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 (SiNx) 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 substrate. A RF power supply generates plasma at the frequency of 13.56 MHz. A RF sputtering system is shown in Fig. 2-2. [39]
DC sputtering has the advantage of higher deposition rate and is less expensive than RF sputtering. A DC sputtering is shown in Fig. 2-3 [40], the substrate is located above the target and acts as the anode. DC sputtering is commonly applied to deposit conductive materials.
The plasma creates ions which are accelerated towards the target by a negative DC bias on the target. The ions bombard the target surface and dislodge the target atoms, which then deposit onto the substrate. The sputtering is performed in vacuum, typically between 1 mTorr and 50 mTorr. A lower chamber pressure increases the mean free path, which is the distance between collisions, so that the sputtered target atoms can reach the substrate without scattering away.
Fig. 2-2 Schematic RF sputtering system [39]
Fig. 2-3 Schematic DC sputtering system [40]
2.3 Basic Microwave Heating System
The microwave heating system is made up of four basic components: power supply, magnetron, applicator for the heating if the target material and waveguide for transporting microwaves from the generator to the applicator. Fig. 2-4(a) shows a simplistic diagram of the microwave heating system.
The microwave system mainly contains: [38]
a. Microwave power supply: supplies high voltage power for the microwave source. Supply includes internal alarms to prevent damage the microwave source, i.e. over volt alarm or over temperature alarm.
b. Microwave source: generates the microwave energy required for processing. Internal interlocks prevent overheating, i.e. water flow switch or over temperature sensor.
c. Isolator: eliminates excessive microwave energy form the process chamber to prevent damage to the microwave source.
d. Coupler: a port to measure forward microwave energy going into the process chamber.
e. Waveguide: delivers generated microwave energy into the process chamber.
f. Process chamber: an octagonal prism and vessel designed to isolate wafers from the
atmosphere while gases and microwave as specified by the recipe are applied to the wafers. The chamber’s geometry promoted a uniform microwave energy field. As the Fig.
2-4 (b). [38]
When the process starts, the loading stage under the process chamber sealed. And then, the stage rotates slowly for increase the uniformity of the microwave absorption. After ten minutes N2 gas pre-purge, the microwave power supplies turns on. The susceptors above and below the wafer can prevent particles from the environment during process. The addition of filler wafers (bare silicon) above and below the process wafer can prevent plasma generation.
[39]
Fig. 2-4 (a) Microwave heating system.
Fig. 2-4(b) The setup in the microwave chamber. The distance between adjacent slots was 1 cm only. [38]
2.4 Parameter Extraction Method
The device electrical properties were measured by a Keithley 4200 IV analyzer in a light-isolated probe station at room temperature. In IDS-VGS measurement, the typical drain-to-source bias was swept from VGS=-20 V to VGS=30 V. In this session, we describe the methods of typical parameters extraction such as threshold voltage (Vth), subthreshold swing
(SS) and field effect mobility (μFE) from device characteristics.
Threshold voltage (Vth) was defined from the gate to source voltage at which carrier conduction happens in TFT channel. Vth is related to the gate insulator thickness and the flat band voltage. Plenty of methods are available to determine Vth which is one of the most important parameters of semiconductor devices. This thesis adopts the constant drain current method, which is, the voltage at a specific drain current NID is taken as Vth, that is, Vth
= VG (NID) where Vth is threshold voltage and NID stands for normalized drain current.
Constant current method is adopted in most studies of TFTs. It provides a Vth close to that obtained by the complex linear extrapolation method. Generally, the threshold current NID = ID/(W/L) is specified at 1 nA in linear region and at 10 nA in saturation region; W and L represent for TFT channel length and width, respectively. [41]
Subthreshold swing (SS, V / dec.) is a typical parameter to describe the control ability of gate toward channel which is the speed of turning the device on and off. It is defined as the amount of gate voltage required to increase and decrease drain current by one order of
magnitude. SS is related to the process, and is irrelevant to device dimensions. SS can be lessened by substrate bias since it is affected by the total trap density including interfacial trap density and bulk density. In this study, SS was defined as one-half of the gate voltage required to decrease the threshold current by two orders of magnitude (from 10-8A to 10-10A). [41] The threshold current was specified to be the drain current when the gate voltage is equal to Vth.
Typically, μFE is determined from the transconductance (gm) at low drain bias (VD = 0.1 V). [41] The TFT transfer I-V characteristics can be expressed as
]
COX is the gate oxide capacitance per unit area, W is channel width,
2.5 Measurement of reliability on a-IGZO TFTs
In this session, we will introduce two ways of reliability’s measurement. One is gate DC bias stress condition and the other is light illumination’s measurement.
The DC gate bias stress condition was set to VG = +37.5 V for Positive Gate Bias stress (PGBS) and VG = -37.5 V for Negative Gate Bias Stress (NGBS). Converted into electric field (E) is 2.5 MV/cm, while source and drain electrodes are connected to ground from 0s to 2000s. The sample was stressed at room temperature (25℃).
The devices was place in the dark environment (in the black box) and under room pressure and temperature. Table 2-2 shows the experiment flow of devices under light illumination. The a-IGZO TFTs with different annealing conditions were measured under different wavelength light which ranged from 900 nm (visible light) to 300 nm (UV light).
The light source was a halogen optic lamp from OSRAM Inc. at 150 W generating light intensity about 63315 lx. We find the devices fabricated by the standard manufacturing processes should be placed in the box for few minutes even few hours to get stable electrical performances. And light_1_on means the specific wavelength illumination on device under measuring.
Table 2-2 The experiment flow of devices under light illumination