Chapter 3 Experimental Methods
3.2.1. RF sputtering
RF sputtering can be applied to the deposition of both insulating and conducting materials. Figure 3-2 shows a RF sputtering system, 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.
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. 3-2 Schematic RF sputtering system.
3.2.1. DC Sputtering
DC sputtering has the advantage of higher deposition rate and is less expensive than RF sputtering. A DC sputtering system is shown in Fig. 3-3, the substrate is located above the target and acts as the anode. DC sputtering is commonly applied to deposit conductive materials.
Fig. 3-3 Schematic DC sputtering system.
Chapter 4
Results and Discussion ______________________________________
4.1. The Effects of Oxygen Flow Rate and Post-Annealing on a-IGZO TFTs
4.1.1. Introduction
The carrier source of Si and metal oxide is shown in Fig. 4-1. For Si, the carriers are resulted from impurity doping as shown in Fig. 4-1 (a). Electrons dominate the carrier transport for phosphorous-doped Si. Carrier transport is dominated by holes for boron-doped Si. For metal oxide, the carrier concentration is related to the oxygen vacancy; one oxygen vacancy provides two electrons, as shown in Fig. 4-1 (b).
characteristics are strongly associated with the a-IGZO film. The reactions on film surface dominate the threshold voltage (Vth) shift. The oxygen absorption changes the carrier concentration. The oxygen absorption forms depletion layer, resulting in Vth shift.
The oxygen absorption accompanies partial charge transfer, Vth varies at different oxygen flow rate implies the change in carrier concentration during the absorption and desorption processes. When the oxygen flow rate increases, the channel carrier concentration decreases because of less oxygen vacancies in a-IGZO film. Therefore, higher voltage is needed to turn on the channel.
The electrical characteristics of IGZO film can be controlled by varying the deposition conditions (Ar flow rate and O2 flow rate). When the oxygen flow rate is low, IGZO film is not applicable for TFT channel layer because the film
conductivity is high. When oxygen flow rate is high (over 8 sccm), IGZO film becomes insulator. In intermediate oxygen flow rate (5~8) sccm, IGZO shows semiconductor characteristics and is suitable for channel layer. It is considered that the low oxygen flow rate increases the electrical conductivity of the deposited film.1,5 In the case of Ar gas flow, even though it is not strongly related to electrical property of IGZO film, it is one of the key parameter to control the uniformity of TFT behavior. Fig. 4-2 shows that only in a proper range of PO2 will the a-IGZO exhibit the semiconductor characteristics.4 In our experiments, several oxygen flow rates (0 sccm, 0.2 sccm, 0.4 sccm, 0.6 sccm, 0.8 sccm) were adopted so as to prepare the a-IGZO TFTs with various electrical characteristics.
Fig. 4-2 Electrical property of IGZO TFT as a function of oxygen and argon flow rate during deposition.6
4.1.2. Results and Discussion
4.1.2.1. Determination of the Deposition Rate
First, the deposition rate of IGZO was determined by measuring the film thickness with the AFM. The AFM results of a-IGZO film thickness are shown in Fig. 3-3. The deposition rate is about 5.933 nm/min for oxygen flow rate = 0 sccm device and 3.537 nm/min for oxygen flow rate = 0.6 sccm device.
(a)
(b)
Fig. 4-3 The AFM result of (a) 0 sccm O2 device deposited for 6.5 mins.
(b) 0.6 sccm O2 device deposited for 8 mins
4.1.2.2. The Effects of Oxygen Flow Rate on a-IGZO TFTs
The a-IGZO TFTs were operated in enhancement mode. The SiO2 gate insulator is 100 nm thick and the post-annealing time is 1.5 hours. Good electrical characteristics like large on-state drain current, small threshold voltage, and low threshold voltage were obtained when the oxygen flow rate is 0 sccm, as shown in Fig.4-4.
-10 0 10 20 30
Fig. 4-4 The a-IGZO TFT output characteristics for different oxygen flow rates.
As a result, under the same post-annealing time, Vth is smaller for smaller oxygen flow rate. The decrease of Vth is due to the mobile carrier increase.
The mobile carrier increase in channel relates to the oxygen flow rate and affects the channel/dielectric interface by changing the density of interface states. In other words, less oxygen vacancies are filled when the oxygen flow rate is smaller. The oxygen vacancies provide electrons and increase the channel carrier concentration, leading to a smaller Vth.
4.1.2.3. The Effects of Post-Annealing on a-IGZO TFTs
TFT based on AOSs (e.g. IGO ZTO, ZIO) sputtered in pure Ar requires high annealing temperature (T > 300 ˚C) to exhibit satisfactory electrical characteristics; including better saturation current, smaller hysteresis, and Vth. The post-annealing effects on device with 300 nm SiO2 gate insulator is shown in Fig. 4-5. Vth shifts negatively after post-annealing because post-annealing leads to the lattice structure rearrangement, structural relaxation, and the improved a-IGZO bonding. Post-annealing improves the channel/dielectric interface; and the charge trapping defects are decreased.
The electrical output characteristics for devices post-annealed for 2 and 3 hours are compared in Fig. 4-6. Annealing causes modification of the semiconductor/insulator interface, local atomic rearrangement and improved bonding. Three-hour annealing is the appropriate for a-IGZO TFT with a 300 nm SiO2 gate insulator. Vth shifts negatively to be near 0 V after the thermal treatment.
-20 -10 0 10 20 30 40
Fig. 4-6 The output electrical characteristic comparison for a-IGZO TFTs post-annealed for 2 and 3 hours.
4-1-3. Conclusions
The oxygen flow rate was varied to examine oxygen absorption effect.
The Vth changes with the oxygen flow rate. The oxygen vacancies provide electrons and increase the channel carrier concentration, leading to a smaller Vth. Post-annealing improves the crystallinity in a-IGZO because of the semiconductor/insulator interface modification and local atomic rearrangement and the threshold voltage can be adjusted to be near 0 V.
It is found that IGZO thin film transistors are very sensitive to oxygen and can be used as oxygen or pressure sensors.9
4.2. The Interface Modification for a-IGZO TFTs
4.2.1. Introduction
Research for a-IGZO TFT was focused on the intrinsic limitations of semiconductor. A large Vth is still an issue for a-IGZO TFT. It is noteworthy that a high interface trap density at the semiconductor/dielectric interface increases the Vth. A large Vth leads to the “hard saturation” phenomena in the electrical output characteristics. Besides, a poor dielectric layer quality leads to the increase of gate leakage current.
In this session, the a-IGZO TFT with the dual-stack structure is introduced, as shown in Fig.4-7. A buffer layer and the gate dielectric insulator were stacked up. The buffer layer is an interface modification layer. The buffer layer materials used in this study were hafnium oxide (HfO2), aluminum oxide (Al2O3), and hexamethyldisilazane (HMDS). Device with only the SiO2 gate insulator is for comparison.
Fig. 4-7 The dual stack a-IGZO TFT structure.
The HfO2 and Al2O3 buffer layers are both 10 nm thick, and are deposited by the e-gun evaporation on SiO2 for the a-IGZO/gate dielectric surface treatment, as shown in Fig. 4-7. The series capacitance of the SiO2/buffer layer was dominated by the thicker 300nm SiO2 and the contribution of the thinner buffer layer was estimated to be of less than 10 %.
4.2.2. Result and Discussion
The buffered-TFT output characteristic comparison is shown in Fig. 4-8.
It is noticeable that ID of the HMDS-buffered a-IGZO TFT is increased.
-20 0 20 40 10-13
10-11 10-9 10-7 10-5
I D
VG SiO2
Al2O3 HfO2
HMDS
Fig. 4-8 ID-VG curve comparison of the buffered-a-IGZO TFTs.
The contact angle measurement result is shown in Fig. 4-9. The water drop on HMDS has the largest contact angle on the HMDS surface, indicating that a-IGZO has improved interfacial condition when it is stacked with a hydrophobic material.
(a) (b)
(c) (d)
Fig 4-9 Contact angle image for water drop on the (a) Al2O3, (b) HfO2, (c) HMDS, and (d) SiO2 surface.
Table 4-1 Contact angle of different buffer layer surface.
The performance comparison of a-IGZO TFT with the interface modification layer is shown in Table 4-2. The mobility of the HDMS-buffered TFT was enhanced to 13.67 cm2/Vs.
Table 4-2 The performance of the a-IGZO TFTs with the interface modification layer.
4.2.3. Conclusion
A dual stack TFT structure is introduced to examine the properties of the a-IGZO/gate insulator interface. The interface modification materials adopted in this session are HfO2, Al2O3, and HMDS. As a result, HMDS improves the field-effect mobility of the a-IGZO TFT. All buffered-TFTs have higher mobility than TFT without the interface modification, indicating that the gate insulator/active layer interface is improved.
From the contact angle measurement; HMDS surface has the largest contact angle for the water drop, indicating that a-IGZO has improved interfacial condition when it is stacked with hydrophobic material HMDS.
The proposed dual layer structure has shown a great potential for the advanced AMLCD technology since the output electrical characteristics of a-IGZO TFTs are greatly related to the conditions of the a-IGZO semiconductor/gate insulator interface.
4.3. The Effects of Source and Drain Electrodes on a-IGZO TFTs
4.3.1. Introduction
The unmatched source/drain electrodes lead to the high series contact resistance, causing the current suppression phenomena. Furthermore, a high contact resistance induces the current crowding effect and increases the Vth. Thus, a high efficient contact is necessary to attain electrical properties.
For a-Si:H TFTs, the source to drain resistance is influenced by the contact resistance between n+ a-Si:H and the source/drain metal; by the bulk resistance
of the n+ a-Si:H film, by the interface effect between n+ a-Si:H and source/drain electrodes, and by the intrinsic a-Si:H layer sheet resistance.
Unlike a-Si:H TFTs, the proposed a-IGZO TFT does not have a highly doped ohmic layer, the source to drain resistance is affected by the interface properties between intrinsic a-IGZO and the source/drain metal. These properties are the interface trap, the effective contact area, and the barrier height and width between the a-IGZO semiconductor and the source/drain metal.
Compared to a-Si:H TFT, the a-IGZO TFT source to drain resistance is more dependent on the channel resistance, because the interface properties are related to the gate voltage.
In our study, the a-IGZO TFTs were fabricated using materials with different work function for the source/drain electrodes. These materials are as follows: titanium (Ti), gold (Au), aluminum (Al), and indium-tin-oxide (ITO), which is widely used in a-IGZO TFT researches. The chosen electrode materials are widely used not only in the integrated circuit (IC) industry but also in the display applications.
First, the effect of different source and drain electrodes on a-IGZO TFT is investigated. Following, the effect of ambient interactions on a-IGZO TFT without passivation is discussed.
4.3.2. Results and Discussion
4-3-2-1. The Effects of Source/Drain Electrodes on a-IGZO TFTs
The ID-VG characteristics of a-IGZO TFTs with different source/drain electrodes are shown in Fig. 4-10. The a-IGZO TFT with Ti source/drain electrodes has the highest on-state current (Ion) and the smallest Vth.
-20 -10 0 10 20 30 40
Fig. 4-10 The electrical characteristics comparison for a-IGZO TFTs with different source/drain electrodes.
The a-IGZO TFT output characteristics can be related to the work function difference between the a-IGZO semiconductor and the source/drain electrodes.
The work functions of several metallic materials are shown in Fig. 4-11. As a
result, Ti has the lowest Vth and Au has the highest Vth.
Figure 4-11 The work functions if some metallic materials.
The work function difference can be related to the contact resistance. The larger the work function difference, the larger the contact resistance. Figure 4-10 shows the contact resistances (Rc) of various source/drains. Generally, Au has the highest Rc, Al has larger Rc than ITO. Ti provides the lowest Rc, indicating that Ti provides the smallest work function difference between the source/drain materials and the a-IGZO thin film.
0 20
Fig. 4-12 The contact resistance comparison for different source/drain electrodes.
The method adopted for extracting Rc is as shown below:
For linear region drain current formula:
ID = (W/L)µCox(VG-Vt)VD
Partial differential to drain voltage was taken:
∂ID/∂VD = (Rtotal)-1 Rtotal = Rc + Rch
If under the same VG , with two different lengths a and b:
Rta = Rc + aK ….(1) Rtb = Rc + bK ….(2) (2) x a/b ….(3)
From (1) – (3), Rc was obtained.
The comparison for a-IGZO TFT output electrical characteristics with varied source/drain is shown in Table 4-3. A large work function difference between channel and electrodes leads to a large Schottky Barrier and increases Vth. As a result, Au has the largest threshold voltage because Au has the largest contact resistance for a-IGZO film. In other words, a large contact resistance causes the current crowding effects, leading to a large Vth. On the other hand, a large contact resistance increases the Schottky Barrier and reduces mobility. In our experiments, for Al, Au, Ti, ITO, the different source/drain electrode materials, the mobilities of a-IGZO TFTs are appropriate for display applications. As a result, the a-IGZO TFT with the Ti source/drain electrodes has high µ, the lowest Vth, and the smallest SS. Therefore, Ti is an appropriate source/drain material for the high performance a-IGZO TFT.
Table 4-3 Electrical properties comparison for varied electrodes.
4.3.2.2. The Ambient Effects on a-IGZO TFTs
The a-IGZO film is sensitive to the surface absorption of oxygen and water molecules. When the oxygen molecules in atmosphere fill into the oxygen vacancies, they decrease the electrical conductivity of a-IGZO film. In other view, the oxygen vacancies can be assumed as holes and they can assist the electrical conduction of a-IGZO film. Therefore, the filling of oxygen vacancies reduces the electrical conduction of a-IGZO film. On the other hand, there are also oxygen molecules carrying electrons in ambience. They also fill in to the oxygen vacancies, affecting electrical characteristics.
Fig. 4-13 The schematic diagram showing the role of (a) oxygen as an electron acceptor and (b) water molecules as a electron donor onto a-IGZO surface.14
The shift of the Vth over time for a-IGZO TFTs with different source/drain electrodes is shown in Fig. 4-13. For a-IGZO TFT, the water molecule absorption is the main cause for the shift of threshold voltage. The oxygen molecules are absorbed on both channel and electrodes, but are electrically active only on channel. In atmosphere, the metastable hole traps form in oxide semiconductors. The water molecules diffuse in and out a-IGZO film adversely affects a-IGZO properties.13
-20 0 20 40
-20 0 20 40
Fig. 4-14 The negative shift of the threshold voltage over time for a-IGZO TFT with (a) Ti, (b) ITO, (c) Au, and (d) Al source/drain electrodes
The absorbed oxygen molecules undergo the partial charge transfer, and form the depletion layer below the a-IGZO active layer surface, hence shifts the threshold voltage positively.
On the other hand, the water molecules on a-IGZO surface act as the electron trapping centers.13 The water molecules absorbed by a-IGZO film form the accumulation layer below the a-IGZO film surface, hence shifts the Vth
negatively. The surface interactions on a-IGZO thin film with the water molecules involves the trap creation/removal in active layer. As a result, the negative Vth shift was observed, indicating that in atmosphere, the interaction with water molecules is the dominant mechanism for a-IGZO TFT.
4.3.3. Conclusions
The a-IGZO TFTs with varying source/drain materials, which are Ti, Au, Al, and ITO, were fabricated without passivation for investigating electrical performance of the a-IGZO TFT.
First, the output electrical characteristics for a-IGZO TFTs with different source/drain electrodes were compared. The work function difference between a-IGZO active layer and the source/drain electrodes can be considered as the
contact resistance. The larger the work function difference between a-IGZO thin film and the source/drain electrodes leads to the larger contact resistance.
The contact resistance is related to the mobility and the threshold voltage. In our experiments, a-IGZO TFT with Au source/drain electrodes has the largest contact resistance. As a result, the higher contact resistance leads to the higher mobility because of the higher carrier injection efficiency. On the other hand, the higher work function difference induces the current crowing effect, leading to the larger threshold voltage. The a-IGZO TFT with Ti source/drain electrodes has the lowest contact resistance; and the device performance is appropriate for the display applications.
Second, since the a-IGZO device without passivation interacts with the atmosphere, the ambience effects were investigated. The oxygen molecules form the electron trapping centers on the a-IGZO surface, thus reduce the carrier concentration. In contrast, the absorbed oxygen molecules deplete electrons while water molecules induce the electron accumulation layer. The water molecule absorption donates the partial electron transfer to the a-IGZO film surface. The water molecule absorption dominates the a-IGZO surface interaction in ambience. As a result, the threshold voltage shifts negatively.
Therefore, the adoption of the passivation layer prevents the degradation under humidity. Passivation is the key factor for sustaining the a-IGZO TFT subthreshold properties and the devices reliability.
Chapter 5
Conclusion ______________________________________
5.1. Conclusions
We report on the properties of a-IGZO TFTs fabricated on Si substrate at room temperature using sputtering process.
In the first part, the oxygen flow rate was varied to examine oxygen absorption effect. Vth is smaller for smaller oxygen flow rate, since the depletion layer underneath the absorption layer forms due to the charge transfer between absorbed O2 and a-IGZO forms.
Post-annealing improves the crystallinity in a-IGZO because of the semiconductor/insulator interface modification and local atomic rearrangement.
As a result, the field mobility can be up to 11.42 cm2(Vs)-1 and the threshold voltage can be adjusted to be near 0 V. Our device performance is appropriate for display applications.
In the second part, the dual stack a-IGZO TFT structure is introduced to
examine the properties of the a-IGZO/gate insulator interface. The interface modification materials adopted are HfO2, Al2O3, and HMDS. HMDS greatly improves the field-effect mobility of the a-IGZO TFT. As a result, HMDS-buffered a-IGZO TFT has larger mobility, implying improved channel/dielectric interfacial condition when it is stacked with the hydrophobic material HMDS. performance of the a-IGZO TFT with Ti source/drain electrodes is appropriate for the display applications.
Second, since the a-IGZO device without passivation interacts with the atmosphere, the ambience effects were investigated. The oxygen molecules reduce the carrier concentration. In contrast, the water molecule absorption provides electrons. The water molecule absorption dominates the a-IGZO surface interaction in ambience and the threshold voltage shifts negatively.
5.2. Future Work
Recently, the issue of the threshold voltage shift for a-IGZO TFT is a hot topic for fabricating a-IGZO TFT backplane for display application. The Canon’s proposed devise structure on SID’ 08, as shown in Fig. 5-1.
Fig. 5-1 The Canon’s proposed devise structure on SID’ 08.
However, in our future work, the direct Si3N4 passivation on a-IGZO by the PECVD process will not be adopted since the hydrogen doped Si3N4 will increase the a-IGZO conductivity, as shown in Fig. 5-2.
Fig. 5-2 The a-IGZO layer with the Si3N4 passivation layer.
In our proposed TFT structure with passivation layer, an oxygen rich SiO2 channel protection layer on the a-IGZO thin film can prevent a-IGZO from losing the O2. A Si3N4 layer can further suppress the active layer surface interaction with the ambience, as shown in Fig. 5-3.
Fig. 5-3 The proposed structure for a-IGZO passivation.
In addition, the comparison for organic/inorganic passivation will be futher studied and discussed. Ink-jet printing technology will be utilized to process organic materials into TFT devices.