to allow a film of glass to dry on the nozzle surface. Connect the direct current or DC power supply to the syringe nozzle with an electrical clip.
Set the flow rate at 10 micro-liters per hour and tune the DC voltage to form a stable Taylor cone. View the spray with a high magnification camera. Once the spray is stable, start computer numerical controlled motion of the spray over the substrate to deposit the film.
Use a serpentine path for uniform thickness. Use passes with a distance longer than the width of the substrate, such that the spray moves completely off of the substrate before making the next pass. This ensures that the flow rate of liquid is the same at every point on the substrate.
Alternatively, use one-dimensional passes for a linear thickness profile. Subject the deposited film to a series of heat treatments under vacuum as described in the text protocol. Take a transmission Fourier Transform InfraRed, or FTIR spectrum, periodically throughout the annealing conditions.
To measure the same location on the sample each time, draw an outline of the substrate on the sample stage and place it within the south line each time a measurement is taken. In the FTIR software, click Experiment Setup and type in the number of scans as 64. Click to the Bench tab and type in the scan range as 7, 000 to 500 inverse centimeters.
Take a background scan with just the sample stage in the instrument by clicking on Collect Background, then place the sample on the stage and click Collect Sample to take the spectrum of the sample. To track the removal of the solvent, estimate the size of the organic absorptions in the film matrix. In the FTIR software, draw a baseline in the spectral range of interest of approximately 2, 300 to 3, 600 inverse centimeters.
The software calculates the area beneath the transmission spectrum of the sample relative to the baseline designated by the user. Scratch the film with fine point tweezers until the dark substrate becomes visible amongst the lighter colored film, which typically occurs in one scratching motion with light pressure. Remove debris caused by scratching with compressed nitrogen.
Load the sample into a contact profilometer and measure the thickness of the blanket films to determine the step height from film to substrate. Open Measurement Setup and type in a scan rate of 0.1 millimeters per second and a scan length of 500 microns. Locate the scratch and rotate the sample, such that the scratch is oriented in the left to right direction.
Move the state, such that the crosshairs are just below the scratch, and begin the surface scan by clicking Measurement. Once the scan is finished, drag the R and M cursors so that they are both on the film surface, and click Level Two Point Linear to level the surface profile. Move one cursor to the bottom of the scratch and write down the distance between each cursor position in the Y dimension.
Measure the thickness as multiple locations to obtain an average thickness and variance in the data. Determine the thickness profiles of the non-uniform thickness films by scanning the profilometer across the entire film. Use this surface profile to create a graph of film thickness versus position.
Scan across the entire film by entering an appropriate scan length greater than the width of the film, usually 10 to 20 millimeters, in Measurement Setup. Place the crosshairs on the uncoated substrate on one side of the film and click Measurement, allowing the profilometer to complete the scan on the uncoated substrate on the other side of the film. Right click on the surface profile and export as a csv file.
Next, measure the surface roughness with a white-light interferometer. Adjust the focus and stage tilt to generate interference fringes over the entire measurement area using the 5X objective. Take five measurements across the uniform thickness film to determine the average roughness and variance of the data.
Load the sample into an ellipsometer and measure the refractive index in the range of 600 to 1, 700 nanometers wavelength. In this case, use an angle of incidence of 60 degrees, and focus the beam to a spot size of 35 microns. Representative results of electrosprayed film properties are shown here.
FTIR transmission spectra of pure ethanolamine solvent and a partially cured arsenic trisulfide film with residual ethanolamine showed the transparency of the films in mid-infrared and also the method by which the solvent peak absorption area is characterized. By characterizing the solvent peak absorption area, and film thickness throughout heat treatments, the evolution of solvent removal and film densification is demonstrated. Minimal residual solvent is achieved by vacuum annealing.
Following deposition and heat treatment, electrosprayed films have a generally smooth appearance. On the left is a film with a uniform thickness region deposited with a serpentine path of the spray. On the right are films made with one-dimensional motion in order to achieve controlled non-uniform thickness profiles.
One-dimensional motion allows linearly-sloped film thickness profiles with film thickness and coverage area controlled by the working distance of the spray. In conclusion, electrospray is the most treated as a method of infrared transparent chalcogenide glass fume deposition with the ability to make localized depositions with engineered non-uniform thickness profiles. Electrospray of chalcogenized glass is a new tool for the field of mid-infrared microphotonics, and it is envisioned that the additional flexibility allowed by this process will lead to novel device designs.
