JP2727578B2 - Method for evaluating characteristics of thin film transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] The present invention relates to a method for evaluating characteristics of a thin film transistor for driving a liquid crystal,
In particular, regarding the method of non-destructively evaluating the TFT characteristics in the panel state in which liquid crystal is sealed, the TFTs for driving the liquid crystal display device are electrically opposed to each other, with the aim of enabling non-destructive electrical evaluation. A plurality of pixel electrodes and thin film transistors are provided in association with one substrate surface of a pair of substrates, and a counter electrode is formed on the other substrate surface so as to face the pixel electrode,
In addition, in the characteristic evaluation of the thin film transistor in the liquid crystal display device having the configuration in which the liquid crystal is sandwiched between the substrates, it is detected on the bus line to which the drain electrodes of the plurality of thin film transistors are commonly connected in a state where the AC signal is applied to the counter electrode. The phase shift of the AC signal with respect to the applied AC signal is measured, and the channel resistance of the thin film transistor is evaluated based on the phase shift.

[Industrial applications]

The present invention relates to a method for evaluating characteristics of a thin film transistor (TFT) for driving a liquid crystal, and particularly to a panel in which a liquid crystal is sealed.
The present invention relates to a method for non-destructively evaluating TFT characteristics.

Since liquid crystal display devices have characteristics such as low power consumption, light weight, and easy color display, they are gaining a wide market as flat display devices such as pocket TVs and OA terminal devices.
In particular, a thin film transistor driven active matrix type liquid crystal display device capable of obtaining a large-capacity and clear gradation display is partially put into practical use and is being actively developed and researched.

The current-voltage characteristics of a TFT are generally gate, drain,
The measurement is made by making an electrical connection to the source terminal.However, since a completed panel with liquid crystal cannot be connected to an external circuit at the source terminal, if a display abnormality occurs during a life test, the panel is destroyed. TFT measurement.

[Conventional technology]

To evaluate the TFT characteristics, as shown in FIG. 7, the gate electrode G and the drain electrode D are connected to power supplies 11 and 12, respectively, and when this power supply is set to a predetermined voltage, the source electrode S
The current is measured by the ammeter A connected to the.

In an actual liquid crystal panel, the TFTs are arranged in a matrix and are further filled with liquid crystal, so that electrical contact cannot be made with the source electrodes of the TFTs, so that a direct current cannot be measured. Therefore, when an abnormality occurs in the display due to a life test or the like, the only method is to destroy the panel and conduct an electrical evaluation of the TFT characteristics.

[Problems to be Solved by the Invention] Conventionally, as described above, there is no other method for evaluating the TFT characteristics of a completed panel except for measuring by destroying the panel. However, once the panel is destroyed, it cannot be evaluated again as a panel, and the life test must be completed at that point.

The present invention provides an electrical evaluation of a driving TFT for a liquid crystal display device,
The purpose is to be able to perform non-destructively.

[Means for solving the problem]

The present invention, as shown in FIG. 1 (a), facing the counter substrate P AC signal S _I to the 'counter electrode E formed on the' added to sandwich the TFT substrate P and the liquid crystal, the drain electrode D of the TFT Is measured with respect to a predetermined gate bias voltage for a phase shift φ of the AC signal S _O detected on the bus line to which the AC signal S _O is connected, for example, the drain bus line _BD, with respect to the AC signal S _I.
From the obtained phase shift φ, the channel resistance is determined by the relationship described later.

When the counter electrode E 'is a common electrode as shown in FIG. 1A, the sum of the characteristics of all the TFTs connected to the measurement bus line is measured.

As shown in FIG. 1 (b), when the counter electrode E 'is an electrode having a stripe pattern perpendicular to the bus line _BD connected to the drain-in electrode D, of the stripe-shaped counter electrode E', by applying an alternating signal S _I only to certain electrodes, measuring a specific one TFT characteristics at the intersection of the specific stripe shaped counter electrode E 'and the bus line B _D of the TFT substrate P side can do.

Note 1 (a), (b), the the V _G bias power, G is the gate electrode, S is the source electrode, E is a pixel electrode.

(Operation)

One pixel which is the circuit under measurement in the above measurement is represented by an equivalent circuit shown in FIG. That is, one pixel has a liquid crystal capacitance C _L of a pixel electrode E and a channel resistance R of a TFT connected in series, and a liquid crystal capacitance C _B of a bus line part connected in parallel to the pixel. a configuration in which bus line resistance R _B are connected in series.

Complex impedance Z of the _{circuit, Z = [R B {R} 2 + (Z L + Z B) 2} + Z B 2 R -i {R 2 Z B + Z B Z L (Z L + Z B)}] / { R ² + (Z _L + Z _B ) ² ｝ here, ω = 2πf (f: frequency of AC signal).

From the above equation, a phase shift φ occurs between the input AC signal S _I and the output AC signal S _O. Since there is a relationship of φ = (π / 2) + δ between the phase shift φ and δ of the dielectric loss tanδ, either φ or tanδ is measured.
This dielectric loss tanδ is Therefore, the channel resistance R can be obtained from tan δ.

From the equation, the channel resistance R is In this case, the dielectric loss tan δ is maximized, that is, the phase shift φ is maximized.

FIG. 3 shows the frequency dependence of the dielectric loss tan δ on the channel resistance R.

On the other hand, channel resistance R of the TFT varies depending on the change of the gate bias, the gate bias value corresponding to the fifth (the frequency of the applied AC signal S _I) measurement frequency, as shown in FIG f
tanδ has a maximum value. Therefore, assuming that a tanδ at a certain gate bias value maximum, the channel resistance R in the gate bias value, the measurement frequency f and C _L, can be obtained immediately from the C _B using the above equation. In this case, the value of the connected bus line resistance R _B to the drain electrode need not be known.

By using this relationship, you can change the gate bias
By measuring the frequency dependence of the gate bias value at which tan δ takes an extreme value, it is possible to easily evaluate the TFT characteristics.

That is, by changing the gate bias, the frequency dependence of the gate bias value at which tan δ takes an extreme value is obtained in advance. This relationship shows the frequency f of the applied AC signal S _I for taking tanδ extrema in some gate bias.
If C _L and C _B are known, the above equation determines the channel resistance R with respect to the frequency f. Therefore, the channel resistance with respect to the gate bias can be easily obtained from the correspondence between the frequency f and the gate bias, and the TFT characteristics can be evaluated.

In this way the present invention, by an alternating signal S _I in addition to the counter electrode E ', to measure the phase shift φ of the AC signal S _O detected by the bus line B _D connected to the drain electrode of the TFT, Since TFT characteristics can be evaluated, T
FT characteristics can be measured.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to the drawings.

4 (a) and 4 (b) show a phase shift measuring circuit used in this embodiment.

In this embodiment, the measurement of the phase shift φ is performed by using an LCR meter 4274A manufactured by YHP for frequencies of 100 Hz or more, as shown in FIG. As shown in b), this was performed using a lock-in amplifier LI-575 manufactured by NF Block.

 Hereinafter, the measurement method will be described.

First, using the measurement circuit shown in 4 (a), is measured and the liquid crystal capacitance C _L of the pixel electrode portion, a liquid crystal capacitance C _B of the bus line part. The channel resistance R is large when the gate bias is low, and small when the gate bias is high. Therefore, when the capacitance C is measured by changing the gate bias, the capacitance C when the gate bias is low is substantially equal to the capacitance C _B of the bus line portion, and the capacitance C when the gate bias is high, as is clear from the equivalent circuit of FIG. is the sum of the capacitance C _L of the C _B and the pixel electrode portion. Therefore, from both, C _B and
C _L is found.

The resistance R _B of the bus line connected to the drain electrode is a value determined in design, is substantially constant for each panel. Therefore, it can be determined by measuring the same type of liquid crystal panel.

As described above, three of C _L , C _B , R _B, and R shown in the equivalent circuit of FIG. 2 are known, so that the measurement circuit of FIG. , by applying an AC signal S _I to the counter electrode E 'drain bus line B _D
The tan δ is measured above, and the channel resistance R can be obtained from the measured values and the above-mentioned C _L , C _B , and R _B by the above-described formula.

Further, if the gate bias value at which tan δ becomes a maximum is measured by changing the frequency, the gate bias and the channel resistance R
3 and FIG. 5, and the channel resistance R can be easily obtained from the equation.

Since the LCR meter cannot measure a frequency range below 100 Hz, the measurement circuit shown in FIG. 4 (b) was used for such a low frequency range. In this case, the phase shift φ of the signal having the same frequency as the output signal of the signal generator 4 is obtained by the lock-in amplifier 2. As described above, φ immediately becomes δ
Thus, as in the case of using the measurement circuit of FIG. 4 (a), tan δ is obtained while changing the gate bias, or the frequency dependence of the gate bias value at which tan δ takes an extreme value is obtained. , TFT channel resistance R can be obtained. If a lock-in amplifier is used, measurement in a frequency range of 100 Hz or more is possible, and measurement can be performed over the entire range by the measurement circuit (b). However, in this measuring circuit can not determine the C _L and C _B above,
To obtain these, the measurement circuit using the LCR meter of (a) was used together.

FIG. 5 shows an example of the dielectric loss tan δ with respect to the gate bias obtained in this manner. The relationship between the gate bias and the channel resistance / conductance (reciprocal of the channel resistance R) of the TFT, that is, the current, is shown in FIG. Shown in the figure.

Thus, according to this embodiment, the TFT characteristics can be evaluated without breaking the liquid crystal panel.

As described in the one embodiment, the applied AC signal S _I in the present invention, the phase shift between the AC signal S _O that is detected on the bus line B _D connected to the drain electrode D phi, or the phi
By measuring the tan δ corresponding to the above, the characteristics of the internal thin film transistor can be evaluated without breaking the liquid crystal display panel.

The method for evaluating the characteristics of a thin film transistor according to the present invention can be applied to an active matrix type liquid crystal display device having any structure. For example, a gate connection type liquid crystal display in which an independent drain bus line is omitted, a gate electrode G is connected to one of two adjacent scan bus lines (gate bus lines), and a drain electrode D is connected to the other. In the device (see Japanese Patent Application No. 61-212696), an AC signal may be detected on the scan bus line to which the drain electrode D is connected.

〔The invention's effect〕

As described above, according to the present invention, it is possible to evaluate the characteristics of the liquid crystal driving TFT without breaking the liquid crystal panel, which greatly contributes to analysis of a life test and improvement of efficiency.

[Brief description of the drawings]

1 (a) and 1 (b) are diagrams illustrating the configuration of the present invention, FIG. 2 is a diagram illustrating an equivalent circuit for one pixel of a liquid crystal display device, and FIG. 3 is a diagram illustrating a relationship between a channel resistance and a dielectric loss. FIG. 4 is a diagram showing a measurement circuit according to one embodiment of the present invention, FIG. 5 is a diagram showing an example of measurement results of the above-mentioned embodiment, FIG. 6 is a diagram showing an example of characteristic evaluation according to the above-mentioned embodiment, FIG. 7 is an explanatory view of a conventional measuring method. In the figure, P is a TFT substrate, P 'is a counter substrate, E is a pixel electrode, E' is a counter electrode, T is a thin film transistor, G is a gate electrode, S
Source electrode, D is a drain electrode, B _D bus line connected to the drain electrode, B _G is the gate bus line, S _I AC signal applied, S _O is detected AC signal, phi is a phase shift, tanδ is dielectric loss, C _L is the liquid crystal capacitance of the pixel electrode part, C _B
The liquid crystal capacitance of the bus line portion, R represents a channel resistance, R _B represents a resistance of the bus line connected to the drain.

Claims (3)

(57) [Claims] A plurality of pixel electrodes (E) and a thin film transistor (T) are provided on one substrate surface of a pair of substrates arranged opposite to each other.
The characteristic evaluation of a thin film transistor (T) in a liquid crystal display device having a configuration in which a counter electrode (E ′) is formed on the surface of the other substrate so as to face the pixel electrode, and a liquid crystal is sandwiched between the substrates. In the above, in a state where an AC signal (S _I ) is applied to the counter electrode (E ′), an AC signal detected on a bus line (B _D ) commonly connected to drain electrodes (D) of a plurality of thin film transistors (T). Measuring a phase shift (φ) of the signal (S _O ) with respect to the applied AC signal (S _I ), and evaluating a channel resistance (R) of the thin film transistor based on the phase shift. Characteristic evaluation method. 2. A frequency of an applied AC signal (S _I ) at which the phase shift (φ) becomes an extreme value is detected corresponding to a gate bias value applied to a gate electrode (G) of the thin film transistor (T). 2. The method according to claim 1, wherein the channel resistance (R) of the thin film transistor is evaluated using the detected frequency. 3. The counter electrode (E ') is composed of a plurality of stripe-shaped electrodes orthogonal to a bus line (B _D ) in which a plurality of drain electrodes (D) of the thin film transistor (T) are connected in common. 3. The method for evaluating characteristics of a thin film transistor according to claim 1, wherein other stripe-shaped electrodes except for the stripe-shaped electrode to which an AC signal (S _I ) is applied are grounded.