Titanium dioxide TiO2 (Titania) and Silicon dioxide SiO2 (Silica) dielectric materials which they have been used in fabricating the multilayer thin films. These Dielectric films are often used as elements of optical devices in the visible and the infrared (IR) ranges due to their high thermo-stability, high resistance against wear and corrosion, and trivial absorption [66, 67].
5.1.2 Substrates needed for deposition process
The optical filters including a plurality of dielectric layers are provided on a fused silica (SiO2) which it is a synthetic molten, amorphous quartz glass. Quartz glass is advantageous as a substrate material because it has relatively low loss at wavelengths commonly used in optical filters. Moreover, it is thermally and chemically stable, and has good adhesion to most dielectric films used quarter and half wave layers. Further, quartz is mechanically tough, relatively inexpensive and readily available[68,69].
5.2 Fabrication Process
Ion-assisted deposition is an energetic process that has the great advantage that it is easy to implement in conventional equipment. It consists of thermal evaporation to which has been added bombardment of the growing film with a beam of energetic ions. All that is required to put it into operation in a conventional plant, therefore, is the addition of an ion gun. The most common types of ion sources for this purpose are broad-beam, often with extraction grids. Much of the published research and reported successes have been with the Kaufman  or gridded type of ion gun. In that, the source of electrons is a hot filament and the extraction system consists of two closely aligned grids, the inner floating and acquiring the potential of the discharge so that it confines it within the gun, and the second applying a field to draw the positive ions out of the discharge chamber through the apertures in the inner grid. The beam of ions is neutralized outside the discharge chamber by adding electrons, usually from a hot filament, immersed in the beam to avoid space charge limitation, or from a separate hollow cathode electron emitter. The grids are fragile and easily misaligned or damaged and so some effort has been put into the development of sources that do not require extraction grids and they are being used in increasing numbers in production. For further information see Bovard  , Fulton  and Pulker [*18].
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The ionized plasma-assisted deposition process includes features of both ion assisted deposition and low-voltage ion plating. It makes use of what is known as an advanced plasma source [73-75]. The source, which is insulated from the chamber and floats in potential, is of simple construction. A central indirectly heated cathode is made of lanthanum hexaboride. This lies along the axis of a vertical cylinder that is the anode. A noble gas, usually argon, is introduced into the source. The cylinder contains a solenoid that produces an axial magnetic field. The crossed electric and magnetic fields make the electrons move in cycloids with the usual increase in path length and degree of ionization, so that an intense plasma is produced in the source. The fields do not confine the plasma axially and so it escapes from the source into the chamber. There the electrons, that are very mobile, escape preferentially to the chamber structure leaving the plasma charged positively without the need for isolated substrate holders. The deposition sources are thermal, usually electron beam, and they emit evaporant into the plasma where it gains energy and is partially ionized. The evaporant then condenses on the growing film with additional energy, as in ion plating, and is bombarded simultaneously by ions from the plasma as in ion-assisted deposition. For reactive processes, the reacting gas is not fed into the source but into the plasma as it leaves the source. A ring-shower-shaped inlet tube is positioned just above the aperture of the source for this purpose. The process has been very successful in the production optical filters. It seems clear that the major benefit of the energetic processes is an increase in film packing density. The improvements are achieved at comparatively low substrate temperatures which help with the difficult coating of plastic substrates.
Figure 5.1 The addition of ion bombardment of the growing film
transforms conventional thermal evaporation into ion-assisted
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Figure 5.2 The advanced Plasma Source. (Leybold AG, Hanau, Germany.) (a) Diagram of the advanced plasma source (APS) and the arrangement of the machine for plasma ion-assisted deposition (PIAD). The monomer inlet shown is used in the construction of the final anti-smudge coat in the coating of spectacle lenses. (b) Photograph of the interior of the system showing the electron-beam sources and just slightly to the right of the centre the cylindrical advanced plasma source.
Characterization of the fabricated optical filters
Characterization by field emission scanning electron
The surface morphology of the single layer of TiO2 and SiO2 was characterized by field emission scanning electron microscopy (FESEM). In addition, Cross section micrograph of the fabricated optical filters was characterized by FESEM.
The basic principle:
A FESEM is a microscope that uses a beam of energetic electrons to examine objects on a very fine scale. These electrons are liberated by a field emission source. The object is scanned by an electron beam according to a zig-zag pattern . It is used to visualize very small topographic details on the objects surface (entire or fractioned). This technique is applied to observe structures that may be as small as one nanometer.
The FESEM works as follow, electrons are liberated from a field emission source and accelerated in a high electrical field gradient. Within the high vacuum column these so-called primary electrons are focused and deflected by electronic lenses to produce a narrow scan beam that bombards the object. As a result secondary electrons are emitted from each spot on the object. The angle and velocity of these secondary electrons are related to the surface structure of the object . A detector catches these secondary electrons and producing an electronic signal, see Fig. 5.8. Also Fig. 5.9 shows the interaction of electrons with sample surface. The produced signal is amplified and transformed to either a video scan-image that can be displayed on a monitor or to a digital image to be saved and processing for further.
No preparation was needed for the fabricated samples.
The fabricated samples were fixed on the motorized stage inside the specimen chamber.
Figure 5.8 A schematic diagram for FESEM.
Figure 5.9 Interaction of electrons with sample surface.
In the FESEM, the stage can be repositioned in the chamber by means of a joy stick that steers in left right axis, or forward and backward. In addition, it can be tilted, rotated and moved in Z direction.
The electron column and the specimen chamber are then evacuated.
As a result of the primary probe bombards the sample, secondary electrons are emitted from the sample surface with a certain velocity that is determined by the levels and angles at the surface of the sample.
The secondary electrons, which are attracted by the Corona, strike the scintillator that produces photons.
The signal produced by the scintillator is amplified and transduced to a video signal that is fed to a cathode ray tube in synchrony with the scan movement of the electron beam.
The real time image that appears on the screen reflects the surface structure of the object. Parallel to this analog image, a digital image is generated which can be further processed.
The FESEM model used in our characterization was Leo Supra 55, as pictured in Fig.s 5.10 and 5.11.
Figure 5.10 Leo Supra 55 FESEM.
Figure 5.11 The motorized stage.
5.3.3 Characterization by the spectrophotometer
The transmission spectra for all the samples were measured using PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer (Fig. 5.12). The LAMBDA 950 is one of the highest performances UV/Vis/NIR System. Choose this model for ultra-high UV/Vis/NIR performance for wavelengths up to 3,300 nm, high precision measurements, and for applications such as highly reflective and anti-reflective coatings, color correction coatings, band pass characteristics of UV, Vis and NIR filters, and more[79,80].
Figure 5.12 PerkinElmer LAMBDA 950 UV/Vis/NIR research
The Lambda 950 Spectrometer features a reflecting, double-monochromator optical system. The optical components are coated with silica for durability. Holographic gratings are used in each monochromator for the UV/Vis range and the NIR range. The optical system is depicted schematically in Fig. 5.13
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Figure 5.13 The optical system of the spectrophotometer
Two radiation sources, a deuterium lamp (DL) and a halogen lamp (HL), cover the working wavelength range of the spectrometer. For operation in the near infrared (NIR) and visible (Vis) ranges, source mirror M1 reflects the radiation from the halogen lamp onto mirror M2. At the same time it blocks the radiation from the deuterium lamp.
For operation in the ultraviolet (UV) range, mirror M1 is raised to permit radiation from the deuterium lamp to strike source mirror M2. Source change is automatic during monochromator slewing.
Radiation from the respective source lamp is reflected from mirror M2 via mirror M3 through an optical filter on the filter wheel assembly (FW) to mirror M4.
The filter wheel is driven by a stepping motor to be in synchronization with the monochromators. Depending on the wavelength being produced, the appropriate optical filter is located in the beam path to prefilter the radiation before it enters the monochromator. Filter change is automatic during monochromator slewing.
From mirror M4 the radiation is reflected through the entrance slit of Monochromator I. All slits are located on the slit assembly (SA). The radiation is collimated at mirror M5 and reflected to the grating table G1. Depending on the current wavelength range, the collimated radiation beam strikes either the UV/Vis grating or the NIR grating (NIR version only).
The radiation is dispersed at the grating to produce a spectrum. The rotational position of the grating effectively selects a segment of the spectrum, reflecting this segment to mirror M5 and then through the exit slit. The exit slit restricts the spectrum segment to a near monochromatic radiation beam. Grating change is automatic during monochromator slewing.
The exit slit of Monochromator I serves as the entrance slit of Monochromator II. The radiation is reflected via mirror M6 to the appropriate grating on grating table G2 and then back via mirror M6 through the exit slit to Mirror M7. The rotational position of grating table G2 is synchronized to that of G1. The radiation emerging from the exit slit exhibits high spectral purity with an extremely low stray radiation content.
In the UV/Vis and NIR range a choice is provided between a fixed slit width, a servo slit, and a slit program. When the servo slit is selected, the slit widths change automatically during scanning to maintain constant energy at the detector.
From mirror M7 the radiation beam is reflected via toroid mirror M8 to the chopper assembly (C). As the chopper rotates, a mirror segment, a window segment and two dark segments are brought alternately into the radiation beam.
When a window segment enters the beam, radiation passes through to mirror M9 and is then reflected via mirror M10 to create the reference beam (R).
When a mirror segment enters the beam the radiation is reflected via mirror M10Î„ to form the sample beam (S).
When a dark segment is in the beam path, no radiation reaches the detector, permitting the detector to create the dark signal.
The radiation passing alternately through the sample and reference beams is reflected by mirrors M11, M12, M13, and M11Î„, M12Î„, M13Î„, respectively of the optics in the detector assembly onto the appropriate detector. Mirror M14 is rotated to select the required detector.
A photomultiplier (PM) is used in the UV/Vis range while a lead sulfide (PbS) detector is used in the NIR range. Detector change is automatic during monochromator slewing.
At the cell plane, each radiation beam is approximately 12 mm high. The width of the radiation beams is dependent on the slit width. At a slit width of 5 nm each radiation beam is approximately 4.5 mm wide.
To permit minimum sample volumes to be measured in micro cells, the height of the radiation beam must be reduced in the active cell area.
A common beam mask (CBM) is mounted between the slit assembly (SA) and mirror M7. This mask restricts the cross-section of both the sample beam and the reference beam in the respective cell area. The radiation beam can be reduced from the maximum height of 11.7 mm to 0.0 mm in 50 steps.
During all scanning operations, the monochromators stop slewing while a filter, source, or detector change is in progress.
The spectrophotometer is Switched on, then the program "Perkin Elmer UV Winlab" starts (the opposite can't be done).
A new method that contains the parameters is opened by clicking "new-method".
In the "select type of instrument" field, "High performance UV/Vis/NIR instrument" has been chosen then clicking "Next".
The proper instrument is chosen, so "Next" is clicked.
The method type according to the measurement has been chosen, most of the times it is "scan". Then "Next" is clicked.
The accessory type is chosen. If transition or the integrating sphere compartment is used, ticking nothing would be done. If universal reflection compartment is used, ticking its box would be done. Then "Next" is clicked.
Clicking "save/next", naming the method and pressing "OK".
Another window with the method name would be opened. On the tree at left, "select Data Collection".
The spectrum range, step and quantity are typed to measure (A: absorption, E1: sample energy, E2: reference energy, etc...).
Clicking "Sample Info" in the left tree to type the number of samples in the field above the table, additional rows would be added to the table according to the number of samples and renaming the samples from the sample ID is possible.
At first, auto-zero (baseline) is done, by measuring the spectrum of the two beams and eliminating the difference between them to calibrate the reference detector and the sample detector.
No samples inside should be checked then click "run" button to start.
A message would appear asking to put the samples.
The sample in the outermost side and the blank substrate (if needed) in the other one have been put, and then click "OK" to start scanning.
After the graph is plotted, the zooming in a certain area can be done by selecting it and double click. To make all the results in one graph, select "tools menu-options-all samples on one graph".
To print the results, "Reporting" is chosen from the left tree; the wanted template was selected. The most common templates are: "Default scan" to print all results on one graph and "test" to print each result alone. It is possible to preview the report by pressing "preview" button then print it.