JPS63288066A - Protective circuit for thin-film semiconductor - Google Patents

Abstract

PURPOSE:To cause large leakage currents to flow through a protective transistor by the increase of drain voltage, and to operate a protective circuit excellently by lowering driving gate voltage through hydrogen substitution treatment and composing the protective circuit by using an element having structure, in which hydrogen substitution treatment is difficult to work, in the protective transistor. CONSTITUTION:When a sample substrate is positioned in plasma hydrogen discharge at high frequency in a plane parallel electrode in order to conduct hydrogen substitution in a thin-film transistor Tr1, hydrogen passes through an insulating film 8, the polycrystalline silicon of a gate 7 and a gate insulating film 6, and enters a channel region 2. Consequently, the effect of hydrogen substitution treatment is displayed to the Tr1, and the driving gate voltage of the Tr1 is lowered. On the other hand, a shielding electrode 20 covering the upper section of at least the gate electrode 7 during the manufacturing process of a wiring electrode 5 from the upper section of the protective insulating film 8 is shaped to a protective transistor Tr2. The Tr2 is used as the protective transistor in such structure, thus shielding the intrusion of hydrogen. The protective transistor is shaped in structure in which hydrogen substitution treatment cannot be performed, and a protective circuit is formed by employing the transistor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gate insulating film protection circuit for thin film semiconductor devices, and particularly to a protection circuit for thin film semiconductors that is inexpensive to manufacture and has good protection characteristics.

[Conventional technology]

In recent years, line sensors and their circuits have been developed, in which thin-film semiconductor elements are formed on inexpensive insulating substrates such as glass, and these are used as transistors that drive the pixels of liquid crystal displays, or even the circuits that drive the screen are formed on the substrate. Thin film transistors (
Development of TPT) technology is progressing rapidly. Most of the transistor structures to be formed are MOS type transistors, which have a short manufacturing process and are advantageous in terms of cost.

Incidentally, in so-called integrated circuits in which semiconductor elements are formed on a substrate and connected by wiring, a surge voltage of static electricity often jumps into the circuit during handling or mounting during manufacturing. This voltage is a pulse voltage of 50 to 100 V, which destroys the gate insulating film of the MOS transistor and is one of the causes of deteriorating the stability of the device. Connect a single diode. Figure 2 (a) shows a protection circuit using diodes, for example, in ``Integrated Circuit Engineering (2)''.
, Corona Publishing, p. 147.

FIG. 2(b) shows the voltage-current characteristics of the protection diode. Briefly explain the operation. The MOSTr in Fig. 2 (a) is an n-channel type (on-current flows when the gate is positive with respect to the source), and the gate voltage Vz (Vg > O) applied during normal operation is less than the diode avalanche voltage Va. It is. Positive voltage (Vp) as surge voltage
If it takes.

p of the protection diode as Vp > Va > Vt > O
If Va is designed by adjusting the impurity concentration of each n-junction, high voltage will not be applied to the gate insulating film of the MOSTr. In the case of a negative surge (V, <O), a diode-on current flows to lower the voltage.

FIG. 3 shows an example in which a MOS type transistor is used as a protection diode. Trz in FIG. 3(a) is a protection transistor. FIG. 3(b) is a cross-sectional view of Trz. Trl, T
Both rz and rz are n-channel type MOS transistors. 1
1 is a p-type substrate, 12 is a p-type epitaxial channel layer, and 3 is a source region.

4 is a drain region, 5 is a wiring electrode, 6 is a gate electrode,
7 is a gate electrode, and 8 is a protective insulating film.

In this circuit, for positive surge, 4 drain layers and 1
In the case of a negative surge, the gate insulating film is protected by the on-current of the n-type inversion layer by the avalanche current of the breakdown voltage Va of the junction with the channel layer No. 2. By the way, let us consider a protection circuit when a MOS type one-film semiconductor element is formed on an insulating substrate. The cross-sectional structure of a thin film transistor is shown in Fig. 1.
The substrate used is glass. 2 is a non-doped channel layer, 3 is a source region, and 4 is a drain region. Since the substrate is glass, 2, 3, and 4 are made of amorphous silicon or polycrystalline silicon formed by a low-temperature (600''C) process.The applicant himself manufactured a TPT as shown in Fig. 4. The results of investigating its characteristics.

It was found that 6. For example, when polycrystalline silicon TPT is manufactured, the avalanche voltage that occurs at the drain junction is very high. One of the reasons for this is that 2 is a non-doped layer.
This is because ten depletion layers formed at the junction spread to the channel layer 2 and relax the electric field. Polycrystalline silicon, which has a high threshold voltage, has many trap levels at grain boundaries, which increases the threshold voltage. This also increases the voltage used. In order to lower the threshold, it is possible to make the gate insulating film in step 6 thinner, but this will lower the gate withstand voltage.In fact, the avalanche withstand voltage of TFT is higher than the gate withstand voltage, so it was found that it cannot be used as a protection circuit. . 2
Attempts were made to lower the avalanche breakdown voltage by implanting p-type ions into the channel layer of the transistor, but in the future the threshold voltage would become higher, and the margin between the transistor operating voltage and the gate breakdown voltage would become extremely small. Furthermore, p-type ion implantation requires a longer process, resulting in lower yields and higher costs.

[Problem that the invention seeks to solve]

It has been found that the above conventional techniques cannot form a good gate insulating film protection circuit for use in integrated circuits of thin film semiconductor devices. An object of the present invention is to provide a thin film semiconductor device protection circuit that performs good protection circuit operation even for thin film semiconductor devices.

[Means for solving problems]

Assuming that the MOS transistor to be protected is Trl and the protection transistor is Trz, the following conditions are necessary for a thin film semiconductor device to realize a good gate insulating film protection circuit.

T rx : Vt (T rx) < Vta (for positive surge) In other words, when a positive voltage equal to or lower than the driving voltage of Trs is applied, the leakage current flowing to Trz is small;
When a high positive surge voltage is applied, the leakage current must be large.

In the case of a negative surge, the current flowing through the TrZ inversion layer must be large.A detailed study of the characteristics of thin-film semiconductor devices revealed that conventional devices are sufficient for negative surges. If Vg (T r 1) is lowered, the margin becomes larger.a Vt (T r 1) A known method for lowering the T r is to lower the trap density by substituting hydrogen in polycrystalline silicon.

The applicant himself manufactured the device shown in FIG. 4, and as a result of subjecting it to plasma hydrogen substitution treatment, it was confirmed that Vz (T rz) was reduced by half. However, as a result of detailed investigation of those experimental results, new findings were obtained. In other words, when we examine the leakage current before and after the hydrogen replacement treatment, we find that when the driving drain voltage Vo is small, the leakage current remains the same, but when the driving voltage is increased, the leakage current of the element before the hydrogen replacement treatment becomes 20 times higher. It's something that happens. This shows that if the protected Trl is subjected to hydrogen substitution treatment to lower the driving voltage, and the protection transistor Trz is constructed to a structure that cannot undergo hydrogen substitution treatment, it can be used satisfactorily even against positive surges. did. That is, the above object is achieved by making the transistor to be protected a structure that is easily subjected to hydrogen replacement treatment, and making the protection transistor a structure that is difficult to undergo hydrogenation.

[Effect]

By lowering the driving gate voltage of the transistor to be protected by hydrogen replacement treatment and configuring a protection circuit using an element with a structure that is difficult to undergo hydrogen replacement treatment for the protection transistor, high positive surge voltage can be applied to the protected transistor. is applied, the drain voltage of the protection transistor increases and a large leakage current flows to the protection transistor.
Ensure good protection circuit operation.

〔Example〕

Hereinafter, one embodiment of the present invention will be explained with reference to FIG.
rz is a protected transistor and Trz is a protection transistor. First, Trr will be explained. Trl
61 is an ordinary thin film transistor; 2 is an insulating substrate; 2 is a non-doped channel layer; 3 and 4 are source and drain regions, respectively; and 5 is a wiring electrode, which is usually made of AQ or its alloy. 6 is a gate insulating film formed by the CVD method. 7 is a gate electrode which is usually self-aligned and is connected to regions 3 and 4 and 7.
A semiconductor material is used to lower the resistance of the gate electrode.

In this example, the semiconductor material is polycrystalline silicon*
In T r t, 2, 3, 4 and 7 are polycrystalline silicon. The thickness of the polycrystalline silicon of No. 7 is 300 to 5000. 8 is a protective insulating film * In order to replace T r L with hydrogen, as shown by the arrow in Figure 5, for example, when the sample substrate is placed in a high-frequency plasma hydrogen discharge using parallel plate electrodes, hydrogen will insulate 8. film, polycrystalline silicon of the gate of 7 and 6
It passes through the gate insulating film of 2 and enters the channel region of 2. As a result, Trl has the effect of hydrogen replacement treatment, and Trx
The drive gate voltage of is lowered. On the other hand, in the protection transistor Trz, in the step of forming the 5th interconnection IIA electrode from above the 8th protective insulating film, at least 7th gate electrode is covered with 2nd
A shielding electrode of 0 is provided. By using Trz as a protection transistor in such a structure, entry of hydrogen can be blocked. According to the author's experimental results, the hydrogen permeability was 80% or that of other metals (polycrystalline silicon), SiOz, or other insulating films. FIG. 5 shows the MOS characteristics of Trl and Trz of the same size formed on the same substrate after hydrogen replacement treatment: gate voltage (V'') vs. drain current (Io) characteristics @Vso is the drain voltage and channel length.

Channel width W is 50 μm and 20 μm, respectively. T
The characteristics of rl are shown by a solid line, and the characteristics of Trz are shown by a broken line, however,
The characteristics of the Trz were measured as a single MOS without shorting the gate and source. The on-characteristics of the Trt element are significantly improved compared to the Trx, and the gate voltage for obtaining the same drain current is about 172 lower for the Trt, allowing the drive gate voltage to be lowered.

1) During normal operation: A voltage of Vt = 5 to 20 V is applied to Trx, and Trt is turned on. Trz is the source
Since the gate is shortened as shown in Figure 1, Vgo = 5 to 2.
Operates at 0V. The leakage current at this time is small as shown at point A in FIG. 5(a) and does not affect the voltage drop of Vt of the Trs.

2) When applying a positive surge voltage: Assume that a 50V surge is applied from the input terminal. At this time, since Vso of Trz is 50V, the current B in FIG. 4(b) flows through Trz. This is more than 20 times the leakage current of Trt, and lowers the gate voltage of Trl to protect the insulating film.

3) Assume that a knee surge of 40 V is input when a negative surge voltage is applied. Trz is immediately turned on and current C in FIG. 4(b) flows to protect Trl.

A surge voltage is also applied to the gate insulating film of the Trz, but since the surge is a pulse voltage, the voltage will drop if the current flows transiently through the Trz, so there is no problem. As a structure that does not allow hydrogen to enter,
This can be achieved by using gold scrap for the gate electrode 7 of the Trz, or by treating the polycrystalline silicon gate 7 and the surfaces 3 and 4 with silicide electrodes such as platinum or molybdenum. Also, T
The same effect can be obtained by changing the thickness of the polycrystalline silicon gate electrodes 7 for rz and Trz so that Trz is formed thicker. Further, in this embodiment, only an n-channel structure is shown, but even if it is a p-channel type, exactly the same effect can be obtained. Of course, amorphous silicon may be used as the semiconductor material.

〔Effect of the invention〕

According to the present invention, a protective transistor is formed into a structure that cannot be subjected to hydrogen replacement treatment, and is used to form a protective circuit, thereby providing a circuit that can prevent static electricity surges and has good functionality. It has at least 20 times the protection performance against positive surge voltages compared to a protection circuit that does not have this type of protection.

[Brief explanation of drawings]

Fig. 1 is an element explanatory diagram of a protection circuit using a thin film semiconductor element representing one embodiment, Fig. 2 is a conventional protection circuit and its characteristic diagram, and Fig. 3 is an explanation of a MoS transistor constituting the conventional protection circuit. FIG. 4 is a cross-sectional view of a general thin film transistor, and FIG. 5 is an element characteristic diagram of the thin film transistor constituting the protection circuit of the present invention. 1... Insulating substrate, 2... Non-doped channel layer, 3
... Source region, 4... Drain region, 5... Wiring electrode. 2-guchi (α) (b) House 3 C (L) (b) 11-1-bi T1h based 1z---P type f q MU'L4 also 40 yose S figure V% ri $ ( V)

Claims (1)

[Claims] 1. In an integrated circuit in which a plurality of MOS type semiconductor elements are formed using an amorphous or polycrystalline semiconductor on an insulating substrate and are connected by wiring electrodes, the gate insulating film of the MOS type semiconductor elements is As a MOS type semiconductor element used in a protection circuit used to prevent surge damage due to static electricity, an element having a structure in which the element characteristics of the MOS type element used in the protection circuit are not susceptible to the effect of the hydrogenation replacement method is used. A protection circuit for a thin film semiconductor, characterized in that the MOS type semiconductor element to be used is an element having a structure that is easily susceptible to the effect of a hydrogenation substitution method. 2. In claim 1, in the structure of a MOS type semiconductor element that is not easily affected by the hydrogen replacement method,
A protection circuit for a thin film semiconductor, characterized in that an element having a structure that uses metal wiring to block the entry of hydrogen is used as an element of the protection circuit. 3. In claim 1, in the structure of a MOS type semiconductor element that is not easily susceptible to the effect of the hydrogen replacement method,
A protection circuit for a thin film semiconductor, characterized in that an element having a structure in which a silicide-based metal is vapor-deposited on a semiconductor layer serving as a gate electrode and a source and drain semiconductor layer to shield hydrogen from entering is used as an element of the protection circuit.