Thesis Organization

In our work, the TEOS films that are deposited by atmospheric-pressure plasma jet technology (APPJ) as gate dielectric layers of OTFT. Then pentacene films are deposited by a thermal evaporation system as active layers in OTFT. Last we will discuss the influence.

In chapter 1, we describe history of OTFT and motivation of our study.And we introduce to organic semiconductor and structure.

In chapter 2, we will introduce a new process, APPJ, which can be operated under low temperature and atmospheric ambient. And APPJ will make use of deposit dielectric layer SiO2 for our experiment.

In chapter 3, we compare the various methods of MIM fabrication and we select the best parameter to fabricate OTFT. Last we discuss the results.

In chapter 4, we will describe the conclusions and the future works.

Figure 1-1 Semilogarithmic plot of the highest field-effect mobility(μ) Reported for OTFT fabricated from the most promising polymeric and oligomeric semiconductors versus year from 1986 to 2000[16].

Table 1-1 Highest field-effect mobility(μ) values measured from OTFT as reported

Table 1-2 Characterization of materials for OTFT.

Figure 1-2 Classification of semiconductor materials.

(a)

Figure 1-4 Molecular structure of pentacene.

Oligomer materials Channel Thin film Mobility

(cm

/V-s) Comments

C60(fullerene) n-type Vacuum

0.08

Table 1-3 Thin film transistor performances for different oligomer active layers.

Chapter 2 Experiment

2.1 Introduction of APPT

2.1.1 Introduction of plasma[21]

(a)Corona discharge.

A corona discharge appears as a luminous glow localized in space around a point tip in a highly nonuniform electric field. The physics of this source is well understood . The corona may be considered a Townsend discharge or a negative glow discharge depending upon the field and potential distribution . Figure 2-1 shows a schematic of a point-to-plane corona. The apparatus consists of a metal tip, with a radius of about 3μm, and a planar electrode separated from the tip by a distance of 4–16 mm . The plasma usually exists in a region of the gas extending about 0.5 mm out from the metal point. In the drift region outside this volume, charged species diffuse toward the planar electrode and are collected. The restricted area of the corona discharge has limited its applications in materials processing. In an attempt to overcome this problem, two-dimensional arrays of electrodes have been developed. Some applications of coronas include the activation of polymer surfaces and the enhancement of SiO2 growth during the thermal oxidation of silicon wafers.

(b) Dielectric barrier discharges

order of several mm, and the applied voltage is about 20 kV. The plasma is generated through a succession of micro arcs, lasting for 10–100 ns, and randomly distributed in space and time. These streamers are believed to be 100μ m in diameter and are separated from each other by as much as 2 cm . Dielectric barrier discharges are sometimes confused with coronas, because the latter sources may also exhibit microarcing. Dielectric barrier discharges have been examined for several material processes, including the cleaning of metal surfaces and the plasma-assisted chemical vapor deposition of polymers and glass films . However, since the plasma is not uniform, its use in etching and deposition is limited to cases where the surface need not be smooth. For example, in the study of SiO2 deposition, it was found that the surface roughness exceeded 10% of the film thickness.

(c)Cold plasma torch

A “cold” plasma torch was first described by Koinuma et al. in 1992. A schematic of this source is shown in Figure 2-3. The powered electrode consists of a metal needle with a thickness of 1 mm. This needle is inserted into a grounded metal cylinder. In addition, a quartz tube is placed between the cathode and anode, which makes this device resemble a dielectric barrier discharge. Mixtures of rare gases, He and Ar, and other species are fed between the metal needle and quartz tube at flow velocities of about 5 m/s at 200–400℃. The gases are ionized and exit the source as a small jet.

Koinuma and coworkers have employed the cold plasma torch in a number of materials processes, including silicon etching , photoresist ashing , deposition of SiO2 , and TiO2

films , treatment of vulcanized rubber , and the production of fullerenes . Koinuma and coworkers measured the electron temperature in the plasma effluent with a Langmuir probe and found it to be between 1–2 eV depending on the gas composition.

(d) Plasma jet

Shown in Figure 2-4 is a schematic of an atmospheric-pressure plasma jet . This new source consists of two concentric electrodes through which a mixture of helium, oxygen, and other gases flow. By applying 13.56 MHz RF power to the inner electrode at a voltage between 100–250 V, the gas discharge is ignited.

The ionized gas from the plasma jet exits through a nozzle, where it is directed onto a substrate a few millimeters downstream. Under typical operating conditions, the gas velocity is about 12 m/s with the effluent temperature near 150℃. So far, this source has been used to etch polyimide, tungsten, tantalum, and silicon dioxide , as well as to deposit silicon dioxide films by plasma-assisted chemical vapor deposition.

Langmuir probe measurements in the jet effluent indicate that the concentration of charged species is relatively low, on the order of 1 ×10¹⁰cm^-3 . However, inside the jet the electron density should be much higher, as suggested by the intense atomic lines observed in the optical emission spectrum. Based on the impedance measurements and the emission spectra, it is estimated that the electron temperature inside the plasma jet averages between 1–2 eV.

Jeong et al. measured the ozone concentration in the effluent of the plasma jet at different distances from the nozzle and found that it varied from 2–5×10¹⁵ cm ^-3, as shown in Figure 2-5. Using these data as the basis for fixing the other species concentrations, a preliminary kinetic model was developed to determine the concentrations of O atoms and metastable oxygen molecules in the jet effluent. These results are also shown in the figure. The simulation predicts that the O atom

15 -3

be the active species in polyimide etching and in the SiO2 CVD process .

Comparison of various plasma sources show in Table 2-1.We find the average densities of oxygen ions,oxygen atoms, and ozone in the different atmospheric -pressure plasma discharges. Since the dielectric barrier discharge operates as a series of transient microarcs, it is difficult toobtain time-averaged values for the reactive species. However, perusal of the literature suggests that the time averaged concentrations should be similar to those found in a corona. Also, the values shown for the plasma jet correspond to the gas in between the electrodes. In the downstream jet, the distribution of species changes as indicated in Figure 2-5.

In the corona and dielectric barrier discharge, ozone is the main reaction product, whereas in the other plasmas, oxygen atoms represent a large fraction of the reactive species. In a low-pressure glow discharge the concentrations of ions and atoms are lower than in an atmospheric-pressure plasma. However, the impingement rate of these species on a substrate may be about the same in both cases, since the flux to the surface increases with decreasing pressure. Taking into account all the properties of the plasmas, it appears that the atmospheric-pressure plasma jet exhibits the greatest similarity to a low-pressure glow discharge. Consequently, this device shows promise for being used in a number of materials applications that are now limited to vacuum.

2.1.2 Applications of APPJ

The atmospheric-pressure plasma jet technology (APPJ) is useful for treating and modifying the surface properties of organic and inorganic materials. The APPJ apparatus does not require any vacuum systems, produces high density plasma, and provides treatment of various substrates at low temperatures while operating open to the atmosphere. The plasma system has used for a wide variety of applications including treatment of polymer films, paper, wood, and foils; plasma grafting and plasma polymerization; ash various materials in the microelectronics industry; barrier layer deposition for the packaging industry; and sterilizing biologically contaminated materials.

For polymer films, the technique offers the following advantages:

• Uniform treatment and No backside treatment.

• Improved surface energy with concomitant improved wet ability, printability,and adhesion.

• No additional vacuum system and low cost.

• Continuous fabrication availably and high speed for production.

• High plasma density.

As shown in Fig.2-6, we exhibited the atmospheric-pressure plasma system which was used in our experiment.

2.2 Affect to capacitance of different conditions

2.2.1 Silicon oxide deposited by APPJ on the metal insulator metal (MIM) structure and relation with main gas of APPJ

In this section, we are resolution to deposit silicon oxide dielectric on the bottom contact electrode metal by atmospheric-pressure plasma jet with different main gas. But silicon oxide is not deposited on the metal at room temperature. Accordingly, we heat the bottom of n⁺-Si substrate and enable TEOS to be deposited on the metal.

First, an n-type bare silicon wafer is cleaned by the standard RCA cleaning process. An insulating layer of silicon dioxide is grown by thermaloxidation (wet oxidation) 90min at 950 for isolation purpose. ℃ The thickness of silicon oxide is 5060 Å measured by n&k system. And deposited 50 nm Nickel as the bottom electrode. Heats up the Tetraethoxy silane (TEOS) to 150 was injected by Argon as carrier gases which is the ℃ deposition source of silicon oxide. Silicon oxide is deposited on the top of Nickel layer at room temperature under an atmospheric-pressure with the plasma power is established around 560 W with an appropriate scanning rate (cycle).Silicon oxide is deposited on the Nickel thin film by atmospheric-pressure plasma jet system (APPJ) was plot in Fig. 2.-7. with varied main gases (Qxygen, Nitrogen and CDA respectively).

The detail experimental parameters show in Table 2-2. We adopt the experimental parameters to compare silicon oxide deposited with different main gases of APPJ.

Finally, all top contact electrodes are deposited 50 nm thick Nickel layer defined with shadow mask by E-Gun system. The active region pad of all capacitors is diameter 200μm. The process flow is shown in Figure 2.-8

2.2.2 Silicon oxide deposited by APPJ on the metal insulator metal (MIM) structure and relation with gap distance of APPJ

In this section, we are resolution to deposit silicon oxide dielectric on the bottom contact electrode metal by atmospheric-pressure plasma jet with different gap distance.

But silicon oxide is not deposited on the metal at room temperature. Accordingly, we heat the bottom of n⁺-Si substrate and enable TEOS to be deposited on the metal.

First, an n-type bare silicon wafer is cleaned by the standard RCA cleaning process. An insulating layer of silicon dioxide is grown by thermaloxidation (wet oxidation) 90min at 950 for isolation purpose. The thickness of silicon oxide is 5060 Å measured by ℃ n&k system. And deposited 50 nm Nickel as the bottom electrode. Heats up the Tetraethoxy silane (TEOS) to 150 was injected by Argon as carrier gases which is the ℃ deposition source of silicon oxide. Silicon oxide is deposited on the top of Nickel layer at room temperature under an atmospheric-pressure with the plasma power is established around 560 W with an appropriate scanning rate (cycle).Silicon oxide is deposited on the Nickel thin film by atmospheric-pressure plasma jet system (APPJ) with varied gap distances (1.8 cm, 2.0 cm, 2.2 cm and 2.5 cm respectively). The detail experimental parameters show in Table 2-3. We adopt the experimental parameters to compare silicon oxide deposited with different gap distances of APPJ.

Finally, all top contact electrodes are deposited 50 nm thick Nickel layer defined with shadow mask by E-Gun system. The active region pad of all capacitors is diameter

2.2.3 Silicon oxide deposited by APPJ on the metal insulator metal (MIM) structure and relation with Ar flow rate of APPJ

In this section, we are resolution to deposit silicon oxide dielectric on the bottom contact electrode metal by atmospheric-pressure plasma jet with different gap distance. But silicon oxide is not deposited on the metal at room temperature.

Accordingly, we heat the bottom of n⁺-Si substrate and enable TEOS to be deposited on the metal.

First, an n-type bare silicon wafer is cleaned by the standard RCA cleaning process. An insulating layer of silicon dioxide is grown by thermaloxidation (wet oxidation) 90min at 950 for isolation purpose. The thickness of silicon oxide is 5060 Å measured by ℃ n&k system. And deposited 50 nm Nickel as the bottom electrode. Heats up the Tetraethoxy silane (TEOS) to 150 was injected by Argon as carrier gases which is the ℃ deposition source of silicon oxide. Silicon oxide is deposited on the top of Nickel layer at room temperature under an atmospheric-pressure with the plasma power is established around 560 W with an appropriate scanning rate (cycle).Silicon oxide is deposited on the Nickel thin film by atmospheric-pressure plasma jet system (APPJ) with varied Ar flow rates (60sccm, 100sccm, 140sccm, 200sccm and 300sccm respectively). The detail experimental parameters show in Table 2-4. We adopt the experimental parameters to compare silicon oxide deposited with different Ar flow rates of APPJ.

Finally, all top contact electrodes are deposited 50 nm thick Nickel layer defined with shadow mask by E-Gun system. The active region pad of all capacitors is diameter 200μm.

2.3 Fabrication of OTFT

First, the n⁺-Si wafer was used as the substrate, and was rinsed in the deionization water ( DI water ), and was then dipped in dilute HF solution ( HF:DI water = 1:100 ) that to remove the native oxide, the wafer was accomplished the RCA Clean process.

An insulating layer of silicon dioxide is grown by thermal oxidation (wet oxidation) 90min at 950 for isolation purpose. The thickness of silicon dioxide is 5060 Å ℃ measured by n&k system. A 50-nm-thick Nickel gate electrode was then deposited on SiO2/Si, through a shadow mask, using E-Gun. The thin silicon oxide gate dielectric was then deposited by atmospheric pressure plasma jet (APPJ) at substrate temperature of 150 in atmosphere ℃ . We used the Ar as the carrier gas to transport the TEOS vapor into the spray nozzle and the clean dry air (CDA) plasma to decompose the TEOS gas.

The atmospheric pressure plasma jet has operation power about 560 W and the diameter of the plasma nozzle was about 5 mm.

After completing the deposition of gate dielectric, a 50 nm-thick pentacene active layer was evaporated on the gate insulator through a shadow mask in high vacuum about 2 x10^-6 toor and the temperature was about 70 . The source and drain electrodes ℃ were then deposited on the surface of pentacene through shadow mask with a channel width W of 2000 μm and channel length L of 500 μm. The schematic diagram of OTFT with top contact structure was shown in Fig. 2-10.

2.4 Characteristic measurement of devices

We use HP 4284A precision LCR meter parameter to analyze Capacitance-Voltage (C-V) characteristic diagrams at 1MHz and the characteristic curves of Current-Voltage (I-V) are measured with semiconductor parameter analyzer by HP 4156. We measure all measurements are at room temperature in an air atmosphere.

Figure 2-1 Schematic of a corona discharge.

Figure 2-2 Schematic of a silent discharge (1) metallic electrodes and (2) dielectric barrier coating.

Figure 2-4 Schematic of the atmospheric-pressure plasma jet for the deposition of silica films.

Figure 2-5. Numerical simulation of the concentrations of species in the effluent of the plasma jet as a function of the distance from the nozzle in a 1.0%

O2/He plasma at 760 torr.

Source

Breakdown Plasma density Density(cm^-3) voltages(KV) (cm^-3) O+,O2+,O^- O O3

Corona plasma 10~50 10⁹~10¹³ 10¹⁰ 10¹⁰ 10¹⁸

Dielectric barrier discharge 5~25 10¹²~10¹⁵ 10¹⁰ 10¹² 10¹⁸

plasma torch 10~50 10¹⁶~10¹⁹ 10¹⁵ 10¹⁸ <10¹⁰

plasma jet 0.05~0.2 10¹¹~10¹² 10¹² 10¹⁶ 10¹⁶ Table 2-1 Densities of oxygen species in the various plasma discharges.

Figure 2-7 Schematic of the atmospheric-pressure plasma jet for the deposition of silicon oxide.

Experimental parameters

Main gas CDA, N2, O2

Speed (mm/sec) 30

Gap distance (cm) 2.2

Substrate temperature (℃) 150

Ar flow rate (sccm) 100

Scanning times 60 or 15 (N2, O2)

Table 2-2 shows the detail experimental parameters of silicon oxide deposited with different kinds of main gases.

Experimental parameters

Table 2-3 shows the detail experimental parameters of silicon oxide deposited with different kinds of gap distances.

Table 2-4 shows the detail experimental parameters of silicon oxide deposited with different kinds of Ar flow rates.

Figure 2-9 shows the schematic of organic thin film transistor structure with top contact.

Chapter 3

Results and Discussion

Low temperature processes and good quality gate insulator are urgent for organic thin film transistor . We utilized the atmospheric pressure plasma jet to deposit silicon oxide as the gate insulator of organic thin film transistor due to the cold chemically active species could reduce the thermal damage on the substrate. In addition, APPJ dose not require vacuum system which could overcome size limit and reduce cost of equipments.The gate dielectric deposited by APPJ shows low leakage current density at the range of 1.59E-8~3.85E-8 A/cm² at 0.5 MV/cm, and it is comparable to some gate dielectrics deposited in a vacuum chamber such as sputter and electron-beam evaporation .

3.1 Result of different conditions

3.1.1 The influence of different main gases

We utilized oxygen, nitrogen, and CDA as the main gas of APPJ, to decompose tetraethoxysilane (TEOS), Si(OC2H5)4. Oxygen, nitrogen, and CDA were used for investigating the influence of different main gases of APPJ on deposition rate and quality of silicon oxide. Leakage current density versus electric field of silicon oxide deposited with different kinds of main gases was shown in Figure 3-1, and The deposition rate and thickness show in Table 3-1. The leakage current density (A/cm²) at 0.5 and 1 MV/cm with different main gases show in Table 3-2. We could find that the

O atom, metastable oxygen, and ozone [22] Show the densities of oxygen species in the plasma discharges show in Table3-3 [21]. The deposition rate of silicon oxide deposited by oxygen main gas is the fastest because oxygen has better ability to decompose TEOS vapor. However, the faster deposition rate may decrease the denseness of silicon oxide and increase the surface roughness of silicon oxide generate higher leakage current density. The silicon oxide deposited with varied main gases (CDA, Nitrogen and Oxygen respectively) and, the SEM images Figure 3-2 had the same tendency as the AFM images Figure 3-3 .We could find that the uniformity of silicon oxide fabricated with CDA main gas is better than oxygen main gas.The schematic illustration of deopsited rate versus RMS are shown in Figure 3-4 and we found that the increase of deposition rate will increases RMS of surface.The structure of silicon oxide fabricated with oxygen gas is loose, and this may cause higher leakage current density.

Silicon oxide deposited with CDA main gas has the best quality but the deposition rate is the slowest. The oxygen percentage of CDA is about 20 % which is much higher than nitrogen, but the deposition rate of silicon oxide deposited with CDA is the slowest.

It may due to that the higher oxygen percentage in nitrogen plasma has strong electronegativity which may decrease the electron density of plasma [23] and cause the decrease of the excited species. Although some researches [22] indicated that the deposition rate of silicon oxide would be proportion to oxygen partial pressure but the percentage of oxygen to total gas was below 2% in these studies. The use of pure nitrogen main gas can generate a lot of ozone because the high density nitrogen plasma generated by APPJ could excite air to create ozone and excited oxygen atoms.

X-ray photoelectron spectroscopy was used to analyze the composition of the films.

Show in Table 3-4 .The silicon oxide deposited by CDA main gas were composed of 33.12 at.% silicon, 62.45 at.% oxygen, 4.1 at.% carbon. Oxygen to silicon ratio (O/Si ratio) was satisfied nearly 2. The presence of carbon in the films was convinced to be