![Atif Syed
Nanofabrication
2
1. Introduction:
One of the fastest growing markets in consumer electronics is the Flat Panel Displays
(FPDs). The market value for FPD in 1999 was estimated to be around 18.5 billion dollars
and it was predicted to reach $70 billion by 2010 which would surpass the production of
CRTs by a huge margin [1]. There are many different types of FPD such as LCD, TFT,
Plasma and Projection Displays which can be found in the market. This report will focus
on one such technology known as the Field Emission Display (FED). A basic flow chart for
display fabrication process is given below:
Figure 1.1: Display Fabrication flowchart
The information display has always been a very critical human interface with the
computer/electronic systems. FPD’s allows us to produce HDTV at an affordable cost.
One of the most prominent features of FPD is that each pixel generates its own light
energy which allows the display to have high resolution.
Cathode
Fabrication
Grid
Attachment
Spacer
Placement
Vacuum
Seal
Anode
Fabrication
Sidewall
Attachment
Getter
Attachment
Tip off and
gettering](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Nanofabrication
- 3. Atif Syed Nanofabrication 3 2. Principle and Working of FED: FEDs work on the similar principle of the Cathode Ray Tube (CRTs) and Vacuum Fluorescent Display (VFD) where an electron gun is used with high voltage of up to 10‐ 30 kV which accelerates the electrons and those electrons excite phosphors. The only difference in FED is that instead of a single electron gun, there is a matrix‐addressed array of millions of cold emitters [1]. These arrays in short are known as the Field Emission Array (FEA). These arrays are arranged in a very close proximity usually around 0.2‐2.0 mm to a phosphor faceplate 1 (Anode) so that each phosphor pixel has individual or dedicated field emitters. FEDs consist of hermetically2 sealed glass envelope which is evacuated to form a vacuum space which allows the electrons beam to accelerate [2]. In addition to the faceplate, a back‐plate (Cathode or source of electrons), sidewalls, spacers and getters are also used in the working of FEDs. The anode and cathode are fabricated separately as described in Figure 1. The metallic stripes on the faceplate form a series of cathode lines and by the process of Photolithography, the cathode lines are fabricated in such a way that they are arranged at 90 degree angles which forms rows of switching gates and these arrays form a perfect addressable grid. After the Photolithography process, the grid can be addressed individually and the emitters in between the cathode and anode/gate lines will have enough power to produce a visible spot and all other leakage will not be visible due to high power of illumination of the visible spot. Figure 2.1: Field Emission Working Principle Diagram 1 Generally Faceplate is a type of metal or plastic plate which is designed to cover or fit on a device or a component to enhance the device’s functionality or its appearance. In this context it’s referring Anode. 2 Completely sealed or air tight so that the interior/material is not affected by outside influence. Anode Plate Cathode Plate Phosphor Dots Spacer Electrons Anode Addressing Gate Addressing Cathode Addressing Interlayer Dielectric MicroTip Emitters
- 4. Atif Syed Nanofabrication 4 The first step of the working of Field Emission is done by emitting electrons parallel to the cathode in between the space which contains ceramic spacers and they withstand any atmospheric pressure changes and adding ceramic spacers reduces the structure from collapsing. These spaces should also be able to withstand high voltage and should be invincible to users at normal viewing range. The electrode gap which is about few nanometers in length is still is a vacuum gap which required an electric potential to extract electrons from one electrode to other electrode in other words from cathode to anode. The electrons which are emitted from cathode are sent directly to the anode but one has to be careful to block the electrons from striking other surface apart from anode because this will lead to dead pixels. The current flow within the electrode gap follows Nordheim Law which determines the emitted current density in its simplest approximation and it’s given by the following equation [3]: / Equation 1 Where is the surface electric filed, B is the emitting material, J is the current density and is the initial surface electric field when the electric field is 0. With the help of Nordheim Law the addressability of the matrix is determined even with the emission being non linear in nature. Secondly, the electrons travel across the gap and the flow of electrons is from cathode to anode which in turn is accelerated to a single phosphor dot. This creates the three basic colors Red,Green and Blue. This can be further explained through the figure below: Va Figure 3.1: Tunneling of Electrons across the nano gap Electric Field Carbon Nano gap Vf Upward Force Equi‐Potential lf le
- 6. Atif Syed Nanofabrication 6 3. Nano sphere lithography process: The process was carried out with the described method in the lab script. Although the method was followed as said but there were experiments done with different chemicals and concentrations. This section will explain each chemical used which is followed by the SEM images before and after the evaporation step. The first and second steps are common for all the chemicals used. The steps and results are given below: ‐ Step 1: The silicon wafer of <100> has been cut carefully in a square of 1cm dimension. ‐ Step 2: The samples of silicon wafers are then are held over boiling acetone and propan‐2‐ol. The samples are then kept in a beaker and agitated in the ultrasonic cleaner for an hour. These samples are then kept under water so that it can be hydrophilic3 . ‐ Step 3: Polystyrene Nanosphere suspension has been used with a suspension of 0.8 micron Nanospheres (as opposed to 0.52 microns mentioned in the lab script. The reason behind this is that the Nanospheres were too small to be viewed clearly through the SEM). The above steps are common for all the chemicals used. The following chemicals and solutions were used in the experiments: De‐ionized water Ammonia ( ) Methanol ( ) Toluene ( ) [This is a substitute to Benzene] The remaining steps of the preparation of the nano beads with the chemicals mentioned above will be explained in details with the results and comments. The next sub‐section 3 The hydrophilic property has been experimented with different chemicals explained later in the report. It has been done to further investigate the possibility of making better nano sphere arrays and also by making the surface tension minimum or low enough to make a perfect array.
- 7. Atif Syed Nanofabrication 7 will include results and SEM images before the evaporation and annealing stage for the above chemicals. 3.1. Preparation of Nanosphere process: i. Water: As mentioned in the lab script, de‐ionized water was used at different concentrations as mentioned in the table 3.1 below so as to better experiment and argue on the results. Chemical Concentration(Water: Suspension) Water 1:1 Water 10:1 Water 20:1 Table 3.1: Concentration Level for Water and Suspension After experimenting with the concentrations, the substrates were put into a Petri dish and tilted at a shallow angle and one drop from each of the above concentrations have been added to each individual substrate and kept it there for few hours to dry. After the drying process has been completed, the silicon substrates were examined in the SEM. Image 3.1: SEM image with concentration 20:1 (20 drops of water into 1 drop of nano suspension)
- 10. Atif Syed Nanofabrication 10 ii. Ammonia: This was used to further experiment with the surface tension of the substrate. Ammonia is known to have the highest specific heat capacity4 which should give a large degree of freedom of molecules of the material. Also Ammonia’s molar mass is lighter than water (17.031 g/mol) and this was used to make the nano beads spread out evenly in an array. The following concentrations were used with different proportions: Chemical Concentration(Ammonia: Suspension) Ammonia 15:1 Ammonia 20:1 Ammonia 40:1 Table 3.2: Concentrations of Ammonia with different proportions Image 3.5: SEM image with concentration 15:1 (15 drops of ammonia into 1 drop of nano suspension) 4 It is the heat capacity per until mass of a material
- 16. Atif Syed Nanofabrication 16 iii. Methanol: Methanol is known to reduce the surface tension of the substrate and apart from that methanol has higher molar mass (32.04 g/mol) which is higher than the previous chemicals used. The following concentrations of methanol and polystyrene has been used: Chemical Concentration (Methanol: Polystyrene) Methanol 15:1 Methanol 20:1 Methanol 40:1 Table 3.3: Concentrations of Ammonia with different proportions The SEM images if the above concentrations are shown below: Image 3.11: SEM image with concentration 40:1 (40 drops of methanol into 1 drop of nano suspension) In the above Image 3.11, the nano beads are arranged in a much better way as compared to the previous images using other chemicals. Although the spacing between
- 22. Atif Syed Nanofabrication 22 iv. Toluene: The main reason to experiment with toluene was to make use of the property of toluene which is that its water insoluble. The following concentrations have been used and examined in the SEM. Chemical Concentration (Toluene: Polystyrene) Toluene 1:1 Toluene 5:1 Toluene 15:1 Image 3.22: SEM image with concentration 1:1 (1 drop of toluene into 1 drop of nano suspension) The zoomed out image of toluene shows that the nano beads are having more arrays as compared to the previous images. Toluene’s property of being hydrophobic allows the nano beads to spread out. Since the substrate has lower surface tension, the nano beads are found closer to the edge of the substrate which can be seen in further images.
- 30. Atif Syed Nanofabrication 30 The black spots in Image 3.35 and 3.36 signify that there were nano sphere arrays present but it eventually floated off the substrate after the drying stage. 3.2. Evaporation and Annealing Process: 3.2.1. Evaporation: The main aim of this process is to evaporate nickel in a vacuum. The vacuum allows the vapor particles to travel to the substrate directly. After this stage, the vapors are then condensed into a solid state. The following substrates were evaporated with nickel: Chemical Concentration Water 20:1(Water: Suspension) Methanol 15:1(Methanol: Suspension) Methanol 20:1(Methanol: Suspension) Methanol 40:1(Methanol: Suspension) Toluene 1:1(Toluene: Suspension) Toluene 5:1(Toluene: Suspension) The nickel deposited on the substrates was controlled to a diameter of 32‐38 nm. After inspection from the SEM, the nickel was seen on the substrate making a thin layer film deposition. Water: Image 3.37: SEM image with concentration 20:1 after evaporation (20 drops of water into 1 drop of nano suspension)
- 39. Atif Syed Nanofabrication 39 Major problems faced during the experiment: ‐ During evaporation, the substrates which were stuck on the glass slab fell off and some were hanging in the chamber and some samples didn’t had the evaporation done properly, the vertical evaporation can be seen in the following image of toluene sample. Due to this not all the evaporated samples were annealed. Image 3.54: SEM image with concentration 1:1 after annealing (1 drop of toluene into 1 drop of nano suspension) ‐ The clean room atmosphere had dirt and dust which can be clearly seen in the SEM images of the samples. ‐ The heating plate during the annealing process didn’t allowed the samples to be annealed at a temperature more than 550 C despite the fact that the results are proven to be better at temperatures more than 750 C [4] [5] and the Low‐ Pressure Chemical‐Vapor‐deposited (LPCVD) oxide thermal conductivity increases by 23% at 1150 C [6]. ‐ If the experiment was carried out with no interference with the samples (including people moving the samples while the samples are drying), much better results could have been achieved.