The Carrier Transport Mechanism of AOSs

The mobility of a-Si:H (~1 cm²(Vs)^-1) is much smaller than that of single crystalline Si (~200 cm²(Vs)^-1) due to the intrinsic chemical bonding nature. The average carrier paths in covalent semiconductors, such as a-Si:H, consist of strongly directive sp³ orbital. The bond angle fluctuation significantly alters the electronic levels, causing high density of deep tail-states, as shown in Fig. 1-3. [12]

Fig. 1-3 Schematic orbital drawing of electron pathway (conduction band bottom) in conventional silicon-base semiconductor and ionic oxide semiconductor. [12]

In contrast, transport oxides constituting of heavy post transition metal cat-ions with the (n-1)d¹⁰ns⁰ electron configuration, where n≧5, are the transparent AOS (TAOS) candidates having large mobility comparable to those of the corresponding crystals. [13] The electron pathway in oxide semiconductor is primarily composed of spatially spread ns orbital with an isotropic shape, as shown in Fig. 1-3. [13] The direct overlap among the neighboring ns

orbital is possible. The degree of overlap of the ns orbital is insensitive to the distorted metal-oxygen-metal bonding. This feature shows why the Hall mobility of AOSs is similar to the corresponding crystalline phase, even under the room temperature deposition of thing-films.

1.2.3 Amorphous In-Ga-Zn-O TFTs

Among the TAOS materials, amorphous indium gallium zinc oxide (a-IGZO) is one of the most glaring candidates serving as semiconductor layer in thin film transistor (TFT). [14]

However there are still many critical issues existed in a-IGZO TFT, especially for the easy absorption and desorption reaction of the oxygen atom with the surrounding atmosphere. As the oxygen species are absorbed from the ambient atmosphere, they can capture electrons in the conducting channel and form a depletion region beneath back channel layer. By following

the equation of O₂₍^g₎ e^ O₂^{ s}

, the resultant buildup of absorbed negative space charges O₂^-_(s) easily repells conduction electrons and positively shifts V_th of a-IGZO TFT. [14]

Whereas the desorption of oxygen atoms in a-IGZO back channel will result in the left shifts of V_th. This random reaction of absorption and desorption happening also leads to some uniformity problems. Some researching groups have applied passivation layer method to shield the back channel from the contact with ambient air. But the sequel processes would strongly affects the original properties of a-IGZO film. Therefore, the fundamental method to release the issue of environmental influence should be the improvement of the film quality of itself.

TAOSs have attracted keen attention since the high performance thin-film transistors can by obtained by using the amorphous In-Ga-Zn-O (a-IGZO) thin films for the semiconductor layers deposited on plastic substrates by using the sputter deposition at room temperature. The

TFT performance is also confirmed by using the sputter deposition [15], which demonstrates the possibility of the large-area applications. The dependence of the TFT characteristics on the metal composition is investigated in detail by a novel combinational approach, since the multi-metal AOSs can take any ratios of the composition.

The a-IGZO film has electrons as majority carriers, which is mainly affected by the oxygen vacancies and oxygen interstitials during deposition processes. [16] The ion bonding structure makes the a-IGZO TFT exhibit high field-effect carrier mobility even in the amorphous phase. [17] Even if the a-IGZO TFT owns many superior characteristics, the sensitivity to atmosphere is a extremely critical issue for the a-IGZO TFT application. [18]

The environment-dependent metastability was attributed to oxygen adsorption/desorption reactions to the backchannel of the a-IGZO TFT device. The random reactions between the ambient air and the a-IGZO backchannel layer can not only change the oxygen vacancies in the a-IGZO film but result in a threshold voltage shift with days going by, and even device uniformity problems. [19] In addition to isolate the a-IGZO layer from exposing to the atmosphere, the electrical stability and uniformity of the a-IGZO film can be improved by the optimization of the chemical stoichiometry or adjusting oxygen content inherently.

For the In2O3-Ga2O3-ZnO ternary system, the incorporation of cat-ions with large ionic valance such as Ga³⁺ and Al³⁺ to high conductive oxides such as In2O3 and ZnO is effective to control the carrier concentration due to their strong metal-oxygen bonds. [20] Other AOSs,

such as a-ITO and c-ZnO have high density carrier density, hence is difficult to control the device characteristics. Besides, amorphous In-Ga-Zn-O (a-IGZO) is transparent throughout the visible spectrum. The transmittance is greater than 80 percent from 400 nm to 850 nm as shown in Fig. 1-4. [21] Because only In³⁺ meets the electron configuration criterion (n-1)d¹⁰ns⁰ (n≧5) of heavy post transition metal cat-ion for ionic AOS (IAOS) among the three cations. The mobility is primary determined by the fraction of In₂O₃ content and the highest value of ~40 cm²(Vs)^-1 is obtained for the samples containing the maximum In₂O₃ fraction. The large ionic valence ions such as Ga³⁺ combine with high conductive oxides such as In₂O₃ and ZnO to control the carrier concentration effectively because of the strong metal-oxygen bonds. [22] In other words, Ga³⁺ suppresses carrier generation via oxygen vacancy formation because Ga ion forms stronger bond with oxygen than Zn and In ion. [23]

Therefore, the InGaZnO₄ composition was chosen as the AOS for channel layer of the transparent TFT. A critical issue of semiconductor materials for TFT applications is controllability of carrier concentration.

Wavelength (nm)

Fig. 1-4 Transmittance of a-IGZO film in visible light region. [21]

This is particularly important for AOSs because electron carriers can be easily generated.

It is vital to choose materials which control carrier concentration at low levels (~10¹⁴ cm^-3) to achieve low off current (I _off) and large on-off current ratio (I_on/I _off). [24] In practice, the effect of binary amorphous materials in the In₂O₃-ZnO system is employed in commercial flexible transparent conductive films by depositing on plastic sheet. Thus, the effect of partial oxygen pressure was studied on the carrier concentration in a-InGaZnO₄ and a-In₂Zn₃O₆ and the results are shown in Fig. 1-5. [25] The carrier concentration in the a-InGaZnO₄ is distinctively reduced to below 10¹³ cm^-3 by increasing PO₂ to 8 Pa, on the other hand, it remains at 10¹⁸ cm^-3 in the a-In₂Zn₃O₆ deposited under the same condition. It is evident that incorporation of Ga³⁺ is supposed to attract the oxygen ions tightly due to its high ionic potential (+3 valence and small ionic radius), and thereby suppressing electron injection

around the chemical composition of InGaZnO₄ is not sensitive to the variation in the composition, as shown in Fig. 1-5. [26] Thus, the InGaZnO4 system has better electrical properties than the In₂Zn₃O₆ system. [27] In addition, the a-IGZO TFTs’ processes are similar to that of a-Si based TFT. It means the existing production lines can be used. In this case, we can save a lot of money from buying new equipment.

Fig. 1-5 The carrier concentration as a function of O2 pressure during the deposition in a- InGaZnO4 and a-In2Zn3O6. [25]

1.3 Microwave Annealing

1.3.1 Interaction of Microwaves with Matter

Conventional heating usually involves the use of a furnace, which heats the walls of the furnaces by convection or conduction. The core of the sample takes much longer to achieve the target temperature. Microwave heating is able to heat the target compounds without heating the entire furnace, which saves time and energy. It is also able to heat sufficiently thin objects throughout using volumetric heating, rather than through the outer surface. Different materials convert microwave radiation to differing amounts of heat. The selectivity of different materials allows the object to be heated at differing speeds as well. [34] [38]

Microwave heating is the perspective techniques, which heat the volume of the wafer, not just its surface. It used very loosely for electromagnetic radiation in millimeter and radio frequency spectrum. Microwave processing is quite the same in thermal processing of ceramic materials. In 1990, Buchta used a microwave generator operating at 2.45GHz and a power about 1500W to heat 5-inch wafers 125mm to about 1000^oC over period of a few seconds.

Recently Thompson and his group used a resonant chamber with a magnetron source. Dr.

Bykov used a 30 GHz gyration device and a resonant processing chamber. They only demonstrated that microwave can activate at high temperature. In 2007, Jeff M. Kowalski report that microwave can activate of the heavily doped implanted layers in the range of temperature from 400-500^oC. [28] Microwave annealing (MWA) has a good activation

situation, and can keep doped less diffusion. The conventional heating is only limited by thermal diffusivity and surface temperature. So, the deep of surface can’t receive the heating

energy. But microwave generate heat directly inside the exposed material as result of molecular motion. Energy is transferred throughout the entire wafer. [29] Therefore, the effective of activated is more than conventional heating. Electromagnetic (EM) radiation is a very crucial form of energy available to mankind. It consists of electric and magnetic fields that fluctuate sinusoidally in planes perpendicular to each other and propagate at the speed of light. EM radiation does not need a medium to in which to travel. [30] The dual nature of EM radiation is evident through its wave-like behavior in the case of interference and diffraction and its particle-like behavior in the case of phenomena like the photoelectric effect. The quanta of EM radiation are termed as photons. The frequency ν and the wavelength λ are inversely proportional to each other, related by ν = c/λ, where c is the speed of light in vacuum. The energy E of the EM radiation depends linearly on the frequency ν, given by E = h ν where h is the Planck’s constant. [31]

The electromagnetic spectrum is classified into regions of increasing frequencies (or equivalently, energies): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Microwaves are generally taken to have frequencies from 300 MHz to 300 GHz which correspond to wavelengths of 1 m down to 1 mm, respectively. Microwaves have found their application in diverse fields such as microwave heating, communications,

RADAR, electronic warfare, radiation therapy, non-destructive testing of materials, etc. [32]

The interaction of microwaves with materials takes place through the two components of the microwave radiation: the electric field E and the magnetic field H. The response of a material when exposed to an electromagnetic radiation may be understood through the dielectric constant ε of the material. The dielectric constant, also known as the permittivity of the material, describes the ability of the material to be polarized in the applied electric field.

To understand the dielectric response to sinusoidal fields such as the microwaves, complex permittivity ε* is employed: ε* = ε’ + i ε”. The real part of the dielectric constant is a measure of the penetration of microwave energy in the material; while, the imaginary part indicates the ability of the material to store the energy. The dielectric properties vary with temperature and frequency. [33]

In any given material, various entities such as the free electrons, valence electrons, ions, molecular dipoles, and interfacial charges respond to the applied electric and magnetic field.

The sinusoidal fields cause the charged species to polarize and vibrate. Different charged species all have different natural frequencies of vibration. The conversion to heat occurs because of the lag of the response of the material to the applied electromagnetic field. In the heating of dielectric materials, it is assumed that the magnetic field does not contribute to microwave absorption and the heating occurs entirely due to the electric field.

There are four principal polarization mechanisms in dielectric solids: [34]

a. Electronic polarization: When an atom is subjected to an external electric field, displacement of the electron cloud with respect to the nucleus gives rise to formation of a dipole. Valence electrons shift much more easily than the tightly bound core electrons.

Covalent crystals have large dielectric constants owing to the displacement of the valence electrons. Thus, materials like silicon (εr’=11.9) and germanium (εr’=11.9)

have high real components of the dielectric constant; hence, microwaves easily penetrate these materials.

b. Dipole polarization: Under the application of an external electric field, polar molecules orient themselves with the field. The lag associated with this response and the inter-molecular collisions lead to dielectric heating. In some materials, the polarization can be retained due to the need for thermal activation for molecular rotation, which gives rise to the formation of “electrets”.

c. Ionic or atomic polarization: Relative displacement of the positive and negative ions or atoms within molecules and crystal structures from their equilibrium lattice sites gives rise to ionic polarization.

d. Interfacial polarization: This involves the accumulation of free charges at interfaces located within the material: grain boundaries, phase boundaries and defect regions.

Under the application of an electric field, the mobile charges are displaced and accumulated at such interfaces.

1.3.2 Microwave Processing of Materials

Microwaves generate rapidly changing electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid. Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions. Microwave heating a material depends to a great extent on its ‘dissipation’ factor, which is the ratio of dielectric loss or ‘loss’ factor to

dielectric constant of the material. The dielectric constant is a measure of the ability of the material to retard microwave energy as it passes through; the loss factor is a measure of the ability of the material to dissipate the energy. In other words, ‘loss’ factor represents the

amount of input microwave energy that is lost in the material by being dissipated as heat. [35]

Therefore, a material with high loss factor is easily heated by microwave energy. In fact, ionic conduction and dipolar rotation are the two important mechanisms of microwave energy loss (i.e. energy dissipation in the material).

Microwaves are reflected from the surface and therefore do not heat metals. Metals in general have high conductivity and are classed as conductors. Conductors are often used as conduits (waveguide) for microwaves. Materials which are transparent to microwaves are classed as insulators. Insulators are often used in microwave ovens to support the material to be heated. Materials which are excellent absorbers of microwave energy are easily heated and are classed as dielectrics. [36]

Non-homogeneous material (in terms of dielectric property) may not heat uniformly, that is, some parts of the materials heat faster than others. This phenomenon is often referred to as thermal runaway. This condition can be minimized by keeping the sample in mixing or fluidized condition. Volumetric heating is the key characteristic of microwave processing. In conventional heating, the thermal energy is transferred to the material from the outside to the inside, creating a temperature gradient. Small penetration depth of infrared (less than 0.1 mm) leads to energy deposition being limited to the surface layers. [37] Microwave heating overcomes this through absorption of the microwave energy throughout the volume of the material. Since the surface loses energy by radiation, the core of the material is usually hotter and the temperature profile is the inverse of that seen in conventional heating. Volumetric heating has the advantage of uniform and rapid processing of materials leading to an increased throughput. Rapid heating in semiconductors provides the advantage of minimal diffusion of various species into the substrate.

Poorly absorbing materials (those with small values of ε”) can be hard to heat using

microwaves. One common solution to this is the use of microwave susceptors to provide hybrid heating. [38] Microwave processing can also be employed for selective heating of materials, which is not possible with conventional heating.

1.4 Thesis Organization

This thesis is divided into four chapters. The main purpose of my thesis is to develop an new process method to improve the TFTs’ characteristics. In my thesis, I use a-IGZO as my active layer for increasing the performance of device. Then developed new processing method:

MWA for enhancing TAOS and transistor’s characteristics. We will discuss the intrinsic electrical characteristics, stability and reliability in the following pages.

In chapter 1, the brief overview of flat display panel industry, operations of the TFT-LCD, the carrier transport mechanism of AOSs, amorphous In-Ga-Zn-O TFTs and microwave annealing method are introduced.

In chapter 2, the experiment procedures are introduced. The sputtering system and microwave heating system are also described. The measurement and extraction of electrical parameters are also described. The measurement of reliability on a-IGZO TFTs is described.

In chapter 3, the intrinsic electrical characteristics of a-IGZO TFTs with different MWA time and power with two kind of gate insulator (SiO₂ and SiN_x) were discussed. Then we will discuss the comparison of MWA annealing and furnace annealing. Then we will show the results of material analysis with UV-visible, X-ray diffraction, scanning electron microscope and X-ray photoelectron spectroscopy.

In chapter 4, we summarize our all experimental results and give a brief conclusion.

Chapter 2 Experiment Procedures

2.1 Experiment Procedures

Table 2.1 is the experiment flow path in my experiment. First of all, we fabricate the TFT device we want. Then we do different treatment to our device. The special part is we will do microwave annealing to our device after they are fabricated. The treatment can be classified to two ways, one is electrical analysis and the other is material analysis.

The TFT devices were chosen as a bottom-gated passivation-free inverted staggered structure and fabricated on a glass substrate. First, a 100-nm-thick Mo layer was formed as a gate electrode in a dc sputtering system and a 150-nm-thick silicon nitride (SiN_x) was subsequently deposited on the patterned gate electrode by plasma-enhanced chemical vapor deposition (PECVD). The active channel layer of a 50-nm thick IGZO layer was formed by dc sputtering with a power of 100 W at room temperature in argon (Ar) ambiance with flow rate 10 SCCM (SCCM denotes cubic centimeter per minute at STP) with target of In:Ga:Zn:O

= 1:1:1:4 at.%. The sputtering was carried out at a working pressure of 3 x 10^-3 torr and the base pressure was below 5 x 10^-6 torr. Then, a 100-nm-thick indium tin oxide (ITO) was formed serving as source/drain electrodes by RF sputtering system and all the layers were defined by shadow mask. The channel width and length of a-IGZO TFTs were varied from 200 to 1000 μm. As we see at Fig. 2-1. Sequentially, all samples were microwave annealed at

was denoted as microwave power 600~700 W for annealing time 100s. The MWA processing time is defined as the period when the microwave power was turned on. In our experiment, the 5.8GHz microwave source has been employed for annealing process. A sample with conventional thermal annealing process was also fabricated at 450 °C for 1 h in a furnace with N₂ gas flow rate of 10L/hr under atmosphere pressure for comparison. All electrical and reliability measurements were carried out by using the semiconductor parameter analyzer, Keithley 4200. For the X-ray photoelectron spectroscopy measurement, X-ray diffraction, scanning electron microscope, 50-nm a-IGZO thin films with different microwave annealing conditions were deposited separately on n-type Si wafer. And measure transmittance with 50-nm a-IGZO films with different microwave annealing conditions were deposited on glass .

Table 2-1 Experiment flow chart

Fig. 2-1 The cross-section of a-IGZO TFTs

2.2 Sputtering systems

The experimental methods of the fabrication of the a-IGZO TFT are described. Besides, the principle of sputtering system including the RF sputtering and the DC sputtering is described. In this chapter, we will introduce two sputtering systems. One is RF sputtering and the other is DC sputtering.

RF sputtering can be applied to the deposition of both insulating conducting materials.

The substrate is located above the target so that the sputtered atoms can be deposited on to the

The substrate is located above the target so that the sputtered atoms can be deposited on to the