Motivation

In fact, we can deposit thin film composed with zinc oxide by several ways, including chemical methods and physical methods and. Different thin film fabrication mechanisms would influence the characteristics of zinc oxide like grains size, defect kinds, and thin film quality very much. Another aspect we need to consider is prime cost during all the device manufacturing process. And the process integration of ZnO TFTs must be suitable for several different substrate materials like silicon wafer for research and glass for real mass manufacturing.

ZnO thin film deposited for TFTs reveals polycrystalline with a hexagonal wurtize structure and has a preferred orientation with the c-axis

perpendicular to the substrate [6]. Intrinsic ZnO film behaves as an n-type transparent semiconductor due to the defects such as oxygen vacancies. In fact, oxygen vacancies in zinc oxide could supply free electrons. It has a wide band gap (~-3.37eV) with optical transmission over 80% in the visible portion of the electromagnetic spectrum. Besides that, thin films based on ZnO have been studied for several years for their low cost, low photo sensitivity, and high field effect mobility.

Recently, many teams announced their reports abort fabrication methods to ZnO TFTs, including RF magnetron sputtering[7], atomic layer deposition (ALD)[8], and pulsed laser deposition (LPS)[9]. But physical deposition and ALD need to be operated in vacuum chamber because reducing the probability that radicals and ions meet collision with vapor molecules during deposition. In this way, RF or DC magnetron sputtering and atomic layer deposition are not feasible for real and mass manufacturing because of high prime cost and low throughput, even ZnO TFTs by using them to fabricate have better electrical properties than normal chemical depositions.

Some chemical solution depositions do not need be operated in vacuum chamber, but there are many impurities and defects which we could not forecast in ZnO thin film. So the quality of ZnO become worse and is not ideal for TFTs. Besides that, chemical solution depositions usually need high fabrication temperature (>400℃ to provide enough energy for ) molecules debonding and chemical reaction. It is difficult to integrate high temperature process in TFTs fabrication if the substrates are glass and flexible material. Synthesizing all above-mentioned fabrication methods

issues, we studied the device properties of TFTs with ZnO film as active layer deposited by atmospheric pressure plasma jet.

We paid much attention to atmospheric pressure jet because this kind of plasma does not require a complicated vacuum system which would reduce the cost of processing and enlarge the size limit [10]. Moreover, atmospheric pressure plasma jet is also a low temperature process. The temperature of plasma could be as low as 200℃ which could reduce the thermal damage of substrate and even be applied for plastic substrate. APP jet is suitable for large area substrate because of no vacuum chamber in APP jet systems. And the most important reason of using APPCVD is the quality of ZnO film deposited by this fabrication method is well enough to be the channel layer of TFTs .

In this paper, studies will be undertaken to interpret the growth mechanism of ZnO films, crystallographic structure, and electrical properties of the films and TFTs. We have explored experimentally as functions of the deposition conditions and defined an optimal deposition condition for TFT including fabrication temperature, thickness of film, main gas and carrier gas during deposition process. And we integrated ZnO TFT with an optimal deposition condition on glass substrate to proof that it is feasible to make a transparent device by our fabrication method. By considering the electrical properties , the mechanisms of film growth, and the influences of different conditions will be reported and discussed later.

Chapter 2

Literature Reviews

2.1Reviews of thin film transistors applications 2.1.1Thin film transistors

A thin-film transistor (TFT) is a special kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate[11]. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays. This differs from the conventional transistor where the semiconductor material typically is the substrate, such as a silicon wafer.

TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon based TFT depend on the crystalline state. That is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or it can be annealed into polysilicon. Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenide and metal oxides such as Zinc Oxide. TFT's have also been made using organic materials (referred to as an Organic TFT or OTFT).

By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent[12]. Because the substrate cannot withstand the high annealing temperature, the deposition process has to be completed under relatively low

temperature. Chemical vapor deposition, physical vapor deposition are applied.

There are two main TFTs structures which are shown in figure2-1. The structure A is top gate structure and structure B is bottom gate structure. We choose one of two structures in TFTs fabrication by practical requirements and issues. The structure of top gate conforms our original ideals naturally because this structure has been used in normal field effect transistors for many years. But majority of TFTs fabrications does not adopt this structure .

We need to consider that process of dielectric layer deposited by PECVD or other deposition ways would maybe influence semiconductor active layer by plasma bombarding or thermal treatment. Defects and interface charges would appear in active layer and the interface between dielectric layer and active layer. In this way, electrical properties of top gate structure TFTs become worse by defects and interface charges.

We usually adopt bottom gate structure in advanced TFTs fabrication for better quality of semiconductor active layer than top gate structure because the active layer is deposited after gate dielectric layer and drain or source electrode. Using this structure avoids plasma bombarding or thermal treatment injuring active layer . And another benefit of bottom gate structure is that we can fabricate devices on silicon substrates as gate electrodes. So the gate leakage would deal little influence to devices because we use thermal oxide to be gate insulator. In this way, we can focus on the active layer of TFTs. But a new problem we need to solve about bottom gate structure is the stability of devices after active layer exposed to the air

directly after a long time. Mists and particles would invade active layer and made the reliability of devices become worse.

In this way, an effective method to solve this problem is depositing preservation layer on active layer like silicon nitride to avoid mists and particles invading active layer. This method would increase the prime cost and influence the active layer, but we choose bottom gate structure to avoid plasma bombarding or thermal treatment injuring active layer .

2.1.2 TFTs as OLED drivers

Passive matrix organic light emitting diode (PMOLED) is a display technology as shown in figure 2-2(a) [13, 14]. OLED describes a specific type of thin display technology which doesn't require a backlight.An OLED is driven by one data line and one scan line at one time order. At one time order only one scan line would be the grounding and other scan lines connect to broken circuit. The next time order the below scan line would be the grounding .This action is ordered from first scan line to last scan line and repeat again and again. When one scan line became grounding, OLEDs vision ,we need to drive OLEDs by large current and high voltage to make them radiating more light. But this movement would make OLEDs having short lifetime and large power consumption. So it is necessary to find out a method to keep OLEDs radiating before scan line connected to grounding

and accepting data line the next time.

Active-matrix OLED (Active-matrix organic light-emitting diode) is a display technology for use in mobile devices and televisions. And Active-Matrix refers to the technology behind the addressing of pixels.

AMOLED technology continues to make progress towards low-power, low-cost, and large size (e.g. 40-inch) for applications such as TV.

An active-matrix OLED (AMOLED) display consists of OLED pixels that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that generate light upon electrical activation as shown in figure 2-3(a), which functions as a series of switches to control the current flowing to each of the pixels as shown in figure 2-3(b).

Typically, this continuous current flow is controlled by at least two TFTs at each pixel ,one to start and stop the charging of a storage capacitor and the second to provide a voltage source at the level needed to create a constant current to the pixel and eliminating need for the very high currents required for passive OLED matrix operation.

Active-matrix OLED displays provide higher refresh rate than their passive-matrix OLED counterparts, and they consume significantly less power. This advantage makes active-matrix OLEDs well suited for portable electronics, where power consumption is critical to battery life. The amount of power the display consumes varies significantly depending on the color and brightness shown.

2.2Reviews of ZnO TFTs

ZnO thin film deposited for TFTs reveals polycrystalline with a hexagonal wurtize structure and has a preferred orientation (002) with the c-axis perpendicular to the substrate. Zinc oxide has several advantages[16, 17]：

1. Cost down and abundant 2. Wide band gap~3.3eV 3. High mobility

4. Bipolarity material (n-type or p-type with different dopants)

And using zinc oxide to replace a-Si as active layer in TFTs has many Advantages[18, 19]：

1. High filed-effect mobility (> 1cm²˙V^-1˙S^-1) 2. Low deposition temperature

3. Wide band gap (~3.3eV) 4. Stability

2.2.1 ZnO TFTs Fabricated by Atomic Layer Deposition

Atomic layer deposition (ALD) is a thin film deposition technique that is based on the sequential use of a gas phase chemical process. The majority of ALD reactions use two chemicals, typically called precursors. These

precursors react with a surface one-at-a-time in a sequential manner. By exposing the precursors to the growth surface repeatedly, a thin film is deposited.

ALD is a self-limiting (the amount of film material deposited in each reaction cycle is constant), sequential surface chemistry that deposits conformal thin-films of materials onto substrates of varying compositions.

ALD is similar in chemistry to chemical vapor deposition (CVD), except that the ALD reaction breaks the CVD reaction into two half-reactions, keeping the precursor materials separate during the reaction. Due to the characteristics of self-limiting and surface reactions, ALD film growth makes atomic scale deposition control possible. By keeping the precursors separate throughout the coating process, atomic layer control of film growth can be obtained as fine as ~0.1 Å (10 pm) per monolayer. Separation of the precursors is accomplished by pulsing a purge gas (typically nitrogen or argon) after each precursor pulse to remove excess precursor from the process chamber and prevent 'parasitic' CVD deposition on the substrate.

The growth of material layers by ALD consists of repeating the following characteristic four steps as shown in figure 2-4:

(1) Exposure of the first precursor.

(2) Purge or evacuation of the reaction chamber to remove the non-reacted precursors and the gaseous reaction by-products.

(3) Exposure of the second precursor or another treatment to activate the surface again for the reaction of the first precursor.

(4) Purge or evacuation of the reaction chamber.

Transparent ZnO thin film transistor was fabricated on glass substrate.

The field effect mobility is about 1~10 cm²/Vs. The active layer (ZnO), gate insulator (Al₂O₃ or SiN), and source–drain electrode (ZnO:Al) were deposited by atomic layer deposition as shown in figure 2-5. The carrier density of the ZnO layer was carefully adjusted to reduce off-current of TFT.

Good contact with small contact resistance was formed between the active layer and the source–drain electrode. The on-off ratio of ZnO TFTs fabricated by ALD is about 10⁶~10⁸.

The four main factors contributing to obtain well-behaved ZnO-TFTs by ALD method were the reduction of the carrier amount of ZnO film by lowering the growth temperature, minimal damage to the surface of dielectric during the S/D and active layer processes, the high quality of the dielectric layer grown by ALD, and the good ohmic contact between the S/D and the active layer resulting in ZnO TFT performance suitable for an OLED driving device. Although the wet etching of the active layer degraded the TFT performance, it showed promise as a large area transparent display.

The active layer process temperature was as low as 100°C and was also compatible with a plastic substrate to realize a flexible display.

The major limitation of ZnO TFTs fabricated by ALD is its slowness and high prime cost; usually only a fraction of a monolayer is deposited in one cycle. Fortunately, the films needed for future-generation ICs are very thin and thus the slowness of ALD is not such an important issue. Although the selection of film materials grown by ALD is wide, many technologically

important materials cannot currently be deposited by ALD in a cost-effective way. ALD is a chemical technique and thus there is always a risk of residues being left from the precursors.

2.2.2 ZnO TFTs Fabricated by RF Magnetron Sputtering

Sputter deposition is a physical vapor deposition process for depositing thin films, sputtering means ejecting material from a target and depositing it on a substrate such as a silicon wafer. The target is the source material.

Substrates are placed in a vacuum chamber and are pumped down to a prescribed process pressure. Sputtering starts when a negative charge is applied to the target material causing a plasma or glow discharge. Positive charged gas ions generated in the plasma region are attracted to the negatively biased target plate at a very high speed. This collision creates a momentum transfer and ejects atomic size particles form the target. These particles are deposited as a thin film into the surface of the substrates.

Magnetron sputtering can be done either in DC or RF modes. DC sputtering is done with conducting materials. If the target is a non conducting material the positive charge will build up on the material and it will stop sputtering. RF sputtering can be done both conducting and non

magnetron sputtering on Si substrates held near room temperature. The best devices had field-effect mobility of more than 2 cm²/Vs and an on/off ratio about 10⁵~10⁷. With high optical transparency ~80% for wavelength 400nm.The combination of transparency in the visible, excellent transistor characteristics, and low-temperature processing makes ZnO thin-film transistors attractive for flexible electronics on temperature sensitive substrates.

2.2.3 ZnO TFTs Fabricated by Pulsed laser deposition

Pulsed laser deposition (PLD) is a thin film deposition (specifically a physical vapor deposition, PVD) technique where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited as shown in figure 2-8. This material is vaporized from the target (in a plasma plume) which deposits it as a thin film on a substrate (such as a silicon wafer facing the target). This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.

While the basic-setup is simple relative to many other deposition techniques, the physical phenomena of laser-target interaction and film growth are quite complex (see Process below). When the laser pulse is absorbed by the target, energy is first converted to electronic excitation and then into thermal, chemical and mechanical energy resulting in evaporation, ablation, plasma formation and even exfoliation. The ejected species expand into the surrounding vacuum in the form of a plume containing many energetic species including atoms, molecules, electrons, ions, clusters,

particulates and molten globules, before depositing on the typically hot leakage current and enabling the ZnO TFT to operate successfully. The Ion /Ioff ratio of ZnO TFTs fabricated on Si wafers was more than 10⁵ and the optical transmittance of ZnO TFTs fabricated on glass was more than 80%.

These results show that it is possible to fabricate a transparent TFT that can even be operated in the presence of visible light. But the deposition temperature of PLD is too high. It makes ZnO thin-film transistors fabricated by pulsed laser deposition not attractive for flexible electronics on temperature sensitive substrates.

2.2.4 ZnO TFTs Fabricated by Sol-Gel Process

The sol-gel process, known as a chemical solution deposition in atmospheric pressure[20], is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically a metal oxide) starting from a chemical solution which acts as the precursor for an integrated network of either discrete particles or network polymers. Typical precursors

are metal alkoxides and metal chlorides, which undergo various forms of hydrolysis and polycondensation reactions.

In this chemical procedure, the 'sol' (or solution) gradually evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. In the case of the colloid, the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time for sedimentation to occur, and then pour off the remaining liquid. Centrifugation can also be used to accelerate the process of phase separation.

Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing.

Afterwards, a thermal treatment, or firing process, is often necessary in order to favor further poly-condensation and enhance mechanical properties and structural stability via final sintering, densification and grain growth.

One of the distinct advantages of using this methodology as opposed to the

more traditional processing techniques is that densification is often achieved at a much lower temperature. Sol-gel process was shown in figure 2-9.

The precursor sol can be either deposited on a substrate to form a film, cast into a suitable container with the desired shape, or used to synthesize powders. The sol-gel approach is a cheap and low-temperature technique that allows for the fine control of the product’s chemical composition. Even small quantities of dopants, such as organic dyes and rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of

The precursor sol can be either deposited on a substrate to form a film, cast into a suitable container with the desired shape, or used to synthesize powders. The sol-gel approach is a cheap and low-temperature technique that allows for the fine control of the product’s chemical composition. Even small quantities of dopants, such as organic dyes and rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of