4.2 Measurement results
4.2.1 Reconstruction of the HR-TFT LC lens
From the experimental setup as mentioned in 2.3, the fringing pattern can be captured by CCD. The fringing pattern (Fig. 4-2) is utilized to observe the phase retardation of the LC lens. According to the fringing pattern, these bright and dark series lines can reconstruct Δ n profile and estimate the optical property. The refractive index distribution of proposed HR-TFT LC lens effectively provides a gradient electric field distribution. The resistance is about 3MΩ/□, as measured the data between source and drain electrodes. The result of refractive index distribution of HR-TFT LC lens is closed to the ideal parabolic curve as shown in Fig.4-3.
Fig. 4- 2 HR-TFT LC lens fringing pattern
Fig. 4- 3 HR-TFT LC lens refractive index distribution
45
4.2.2 Transmittance of the HR-TFT LC lens
The brightness is always a critical parameter for a display. Thus a high transparent HR-TFT LC lens placed in front the display is needed. We fabricated a multi-layer HR-TFT LC lens, and the measurement is shown in Fig. 4-4. Fig. 4-4 shows that the transmittance of the HR-TFT LC lens is above 90% in the visible wavelength. For the a-IGZO absorbs the ultraviolet, the transmittance decreases in the UV wavelength.
Fig. 4- 4 HR-TFT LC lens refractive index distribution
4.2.3 Analysis of the electrical properties
The transfer characteristic of a-IGZO TFTs for inverted-coplanar structure is shown in Fig. 4-5. Fig. 4-5(a) shows the a-IGZO TFTs annealing at 400℃ exhibit a Vth of 6.6 (V), a S value of 0.8179 (V/decade), a μ FE of 6.52 (cm2/Vs), and a Ion/Ioff of 7E5; Similiarly, Fig. 4-5(b) shows the a-IGZO TFTs annealing at 350℃ exhibit a Vth of 5.0 (V), a S value of 0.7122 (V/decade), a μ FE of 9.03 (cm2/Vs), and a Ion/Ioff of 1.2E6. All of the parameters are summarized in Tab. 4-1. The comparison between
70
300 400 500 600 700 800
Transmittance(%)
300 400 500 600 700 800
Transmittance(%)
46
these two annealing temperatures shows 350℃ has a better electrical properties, such as the lower Vth (i.e. the lower power consumption cost), larger current (i.e. the better charge property), and higher mobility.
(a)
(b)
Fig. 4- 5 a-IGZO TFTs transfer characteristics (a) 400℃ annealing (b) 350℃ annealing
Table. 4- 1 Electrical parameters of a-IGZO TFTs
1.0E-12
47
Fig. 4- 6 a-IGZO TFT structure
Moreover, the definitions of each parameter in the TFT part are following:
Vth (Threshold voltage): the value of VGS at which a sufficient number of mobile electrons accumulate in the channel region to form a conducting channel.
S (Subthreshold swing): S, defined as the voltage required increasing the drain
current by a factor of 10, is given by
(Field effect mobility): the mobility is an important parameter of the
semiconductor since it describes how fast a particle can move due to an electric field.
The is defined by the transconductance (gm) at a low drain voltage (VDS=0.1V).
48
Thus, is obtained from Eq. 4-3:
(4-4)
Ion/Ioff (drain-current ratio): on-current (Ion) determines the rate of the charging and the off-current (Ioff) is associated with the leakage of the voltage.
4.2.4 Holding time of HR-TFT LC lens
The equivalent electric circuit of the HR-TFT LC lens and the voltage output are shown in Fig. 4-7. CLC is about 3.98pF after calculating ( ). For the electric potential saving on the drain electrodes, a 5uF storage capacitor is added parallel connection to the CLC [Fig. 4-7 (a)]. The storage capacitor can reduce the electric potential various which is caused by the leakage current. Therefore, the electric potential can be effectively saved on the drain electrodes. The source voltage operates at ±5 voltages 60Hz, and the gate voltage operates at 0~30 voltages 120Hz pulse function.
(a) (b)
Fig. 4- 7 HR-TFT LC lens (a) electric circuit (b) voltage output signal
CLC
49
Fig. 4- 8 Operation of HR-TFT LC lens initial and charging and holding state The operating results are shown in Fig. 4-8. The electrodes are first set at zero volt in the initial state. Following the source electrodes apply 5 volts and the gate electrodes are still set at 0 volt as shown in Fig. 4-8(b). For this state, the drain electrodes cannot be charged by the drain electrodes because the TFT is turned off (i.e., channel off). For the charging state (i.e., channel on), the voltage is applied on the gate electrodes. During this state, the drain electrodes are charged by the source
Initial state
Charging
(Gate on)
Holding
Top view Side view
(a)
(b)
(c)
(d)
(e)
50
electrodes as shown in Fig. 4-8(c). When the gate is turned off, the source electrodes turn back to zero volt. By this time, the charge in the drain electrodes can still be holed due to the TFT is turned off. Therefore, the leakage current between TFT and LC is an important point decided the holding time of electric potential.
From the previous work [35], the electrical properties of a-IGZO will be affected due to the UV absorption and the material ratio of a-IGZO. However, the light-induced in the TFT characteristic is reversible with the recovery time constant.
There are two methods to solve this problem. First, the gate operating voltage of the HR-TFT LC lens could adjust to the exposed light power. Second, since the a-IGZO absorbs mainly in the UV wavelength, an anti-UV layer coating in front of the panel to prevent UV light will be a simple way.
4.3 Summary
The High-resistance Thin-film Transistor Liquid Crystal Lens (HR-TFT LC lens) with a-IGZO active layer on glass substrate had been fabricated and measured.
Supplying specific operating voltages on each electrode, the HR-TFT LC lens performed a refractive index distribution which was closed to the ideal parabolic curve as the resistance is about 3MΩ/□. It also showed that the transmittance of the high transparent multi layer HR-TFT LC lens is above 90% in the visible wavelength.
The better electrical properties is also obtained as the annealing temperature at 350
℃.
The timing of the charging and the holding are important in the operating process.
The charging time responded rapidly when the turn on current is about 10^-5 A. Once the gate turned on, the current easily passed through the resistance layer charging the source electrodes. As the lens holding state, the gate is turned off and the current
51
hardly passed through the high resistance layer. Therefore, the charges can be storage on the source electrode. It also means the lens holding time is depended on the leakage current of the TFT path or the LC path. The transfer characteristic of TFT shows that the off state leakage current is about 10^-10 A. However, the LC leakage current will be affected by the ρLC. Compared these two leakage currents, the LC path probably is the reason for the weak holding time [36]. Besides, the UV effect can be solved by gate operating voltage or anti-UV layer.
Finally, the HR-TFT LC lens can be expected as an active LC lens to obtain the 2D/3D localized by the above operating method.
52
Chapter 5
Conclusions and Future Work
5.1 Application of HR-TFT LC-lens Array
Nowadays, people would like to pursue more realistic display images through developing high quality 3D technologies. The existing work, an auto-stereoscopic display using the LC lens can display high resolution 2D images on 2D mode, and low resolution 3D images on 3D mode. Moreover, the interesting function of locally 2D/3D switching is also required for the users to watch the texts in 2D mode, and figures in 3D modes.
The HR-TFT LC lens has multifunction such as, 2D/3D switchable, 3D rotatable, and 2D/3D localized. The traditional fixed lens array is hard to switch the lens arbitrary. Besides, not all the images are suitable for showing in 3D. Contrary, the lenticular LC lens array can easily switch on and off the lens by electric control, as shown in Fig. 5-1. Nowadays, more and more mobile devices can display in horizontal and vertical directions. However, the 3D images only can show in one direction. The cross design of the top and bottom electrodes coating with semiconductor layer can realize the 3D rotatable display. As Fig. 5-2 (a) shown, the gradient electric field is created on the top electrodes and the bottom can be saw as plate electrode. Fig. 5-2 (a) shows that the gradient electric field is created on the bottom electrodes and the top electrodes are saw as plate electrode. Thus, the lens array can show in two directions. The localized 2D/3D displays are surely to be the mainstream in the future. There is no needed to show all images in 3D modes. For
53
example, the video on Youtube could be the 3D mode, but the other part, like text, could maintain in 2D mode [Fig. 5-3]. Therefore, the HR-TFT LC lens is proposed to obtain 2D/3D switchable, 3D rotatable, and 2D/3D localized.
(a) (b)
Fig. 5- 1 2D/3D switchable LC lens (a) lens off (b) lens on
(a) (b)
Fig. 5- 2 3D rotatable LC lens (a) driving top electrodes for horizontal 3D images (b) driving bottom electrodes for vertical 3D images
2D/3D Switchable
3D Rotatable
54
Fig. 5- 3 3D localized LC lens (a) gate on for side electrodes charging to middle electrodes (b) gate off for holding localized LC lens
5.2 Conclusions
A novel active device, high-resistance thin film transistor liquid crystal lens, of locally 2D/3D switching display had been demonstrated in this thesis. Controlling the LC with switching gate electrode on and off, the drain electrodes can be charging and holding to perform a LC lens function. The HR-TFT LC lens has some benefits: first of all, coating the high resistance layer between the ITO electrodes, the continuous electric field could be generated. Moreover, experimental results also showed that the HR-TFT LC lens can build a lens-like refractive index distribution. Besides, the results also showed that the high transmittance in the visible light (above 90%) is still kept for multi-layer structure of HR-TFT LC lens.
Second, when turning gate electrode on, the charging time responded rapidly for the turn on current is about 10^-5 A; and the leakage current of TFT is about 10^-10 A
3D Localizable
55
when turn gate electrode off. It means the charges can firstly charge into the drain electrodes quickly, and the charges can be storage in the drain electrodes following.
TFT path and LC path are two paths for the leakage current. The leakage current of TFT is only about 10^-10 A. Therefore, the LC leakage current probably is the reason for the weak holding time. Consequently, extending the holding time of the HR-TFT LC lens can be the next research topic.
In this thesis, the HR-TFT LC lens is based on the high-resistance layer and TFTs functions. The structure based on ELC lens and MeD-LC lens have much simpler fabrication than Active LC lenticular lens and polarization active micro-lens (see section 1.6). Adjustable pitch of LC-lens by observers will be another benefit. For example, the three-electrode LC lens can adjust to five-electrode LC lens for the different viewing distance and different 3D viewing numbers. Compared to others LC lens, the HR-TFT LC lens can not only apply on 2D/3D localized but also 2D/3D switchable and 3D rotatable in a structure. Therefore, the extra TFT-plate to fabricate HR-TFT LC lens is indeed needed.
5.3 Future work
To hold the voltage on the drain electrodes as the gate turning off, the TFT leakage current is supposed to be as small as possible. However, the resistance value should be about 3MΩ/□ to perform a gradient electric field for the LC lens. This is the trade-off for the HR-TFT LC lens. To overcome this issue, the storage capacitor is needed. Beside, the LC leakage current should also be small to enhance the lens holding time.
56
5.3.1 Holding time enhancement
There are two paths for the leakage current. One is the TFT path. The dVhold at most is 17V as shown in table.5-1. The other various potential on the charged drain electrodes is acceptable for the LC holding at 60Hz. The second one is the LC path.
The VLC is almost zero as the resistance value of liquid crystal is about 10^9Ω-cm.
The low resistivity may causes by the purity of LC or the fabricated processes.
In order to enhance the lens holding state, the high resistivity value of liquid crystal is needed. The pollution such as moisture, dust particle causes the low LC resistivity value. Otherwise, the charging frequency should be increased and recharged again before the electric potential disappeared.
Table. 5- 1 TFT path various electric potential
10^-12 10^-11 10^-10 10^-09 10^-08
10pF
1.7E-03 1.7E-02 1.7E-01 1.7E+00 1.7E+01100pF
1.7E-04 1.7E-03 1.7E-02 1.7E-01 1.7E+001nF
1.7E-05 1.7E-04 1.7E-03 1.7E-02 1.7E-0110nF
1.7E-06 1.7E-05 1.7E-04 1.7E-03 1.7E-02100nF
1.7E-07 1.7E-06 1.7E-05 1.7E-04 1.7E-031uF
1.7E-08 1.7E-07 1.7E-06 1.7E-05 1.7E-0410uF
1.7E-09 1.7E-08 1.7E-07 1.7E-06 1.7E-05Cst Ioff
dVhold(V) <lleak*dthold/Chold dthold=16.67ms Chold=CLC+Cst CLC=εLCε0(A/d)= 3.98pF
57
Table. 5- 2 LC path various electric potential
5.3.2 Passive layer on a-IGZO film
For the chemical reaction may happen between a-IGZO film and liquid crystal.
The a-IGZO film has better protected by a passive layer to make sure the electrical properties won’t be affected by other materials. In addition, the air or the moisture will also affect the amorphous layer. Therefore, use the passive layer to protect amorphous layer can enhance its steadiness.
Fig. 5- 4 HR-TFT LC lens with passive layer on a-IGZO layer
5.3.3 Summary
As mentioned above, these two methods can improve the HR-TFT LC lens by increasing lens holding time and passive layer. Finally, we may realize these two methods to solve the issues of the HR-TFT lens. For the application on products, the
RLC (Ω—cm) 1.E+13 1E+12 1E+11 1.E+10 1.E+09 1.E+08
R
LC*C
LC(ms)
1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01V
LC(V)
5.E+00 5.E+00 4.E+00 1.E+00 8.E-07 3.E-68 CLC=12*8.85*10^-14 F/cm* 75*0.015/30 VLC(V)=V0*exp[-16.67/(RLC*CLC)]Passive layer SiO2(20nm)
58
electrode design is one of the methods to build storage capacitor into the cell.
Fig. 5- 5 HR-TFT LC lens electrode design
A’
B
B’
A
59
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