JPH0448735A - Stabilizing method for characteristic of thin film transistor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a thin film transistor used as a solid-state photoelectric conversion element and a switching element used for extracting optical signals in facsimiles, etc., and a method for stabilizing its characteristics. Regarding.

[Conventional technology]

Conventional techniques for stabilizing the characteristics of thin film transistors and photoelectric conversion elements using amorphous silicon as a semiconductor film and silicon nitride as an insulating film are based on silicon nitride as described in Japanese Patent Application Laid-Open No. 1-14080. By forming an extremely thin oxide layer on the semiconductor layer (amorphous silicon layer) side surface of the insulating film, it is possible to reduce the level at the interface between silicon dioxide and amorphous silicon, and to remove silicon from amorphous silicon. The idea was to prevent injection of carriers into silicon oxide.

[Problem to be solved by the invention]

The above-mentioned conventional technology does not take into consideration the reduction in the number of manufacturing steps of the thin film transistor or the photoelectric conversion element and the reduction in manufacturing cost, and there is a problem in that the manufacturing cost becomes high.

Further, in the above-mentioned conventional technology, when an element whose threshold voltage differs from an ideal value for some reason is manufactured in the manufacturing process of a thin film transistor or a photoelectric conversion element,
No consideration has been given to adjusting this threshold voltage to return the device to its normal state, and once a device with an unfavorable threshold voltage is manufactured, thin film transistors cannot perform switching operations ( (Figure 7), or for photoelectric conversion elements, the current ratio between light irradiation and dark state cannot be maintained (Figure 8), and the entire substrate containing these elements must be discarded, reducing the overall cost. There was a problem with the increase in

Further, the above-mentioned prior art has a problem in that when a manufactured thin film transistor is used by repeatedly applying a gate voltage, the threshold voltage still shifts.

An object of the present invention is to provide a thin film transistor or a photoelectric conversion element with low manufacturing cost.

Another object of the present invention is to solve the problem even when a thin film transistor or a photoelectric conversion element is manufactured with an inappropriate threshold voltage.
Adjust the threshold voltage quickly and arbitrarily.

The purpose is to provide a method of stabilization.

Another object of the present invention is to provide a stable thin film transistor or photoelectric conversion element that can withstand long-term use, and a method for stabilizing its characteristics.

[Means to solve the problem]

In order to achieve the above object, the present invention applies a voltage between each electrode or to a light shielding layer in order to stabilize or adjust the threshold voltage while the thin film transistor or photoelectric conversion element is heated.

In order to achieve the other objects mentioned above, one aspect of the present invention heats a thin film transistor or a photoelectric conversion element using a laser beam that is irradiated only on an electrode portion or a light-shielding layer.

To achieve the above object, heating of the thin film transistor or photoelectric conversion element is applied in a direction perpendicular to or close to the substrate surface. This is performed using eddy currents generated within the electrodes and the light-shielding layer by varying the magnetic flux density of the external magnetic field.

In order to achieve the above-mentioned other objects, the present invention provides a thin film transistor 2 for stabilizing the characteristics of a photoelectric conversion element.

The adjustment sequence was divided into a time period for measuring the threshold voltage and a time period for stabilizing or adjusting the threshold voltage.

In order to achieve the above-mentioned other objects, one aspect of the present invention is that the laser light or eddy current for heating the thin film transistor or photoelectric conversion element is stopped during the time period when the threshold voltage of the thin film transistor or photoelectric conversion element is measured.

In order to achieve the above-mentioned other objects, the present invention is arranged so that the voltage applied to each electrode or light shielding film of a thin film transistor or a photoelectric conversion element is applied in a state in which a time relationship with a laser beam or an eddy current is selected. .

In order to achieve the above-mentioned other objects, the present invention provides that the thin film transistor 2 is capable of adjusting the magnitude and application time of the voltage applied to each electrode or light-shielding layer of the photoelectric conversion element to the value of the measured threshold voltage of the photoelectric conversion element. Adjusted accordingly.

In order to achieve the above-mentioned other objects, the present invention applies positive and negative voltages alternately between a gate electrode, a source electrode, and a drain electrode of a thin film transistor.

For the photoelectric conversion element, positive and negative voltages were applied alternately between the light shielding film and the other two electrodes.

[Effect]

Heating a thin film transistor or a photoelectric conversion element accelerates the movement of electrons or holes, which are carriers, spatially or between energy levels. Therefore, the threshold voltage of the thin film transistor or photoelectric conversion element can be adjusted at high speed.

Apply a positive or negative voltage between the gate electrode, source electrode, and drain electrode of the thin film transistor.

For example, if a positive voltage is applied to the gate electrode with respect to the source electrode and the drain electrode, the electrons 23, which are carriers injected from the source electrode and the drain electrode into the amorphous silicon film, are further transferred to the insulating film. It is injected into the silicon nitride film, where it is trapped and fixed at the trap level 25. This causes the threshold voltage of the thin film transistor to move in the positive direction. Also in the case of photoelectric conversion elements.

If a positive voltage is applied to the light shielding layer with respect to the other two electrodes,
Through a similar process, the threshold value of the photoelectric conversion element moves in the positive direction. In thin film transistors, conversely,
When a negative voltage is applied to the gate electrode with respect to the source and drain electrodes, the holes, which are carriers, injected from the source and drain electrodes into the amorphous silicon film are further It is injected into the film, where it is trapped and fixed at a trap level. As a result, the threshold voltage of the thin film transistor moves in the negative direction. In the case of photoelectric conversion elements, if a negative voltage is applied to the light-shielding film with respect to the other two electrodes, a similar process will occur,
The threshold value of the photoelectric conversion element will move in the negative direction. Due to the above effects, it becomes possible to adjust the threshold voltage of the thin film transistor or photoelectric conversion element to an arbitrary value (
Figure 6).

For thin film transistors, positive and negative voltages are applied alternately between the gate electrode, source electrode, and drain electrode, and for photoelectric conversion elements.

By alternately applying positive and negative voltages between the light shielding film and the other two electrodes, electrons and holes, which are carriers, are alternately injected into the silicon nitride film, which is an insulating film. As a result, the trap level in the insulating film is filled with the injected carriers, and no further carrier injection occurs. Therefore, by performing this treatment after manufacturing the thin film transistor or photoelectric conversion element, fluctuations in the threshold voltage can be prevented.

Thin film transistors or photoelectric conversion elements with stable characteristics can be manufactured.

By narrowing down the irradiation range of laser light, irradiating only the gate electrode, source electrode, and drain electrode for thin film transistors, and irradiating only the light shielding film and the other two electrodes for photoelectric conversion elements. The semiconductor film and the insulating film are heated by thermal conduction. Therefore, only the targeted thin film transistor or photoelectric conversion element can be heated to stabilize or adjust its characteristics. Furthermore, irradiation with laser light
Since there are only electrodes and a light-shielding film portion, the characteristics can be stabilized or adjusted without causing any performance deterioration due to light irradiation of non-doped amorphous silicon.

By applying an external magnetic field perpendicular to the substrate surface or at an angle close to it and varying the magnetic flux density, it is possible to efficiently generate eddy current inside the metal of the electrode, and the semiconductor film and insulation The membrane is heated.

Therefore, only the target thin film transistor or photoelectric conversion element can be heated to stabilize or adjust its characteristics. Furthermore, since heating is performed using a magnetic field, there is no performance deterioration due to light irradiation of non-doped amorphous silicon.

In the characteristics stabilization sequence of a thin film transistor or photoelectric conversion element, the time period for measuring the threshold voltage and the time period for stabilizing or adjusting the threshold voltage are divided, and heating with laser light or eddy current stabilizes the threshold voltage. By performing the measurement only during the period of time during which the threshold voltage is adjusted or adjusted, it is possible to accurately measure the threshold voltage without being disturbed by noise caused by heating.

This is the time period during which the characteristics of a thin film transistor or photoelectric conversion element are stabilized or adjusted.In the case of a thin film transistor, the voltage applied between the gate electrode and the source and drain electrodes, and in the case of a photoelectric conversion element, the voltage applied between the gate electrode and the light shielding film. and the magnitude and sign of the voltage applied between the two electrodes, as well as the application time.

Since the threshold voltage can be changed at any time depending on the threshold voltage measured during the time period and the desired threshold voltage value, the threshold voltage can be stabilized and adjusted at high speed.

[Example] An example of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 1 is an embodiment of the present invention, and shows the basic components necessary to carry out the present invention.
Although this embodiment intends to apply the present invention to a thin film transistor formed on a transparent insulator substrate (glass etc.) 1, it is also possible to apply the present invention to a photoelectric conversion element. It can be implemented in exactly the same way, except that the name is changed to light-shielding film. A gate electrode lead line 8 (or a prober, etc.) is connected between the gate electrode 2 and the source electrode 6a.
A gate voltage (Vas) applying power supply unit 13 is present through the lease electrode lead line 9 (or prober, etc.). Further, between the source electrode 6a and the drain electrode 6b, a source/drain current (Ios) measuring unit 11 is connected through a source electrode lead line 9 (or a prober, etc.) and a drain electrode lead line 10 (or a prober, etc.). , a drain voltage (Vos) application unit 12 is present. These gate voltage application units 13
The drain voltage applying unit 12, the source/drain current measuring unit 11, and the laser light source 15 are connected to the controller 16 and signal lines 17, 18, and 19, respectively, and are operated by adjusting the time relationship and applied voltage. Ru.

FIG. 2 is a graph showing a typical time relationship of this method. In FIG. 2, the gate voltage Vcs and the drain voltage VOS are drawn with the voltage of the source electrode (source voltage) as zero. After an initial period from to to tl, at time 1. From then on, the time period for measuring the threshold voltage begins.

This time period from tl to t2 is exactly the same as a normal sequence for measuring the static characteristics of a thin film transistor.

That is, a sawtooth waveform gate voltage VGS is applied to the gate electrode 2, and at the same time, a constant drain voltage Vos is applied to the drain electrode 6b, while Ins is measured by the source/drain current measurement unit 11, and the threshold voltage V
Determine T. The time period for stabilizing or adjusting the threshold voltage begins one hour ta after the time period for measuring the dark value voltage ends, but time t2 and time ta may coincide. It goes without saying. Constant gate voltage V applied between time t3 and time ta
O8 and the length of the application time tt-ta, the larger the difference between the threshold voltage VT measured during the time period for measuring the threshold voltage and the target threshold voltage Vto, the larger the absolute value of VCS becomes. , is applied for a long time (with a large ta ts), and when the difference is small, the absolute value is small and the threshold voltage is applied for a short time, thereby making it possible to adjust the threshold voltage quickly and accurately. The sign of the gate voltage Vas may be negative if VT > VTO (71), and positive if VT )Vto (7).
Between time t8 and time t4, from the laser light source 15,
Gate electrode 2 is irradiated with laser light 14 for heating. The laser beam to be irradiated may be a pulsed laser beam instead of a continuous wave one. Also, taking into account the delay in heating due to laser beam irradiation and heat conduction, the irradiation start time and irradiation end time of the laser beam are as follows. A sequence of 0 or more that may precede the application start time and application end time of the gate voltage, respectively, is determined so that the measured threshold voltage is the target threshold voltage value and the prespecified error range. Adjustment of the threshold voltage is achieved by repeating this until the values match.

Although this embodiment is intended to be applied to a thin film transistor or a photoelectric conversion element formed on a transparent insulating substrate 1, it is intended to be applied to a thin film transistor or a photoelectric conversion element formed on an opaque insulating substrate. If the target is
In the figure, a laser beam is applied from above to the gate electrode 2. The source electrode 6a and the drain electrode 6b may be irradiated and heated. By irradiating the non-doped amorphous silicon film 4 with laser light,
If it is necessary to prevent characteristic deterioration, the source electrode 6
a. Only the drain electrode 6b may be irradiated with laser light.

FIG. 3 shows another embodiment of the present invention, showing the basic components necessary to carry out the present invention. This example heats a thin film transistor or a photoelectric conversion element.
The difference is that eddy currents due to fluctuating magnetic fields 22a and 22b generated by magnetic field sources 21a and 21b are used,
According to this embodiment, which is otherwise the same, it is possible to heat a thin film transistor or a photoelectric conversion element regardless of whether the substrate material is transparent or opaque. External magnetic field 2 to be applied
2a and 22b do not have to be perpendicular to the substrate surface, but from the standpoint of eddy current generation efficiency, angles that are perpendicular or close to perpendicular are preferred.

Figure 4 is a graph showing the typical time relationship of this method.The only difference from Figure 2 in this example is the change in the magnetic flux density of the external magnetic field, so a detailed explanation will be omitted. . Note that the temporal period of change in the magnetic flux density of the external magnetic field and the waveform of the change can be selected in accordance with the required heating capacity.

FIG. 5 shows that a thin film transistor,
Alternatively, in Figure 0 showing an example in which the threshold voltage of the photoelectric conversion element is stabilized, vt indicates the initial value of the threshold voltage, and Vta indicates the desired threshold voltage. Figure 2 or 4
If the period from time to to time ti in the figure is called a -cycle, then in Figure 5, from time too to tll to time tx
Three cycles are repeated from 1 to ttz, from time tlz to tta, and from time tza to t1. It is a hot theory that the number of repetitions can be selected arbitrarily. Also, at this time, the voltage is applied between time tδ and t4 in FIG. 2 or 4. Constant gate voltage Va
s is negative between time tio and tll, positive between time txt and txz, negative between time txz and tss, and positive between time tza and ti4, and the absolute value of the gate voltage VaS is equal. As shown in this example, if the present invention is used, the gate electrode.

Alternatively, by alternately applying positive and negative voltages to the light shielding film, the threshold voltage can be easily adjusted to a desired value and stabilized at that value.

〔Effect of the invention〕

Since the present invention is configured as described above, the six thin film transistors or photoelectric conversion elements that produce the effects described below are heated by electric current without laser light irradiation or external magnetic field fluctuations. Therefore, the threshold voltage can be adjusted to an arbitrary value and stabilized without deteriorating the characteristics. Further, since a voltage is applied between each electrode while the thin film transistor or photoelectric conversion element is heated, the threshold voltage can be adjusted or stabilized to an arbitrary value at high speed. Furthermore, since the characteristic stabilization or adjustment sequence is divided into a time period for measuring the threshold voltage and a time period for adjustment or stabilization, it is possible to adjust or stabilize the threshold voltage to an accurate value. Furthermore, since the laser light or eddy current is stopped during the time period during which the threshold voltage is measured, it is possible to efficiently adjust or stabilize the threshold voltage with high precision. Furthermore, thin film transistors,
Alternatively, since the voltage applied to each electrode or light shielding film of the photoelectric conversion element is applied in a state where the time relationship with the laser light or eddy current is selected, the threshold voltage can be adjusted, or Stabilization can be performed with high precision. Furthermore, since the application time of the voltage applied to each electrode or light shielding film of the thin film transistor or photoelectric conversion element is adjusted according to the measured threshold voltage value, the threshold voltage can be adjusted or stabilized with high precision and high speed. can be executed. Furthermore, in the case of thin film transistors, positive and negative voltages are applied alternately between the gate electrode and the source and drain electrodes, and in the case of photoelectric conversion elements, between the light shielding film and the gate and drain electrodes. can be stabilized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the first embodiment of the present invention, FIG. 2 is a representative graph at each step of the time sequence in the first embodiment of the present invention, and FIG. 3 is a block diagram of the second embodiment of the present invention. A block diagram of the embodiment, FIG. 4 is a representative graph at each step of the time sequence in the second embodiment of the present invention, and FIG. 5 is a graph showing threshold stabilization of a thin film transistor according to the embodiment of the present invention. Figure 6 is a characteristic diagram showing the principle of threshold voltage adjustment, Figure 7 is a characteristic diagram showing how the static characteristics of a thin film transistor are affected by fluctuations in threshold value, and Figure 8 is a characteristic diagram showing the static characteristics of a photoelectric conversion element in a light irradiation state and a dark state. FIG. 4 is a characteristic diagram showing how characteristics are affected by fluctuations in threshold values. 1...Substrate, 2...Gate electrode or light shielding film, 4...
Non-doped amorphous silicon, 5...n medium-sized amorphous silicon, 14...laser light, 16...controller,
22...External magnetic field, 24...Electrons trapped in the trap level, 26...Desired static characteristics, 27...
Static characteristics when the threshold value changes negatively, 28... Static characteristics when the threshold value changes positively, 29... Static characteristics preferably in a dark state, 30... Preferably in a light irradiation state. Static properties. 31... Static characteristics in the dark state when the threshold value changes negatively, 32... Light irradiation state when the threshold value changes negatively Figure 81

Claims (1)

[Scope of Claims] 1. An n^+ type amorphous film in which a gate electrode is formed on an insulating substrate, a gate insulating film is formed, a non-doped amorphous silicon film is formed, and an n-type impurity is doped at a high concentration. In a thin film transistor in which a silicon film is formed and a source electrode and a drain electrode are formed, a positive or negative voltage is applied between the gate electrode and the source electrode and the drain electrode while the thin film transistor is heated. The threshold voltage of
A method for stabilizing the characteristics of thin film transistors that changes and stabilizes them to arbitrary values. 2. The thin film transistor according to claim 1, wherein the thin film transistor element is subjected to a characteristic stabilization method. 3. In claim 1, the thin film transistor is heated by applying a laser beam to only the gate electrode, the source electrode, and the drain electrode, or to only one or two of these electrodes. A method for stabilizing the characteristics of thin film transistors using irradiation. 4. In claim 1, the heating of the thin film transistor is caused by varying the magnetic flux density of an external magnetic field applied in a direction perpendicular to or at an angle close to the surface of the substrate on which the thin film transistor is formed. , a method for stabilizing the characteristics of thin film transistors using Joule heating of eddy currents inside electrodes. 5. The method for stabilizing the characteristics of a thin film transistor according to claim 1, wherein a time period for measuring the threshold voltage of the thin film transistor and a time period for stabilizing or adjusting the threshold voltage of the thin film transistor are provided. 6. In claim 1, heating by laser light or eddy current is stopped during a time period for measuring the threshold voltage of the thin film transistor, and heating by laser light or eddy current is stopped during a time period for stabilizing or adjusting the threshold voltage of the thin film transistor. Or a method for stabilizing the characteristics of thin film transistors that uses eddy current heating. 7. The method for stabilizing characteristics of a thin film transistor according to claim 1, wherein positive and negative voltages are alternately applied between the source electrode, the drain electrode, and the gate electrode. 8. In claim 1, the voltage applied to the gate electrode, the source electrode, and the drain electrode stabilizes the characteristics of the thin film transistor by applying the voltage with a selected time relationship with the laser light or eddy current. method. 9. In claim 1, the magnitude and application time of the voltage applied to the gate electrode, the source electrode, and the drain electrode correspond to the value of the threshold voltage measured during the time period in which the threshold voltage of the thin film transistor is measured. A method for stabilizing the characteristics of thin film transistors that has been adjusted accordingly. 10. Form an n^+ type amorphous film by forming a metal film that serves as a light shielding layer on an insulating substrate, forming an insulating film, forming a non-doped amorphous silicon film, and doping it with n-type impurities at a high concentration. In a photoelectric conversion element formed with a silicon film and at least two electrodes, a positive or negative voltage is applied between the light-shielding metal film and the at least two electrodes while the photoelectric conversion element is heated. A method to stabilize the characteristics of photoelectric conversion elements by applying voltage. 11. The photoelectric conversion element according to claim 10, wherein the photoelectric conversion element is subjected to a characteristic stabilization method. 12. In claim 10, heating of the photoelectric conversion element is performed only on the light shielding layer and two other electrodes, or only on one or two light shielding layers or only the electrodes among these light shielding layers and two electrodes. In contrast, this method stabilizes the characteristics of photoelectric conversion elements by irradiating them with laser light. 13. In claim 10, the photoelectric conversion element is heated by varying the magnetic flux density of an external magnetic field applied in a direction perpendicular to or at an angle close to the surface of the substrate on which the photoelectric conversion element is formed. A method for stabilizing the characteristics of photoelectric conversion elements using Joule heating of the resulting eddy current inside the light-shielding layer and electrodes. 14. The method for stabilizing the characteristics of a photoelectric conversion element according to claim 10, comprising a time period for measuring the threshold voltage of the photoelectric conversion element and a time period for stabilizing or adjusting the threshold voltage of the photoelectric conversion element. 15. In claim 10, during the time period when the threshold voltage of the photoelectric conversion element is measured, heating by laser light or eddy current is stopped, and during the time period when the threshold voltage of the photoelectric conversion element is stabilized or adjusted, A method for stabilizing the characteristics of photoelectric conversion elements using laser light or eddy current heating. 16. The method for stabilizing the characteristics of a photoelectric conversion element according to claim 10, wherein positive and negative voltages are alternately applied between the light shielding film and the other two electrodes. 17. The method for stabilizing the characteristics of a photoelectric conversion element according to claim 10, wherein the voltage applied to each electrode is applied in a state in which a time relationship with the laser beam or the eddy current is selected. 18. In claim 10, the magnitude and application time of the voltage applied to each electrode are adjusted according to the value of the threshold voltage measured during the time period in which the threshold voltage of the photoelectric conversion element is measured. Method for stabilizing device characteristics. 19. A characteristic stabilizing device for stabilizing the characteristics of the thin film transistor and the photoelectric conversion element according to claim 1 or 10. 20. Injecting carriers into defect levels in the insulating film of the element by stabilizing the characteristics of the thin film transistor and the photoelectric conversion element according to claim 1 or 10,
Functional element with reduced defect level density.