JP2009099847A - Thin-film transistor, its manufacturing method, and display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an a-IGZO-based thin-film transistor further stable against electrical stress, and to provide a display device using the same. <P>SOLUTION: The thin-film transistor has an active layer formed of an amorphous oxide prepared by containing In and Zn. In the thin-film transistor, energy for stabilizing an electric characteristic is provided to the active layer after forming the active layer, and the active layer provided with the energy desorbs Zn more than 1.05 times as much as the active layer without being provided with energy, at 200-700°C by a temperature programmed desorption method. The method of manufacturing the thin-film transistor having an active layer formed of an amorphous oxide containing In and Zn includes a step of providing energy for stabilizing an electric characteristic to the active layer after a step of forming the active layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a thin film transistor, a manufacturing method thereof, and a display device, and more particularly to a thin film transistor whose active layer is an amorphous oxide including In and Zn, a manufacturing method thereof, and a display device.

  In recent years, research and development of displays using organic light emitting diodes (Organic Light Emitting Diodes, hereinafter referred to as OLEDs) as light emitting elements have been actively conducted.

  As a thin film transistor (hereinafter referred to as TFT) used for driving the OLED, an amorphous Si (Amorphous Si; hereinafter referred to as a-Si) TFT can be used. Since the a-Si TFT is an element generally used in a liquid crystal display (hereinafter referred to as LCD), there is an advantage that the technology can be used. Further, the a-Si TFT can be produced at a lower cost and can be easily increased in size and is more uniform because it is amorphous compared to a polycrystalline Si (Poly-crystal Si, hereinafter referred to as p-Si) TFT.

  In recent years, TFTs using transparent oxide semiconductors (Transparent Oxide Semiconductors, hereinafter referred to as TOS) as active layers have been actively developed.

For example, in a TFT using Zn—Sn—O as an active layer, a TFT having a field effect mobility of 20 cm ² / Vs or more can be obtained by post-annealing at 400 to 600 ° C. after the formation of the active layer. (Non-Patent Document 1). This characteristic change is considered to occur when Zn—Sn—O changes from amorphous to polycrystalline.

  Here, the temperature of 400 to 600 ° C. is near the heat-resistant temperature of the glass substrate and is difficult to handle as a TFT for display. In addition, it is difficult to apply to a flexible display using a plastic substrate.

As another TOS for the active layer of a TFT, amorphous In—Ga—Zn—O (Amorphous In—Ga—Zn—O, hereinafter referred to as a-IGZO) has been proposed. a-IGZO is TOS having a crystallization temperature of 500 ° C. or higher. Furthermore, a TFT using a-IGZO as an active layer has a high field-effect mobility of about 10 cm ² / Vs and a sufficient on / off ratio even in an amorphous state formed at room temperature (Patent Literature). 1, Non-Patent Document 2). Since a TFT using a-IGZO as an active layer is in an amorphous state, it is resistant to bending and is suitable as a TFT for a flexible display.
JP 2006-165527 A RL Hoffman, Solid-State Electronics, 2006, Vol. 50, pp. 500-503 Donghun Kang, Chang Jung Kim, Hyuck Lim, Sunil Kim, Jaechul Park, Ihun Song, Eunha Lee, Jaecheol Lee, and Youngsoo Park, TAOS2006, 2006, P2

A problem of a thin film transistor is a large threshold voltage shift with respect to electrical stress.
TFTs for active matrix (Active Matrix, hereinafter referred to as AM) type OLED displays are more required to have stability against electrical stress than AM type LCDs.
On the other hand, it is well known that the characteristics, particularly the threshold voltage, of the a-Si TFT changes greatly due to long-term electrical stress. Therefore, even if there is no problem in an AM type LCD, it is required to solve this problem in order to use an a-Si TFT in an AM type OLED display.

  On the other hand, the research and development of a-IGZO TFT has just started. As a report example so far, it has been reported that when a constant current of 3 μA is continuously applied to a TFT at room temperature for 100 hours, the threshold voltage shift amount is 1.6 V or less (Non-patent Document 2). .

The thin film transistor of the present invention is a thin film transistor whose active layer is an amorphous oxide composed of In and Zn,
Providing the active layer with energy for stabilizing electrical characteristics after the formation of the active layer;
The active layer imparted with energy desorbs more than 1.05 times Zn in a temperature range of 200 ° C. to 700 ° C. by temperature-programmed desorption analysis with respect to the active layer not imparted with energy. The thin film transistor.

  It is desirable to apply energy by heat treatment. And this heat processing is performed in air | atmosphere, and heating temperature can be made into the temperature higher than 150 degreeC and less than 281 degreeC. The heat treatment is performed in a vacuum, and the heating temperature can be higher than 150 ° C. and lower than 300 ° C.

  The active layer is preferably an amorphous oxide containing In, Zn, and Ga.

  The method of manufacturing a thin film transistor of the present invention is a method of manufacturing a thin film transistor in which an active layer is composed of an amorphous oxide containing In and Zn, and stabilizes electrical characteristics in the active layer after the step of forming the active layer. A method of manufacturing a thin film transistor, comprising a step of applying energy for causing the thin film transistor to be applied.

  The step of applying energy is desirably performed after the step of forming the insulating layer in contact with the active layer. Moreover, it is desirable that energy is applied by heat treatment, and the heating temperature is higher than the heat treatment temperature in a process other than the process of applying the energy.

  The heat treatment is performed in the air, and the heating temperature can be higher than 150 ° C. and lower than 281 ° C. The heat treatment is performed in a vacuum, and the heating temperature can be higher than 150 ° C. and lower than 300 ° C.

According to the present invention, a thin film transistor having good electrical stress resistance can be realized.

  Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

  As a result of studying various stabilization treatments for the purpose of improving the electrical stress resistance of a TFT using an a-IGZO-based semiconductor as an active layer, the present inventors have found that Zn is deeply involved in this characteristic. We found experimental facts to show.

  As will be described in more detail in Example 1, in a TFT using an a-IGZO film as an active layer, when the active layer is heat-treated (stabilized) in the temperature range of 200 ° C. to 250 ° C. The resistance to electrical stress is improved when the stabilization treatment is performed at a temperature lower than 200 ° C. or higher than 250 ° C. In addition, the a-IGZO single film heat-treated (stabilized) at 200 ° C. to 250 ° C. in the air or in vacuum is subjected to heat treatment at temperatures below 200 ° C. and higher than 250 ° C. from the results of thermal desorption analysis. The amount of desorbed Zn is larger than that of the case. That is, the electrical stress resistance and the amount of Zn desorption take the extreme values from 200 ° C. to 250 ° C., and a correlation is observed between these values.

  However, as will be explained below, in the atmosphere, if the temperature is higher than 150 ° C. and lower than 281 ° C., improvement against electrical stress can be seen. In vacuum, the temperature is higher than 150 ° C. and lower than 300 ° C. For example, improvement against electrical stress can be seen.

  An increase in the amount of desorbed Zn indicates that there is a large amount of unstable Zn, which is considered to be derived from the structural change of the a-IGZO film.

FIG. 5 shows the results of temperature programmed desorption analysis of H ₂ O molecules from the a-IGZO single film. Occur desorption of H _{2 O} molecules adsorbed on the film surface up to about 200 ° C., more, in a temperature range of up to about 500 ° C., occurs desorption of H _{2 O} molecules from the interface between the film or substrate ing. The desorption of H ₂ O molecules from the latter film or from the interface with the substrate has a maximum of desorption at about 310 ° C.

Most of the desorbed H ₂ O molecules exist in the form of OH groups in the film or at the interface with the substrate, and are considered to be bonded to metal ions. When the heat treatment is performed, the bond between the OH group and the metal ion is cleaved, and a part of the OH group is desorbed as a H ₂ O molecule through a reaction with another OH group, and the remaining metal ion. Has a dangling bond and an unstable state with a small effective charge. Since Zn has the highest vapor pressure among In, Ga and Zn, it is presumed to be desorbed during temperature programmed desorption analysis.
The characteristic change due to the electrical stress of the TFT mainly appears as a change in the threshold voltage including the TFT of this embodiment. In general, the change in the threshold voltage of the TFT is caused by a change in the charge state at the active layer or at the interface between the active layer and the insulating film.

  Therefore, even in a TFT having an a-IGZO film as an active layer, the threshold voltage changes due to trapping of carrier electrons in the trap level originally formed in the active layer or the interface or in the trap level formed by electrical stress. It is thought that it is caused by being done. That is, the stabilization process has an effect of suppressing a change in threshold voltage by reducing the trap level or suppressing the formation of the trap level. Furthermore, from the results of temperature programmed desorption analysis, it is considered that the structural change of the a-IGZO film achieves the reduction of trap levels or the suppression of trap level formation.

  In the present embodiment, heating is not necessarily required for energy application (stabilization treatment). In the present embodiment, the a-IGZO film is structurally changed as a result of the heat treatment, but other treatments can be used as long as the electrical stress resistance is improved. As a method for imparting energy to the active layer (stabilization treatment), for example, heating such as hot plate, furnace, laser irradiation and infrared irradiation is used, or heating such as ultraviolet irradiation and X-ray irradiation is not used. There are cases.

  Whatever the treatment, the active layer subjected to the stabilization treatment is compared with 1.05 in the temperature range of 200 ° C. to 700 ° C. by the temperature programmed desorption analysis with respect to the active layer not subjected to the stabilization treatment. Any treatment that desorbs more than twice as much Zn may be used.

  As shown in Example 1, when temperature is used as a means for stabilization treatment, more than 1.05 times more Zn is desorbed at a temperature higher than 150 ° C. than when stabilization treatment is not performed. It is time for heat treatment. The effect of improving the electrical stress resistance of the TFT subjected to the stabilization treatment at 150 ° C. is about an order of magnitude smaller than the effect at 200 ° C. and 250 ° C. In addition, the amount of Zn desorbed from the a-IGZO film is 1.05 times the increase in the amount of desorbed when heat-treated at 150 ° C. in the atmosphere as compared to the film not subjected to stabilization treatment. . Furthermore, when heat treatment is performed at 150 ° C. in vacuum, the increase in desorption amount is 1.04 times (FIGS. 2 and 3). The accuracy of Zn desorption amount in temperature programmed desorption analysis includes an error of about 5% at most. Therefore, it is considered that the effect of the present embodiment can be obtained when the amount of desorption is larger than the amount of desorption of Zn from the a-IGZO film heat-treated at 150 ° C.

  When temperature is used for the stabilization process of the electrical characteristics, the effect is obtained in a temperature range in which the amount of Zn desorption can be obtained. From the above, it can be seen that the lower limit of the temperature range is “temperature higher than 150 ° C.”, but it is necessary to determine the upper limit. As will be described in detail later in Example 1, the amount of Zn desorbed from the a-IGZO film that was heat-treated at 300 ° C. in the atmosphere was 0.64 times that of the film that was not subjected to stabilization treatment. Yes, it is drastically reduced to ½ or less of 1.62 times the desorption amount at 250 ° C. In addition, the effect of improving the electrical stress resistance of the TFT subjected to the stabilization treatment at 300 ° C. in the atmosphere is lower than the effect at 250 ° C. Therefore, the temperature at which 250 ° C. and 300 ° C. are linearly approximated and the amount of Zn desorption is the same as that of the film not subjected to the stabilization treatment is set above the temperature range of the electrical property stabilization treatment. When calculated, this temperature is 281 ° C. That is, when heat treatment in the atmosphere is used as a stabilization process for electrical characteristics, an effect can be obtained at a temperature higher than 150 ° C. and lower than 281 ° C. Moreover, the temperature of 200 to 250 degreeC is more preferable.

  On the other hand, the desorption amount of Zn from the a-IGZO film subjected to the heat treatment at 300 ° C. in vacuum is 1.12 times that of the film not subjected to the stabilization treatment, and the desorption amount at 270 ° C. is 1 Although it is reduced compared to 16 times, it is higher than that of the film not subjected to the stabilization treatment. Therefore, it can be seen that the temperature at which the desorption amount is the same as that of the film not subjected to the stabilization treatment is higher than 300 ° C. Further, when heat treatment is performed at a temperature higher than 300 ° C., oxygen is desorbed from the active layer and the carrier concentration is increased, which causes a problem that the TFT is always on. Therefore, when the stabilization process is performed in a vacuum, 300 ° C. is set above the temperature range of the stabilization process of the electrical characteristics. That is, when heat treatment in vacuum is used as a stabilization process for electrical characteristics, an effect can be obtained at a temperature higher than 150 ° C. and lower than 300 ° C. Moreover, the temperature of 200 to 250 degreeC is more preferable.

  The electrical property stabilization process is performed in a vacuum immediately after the active layer is formed, or is performed during the manufacture of the display device after the active layer is formed.

  The element configuration may be either a top gate or a bottom gate, and each may be either a top contact or a bottom contact. Accordingly, the timing of performing the electrical property stabilization process may be after the active layer is formed, and also after the source electrode, drain electrode, gate insulating film, gate electrode, protective film, and the like are formed. Can do.

  However, according to the knowledge of the inventors, when the stabilization process of the electrical characteristics is performed in the manufacturing process of the thin film transistor, it is preferable to perform it after the process of forming the insulating layer in contact with the active layer. When an insulating layer is formed on the active layer, the active layer may be greatly damaged depending on the formation conditions. In the example of forming the protective layer by the sputtering method, the active layer is impacted by neutrality and ionized sputtered particles, sputtering gas, etc., and the characteristics of the active layer including electrical characteristics may change greatly depending on the forming conditions. I know it. This is because a large characteristic change of the active layer occurs after the stabilization process, and instability to electrical stress may occur with this change.

  Further, according to the knowledge of the present inventors, when the stabilization treatment is performed by heat treatment, the heating temperature in the heat treatment is preferably higher than the heat treatment temperature in steps other than the step of performing the stabilization treatment. That is, it is preferable that the heating temperature in the stabilization process is the highest among all processes for forming a semiconductor element (for example, TFT). This is because if there is another process that is heat-treated at a temperature higher than the temperature during the stabilization process, the effect of the stabilization process may be reduced. However, even if there is another process that is processed at a temperature higher than the heating temperature during the stabilization process, the process time is short, and it is acceptable if it does not substantially affect the active layer of the present invention. The

In Patent Document 1, an a-IGZO film is subjected to a heat treatment at 0 ° C. to 300 ° C., more preferably 100 ° C. to 200 ° C. in an oxygen-containing atmosphere, whereby the amount of oxygen vacancies, that is, the carrier density is 10 ¹⁸ (/ cm ³ ) The following control is disclosed.

  FIG. 4 shows the change in conductivity when the a-IGZO film is heat-treated at 120 ° C. to 350 ° C. in the atmosphere.

  Here, the horizontal axis in FIG. 4 is the heating temperature, and the vertical axis is the conductivity. In addition, the electrical conductivity of the non-heat-treated film is shown at the point where the heating temperature is 50 ° C.

A method for preparing the sample used for the measurement is shown below. Single-crystal Si having a thermal Si oxide film formed on the surface was used as the substrate. Single-crystal Si is doped with phosphorus at a high concentration and has a low resistance to n-type. The thermal Si oxide film has a thickness of about 100 nm. Next, an oxide semiconductor film of a-IGZO was formed using an RF sputtering apparatus. The substrate temperature during film formation was room temperature, and a target having an In: Ga: Zn: O composition ratio of 1: 1: 1: 4 was used. Sputtering was performed at a pressure of 0.5 Pa in an Ar atmosphere with an RF power of 200 W during film formation and an O ₂ partial pressure of 5%. The film thickness was set to 20 nm by adjusting the film formation time. Separately, when a thin film X-ray diffraction measurement (incidence angle of 0.5 degree) of a thin film formed under the same conditions was performed, no clear diffraction peak was observed, so the produced In-Ga-Zn-O thin film was It can be said that it is amorphous.

  By changing the film formation time of the film formation method, a-IGZO films having a film thickness of 20 nm and 40 nm were obtained.

Subsequently, the electrode was patterned using a metal mask. Au was used as the electrode material, and after depositing about 5 nm of Ti by electron beam vacuum deposition, about 40 nm of Au was deposited.
In measuring the electrical conductivity of the prepared film, 4-terminal measurement using four electrodes was used in order to obtain the characteristics of the a-IGZO film itself without being affected by the contact property of the electrodes. The width of the electrodes is 0.4 mm, the length is 6 mm, and the distance between the electrodes is 0.4 mm.

  All conductivity measurements were performed at room temperature, in the air, and in the dark.

  After measuring the conductivity, heat treatment is performed. The heating temperature is 120 ° C. and the heating time is 20 minutes. After heating, the conductivity was measured by 4-terminal measurement. Furthermore, the heating temperature was set to 150 ° C., 200 ° C., 250 ° C., 300 ° C., and 350 ° C., and the conductivity was measured in the same manner.

The conductivity of the non-heat-treated film at a thickness of 20 nm was 9.4 × 10 ⁻⁷ (S / cm), and the conductivity at 40 nm was 4.5 × 10 ⁻⁷ (S / cm).

When the heating temperature was 120 ° C. and 150 ° C., the change in conductivity was about one digit.
When the heating temperature reached 200 ° C., a rapid increase in conductivity of 3 digits or more was observed for both film thicknesses of 20 nm and 40 nm. The conductivity was 4 × 10 ⁻² (S / cm) regardless of the film thickness.

When heat treatment is performed at 250 ° C., the conductivity decreases by about two orders of magnitude at a film thickness of 20 nm. In the case of a film thickness of 40 nm, it increased by an order of magnitude and took the maximum value of 4 × 10 ⁻¹ (S / cm).
When heat treatment at 300 ° C. or higher was performed, the conductivity decreased at both the film thickness of 20 nm and 40 nm, and the difference in conductivity of the non-processed film was as small as two digits or less.
As described above, since the change in conductivity due to heat treatment is dependent on the film thickness, the above change in conductivity is not due to a change near the interface between the semiconductor and the atmosphere, but due to a change in the amount of oxygen vacancies in the entire film. it seems to do.
On the other hand, when the stabilization process at 200 ° C. to 250 ° C. is performed on the TFT of this embodiment, the change in TFT characteristics due to electrical stress is reduced, and resistance to electrical stress is improved. This is presumably because the trap level at the active layer or the interface is reduced or the formation of the trap level is suppressed from the above consideration. The TFT production method and measurement means will be described in the examples described later.

In addition, the conductivity of the a-IGZO single film that is the active layer is about the same as that of the TFT that is not subjected to stabilization treatment and the TFT that is subjected to stabilization treatment at 300 ° C. in the atmosphere. From the same level, it is considered that the carrier density of the a-IGZO films of the two TFTs, that is, the amount of oxygen vacancies is the same level. However, in the TFT subjected to the stabilization process at 300 ° C., the threshold change before and after the electrical stress is about 1/10 of that of the TFT not subjected to the stabilization process. This shows that the resistance to electrical stress varies depending on the presence or absence of the stabilization treatment even when the amount of oxygen deficiency is similar.
Therefore, it can be concluded that the oxygen deficiency of the carrier origin and the trap level of the origin of the characteristic change due to electrical stress are not the same.

In this embodiment, any stabilization treatment at 200 ° C. in the air or 200 ° C. in a vacuum is effective in improving the resistance of the TFT to electrical stress. In this case, the temperature is effective, and an atmosphere containing oxygen is not necessarily required. Therefore, the stabilization treatment may be performed in the air, in a vacuum, in an atmosphere under reduced pressure, or in a non-oxidizing atmosphere such as an Ar atmosphere. When the stabilization treatment is performed in air, a non-oxidizing atmosphere such as an Ar atmosphere, or an atmosphere under reduced pressure, the temperature is preferably higher than 150 ° C. and lower than 281 ° C.
In addition, including the results of this time, the presence of oxygen vacancies generally may cause trap levels, but the TFT conductivity and the tendency of electrical stress resistance do not match, so the stability of this embodiment is stable. The chemical treatment does not control oxygen deficiency. Furthermore, the fact that the most preferable temperature range is different from the temperature range shown in Patent Document 1 also proves that the origin of improvement in oxygen deficiency and electrical stress resistance is different.
From the above, it is considered that the stabilization process of the present embodiment does not have a direct causal relationship with the control of the oxygen deficiency amount performed in Patent Document 1.

  The active layer may be made of an oxide containing In and Zn, but an oxide containing In, Zn and Ga is preferable. In addition, since it can be formed at room temperature, there is little variation between elements, and application to a flexible display can be expected, it is preferably amorphous.

The active layer is preferably transparent. In this case, by using the same transparent material for each component of the transistor, a transparent transistor can be obtained, and the aperture ratio when applied to a display can be improved.
In addition, a flexible transistor can be formed by making the active layer amorphous, using a flexible substrate such as a polymer substrate, and further using an amorphous material for each component of the transistor.

  As a method for forming each component of the transistor, there are a sputtering method, a vacuum deposition method, an ion plating method, a dip method, a CVD method, an MOCVD method, a PCVD method, and the like. Among these, this embodiment is particularly effective when the film is formed at a low temperature of 300 ° C. or lower, and a sputtering method suitable for uniform large-area film formation is preferable.

The insulating layer is preferably a composite oxide or a composite oxynitride containing at least one of Al ₂ O ₃ , SiO ₂ , SiON, SiN, and Si ₃ N ₄ , or at least one. Also, the relative dielectric constant is high, Sc ₂ O ₃ , TiO ₂ , ZnO, Ga ₂ O ₃ , SrO, Y ₂ O ₃ , ZrO ₂ , In ₂ O ₃ , SnO, BaO, La ₂ O ₃ , Pr ₂ O It is also preferable to use at least one of ₃ , Gd ₂ O ₃ , Yb ₂ O ₃ , HfO ₂ , Ta ₂ O ₃ , PbO, and Bi ₂ O ₃ as an insulating layer.

  Examples of the substrate include quartz glass, Si substrate, and ceramics. For example, polyimide, polyester, other polymer materials, glass, cloth, paper, and the like can be used as the flexible substrate. This embodiment is particularly effective in the case where the film is formed at a low temperature of 300 ° C. or lower due to problems such as a change in the material of the substrate and a difference in thermal expansion from the active layer or the insulating layer during the film formation. .

Examples of the electrode include Au, Ti, Ni, In, Sn, Zn, Cu, and Ag, and alloys and oxides including at least one of them.
The element configuration may be either a top gate or a bottom gate, and each may be either a top contact or a bottom contact.

  As described above, the thin film transistor subjected to the stabilization process of electrical characteristics has improved stability against electrical stress, and the display device has excellent temporal stability by connecting the source electrode or the drain electrode to the electrode of the display element. It can be.

  Further, as described above, the effect can be most expected when the display device is a display using an OLED element.

  Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited thereto.

  A first embodiment of the present invention will be described below.

  In this embodiment, as shown in FIG. 1, single-crystal Si having a thermal Si oxide film 2 formed on the surface was used as the substrate 1 for the production of the TFT. Single-crystal Si is doped with phosphorus at a high concentration and has a low resistance to n-type. In the TFT, the low-resistance single crystal Si serves as a gate electrode. Further, the thermal Si oxide film has a thickness of about 100 nm and becomes a gate insulating film of the TFT.

Next, an a-IGZO oxide semiconductor film was formed as the active layer 3 using an RF sputtering apparatus. The substrate temperature during film formation was room temperature, and a target having an In: Ga: Zn: O composition ratio of 1: 1: 1: 4 was used. This is a target with a cation ratio of In: Zn: Ga = 1: 1: 1. The substrate was sputtered at a pressure of 0.5 Pa in an Ar atmosphere with a power of 200 W and an O ₂ partial pressure of 5%. The film thickness was set to 20 nm by adjusting the film formation time. Here, apart from this example, thin film X-ray diffraction measurement (incident angle 0.5 degree) of a thin film formed under the same conditions was performed, and no clear diffraction peak was observed. It can be said that the -Zn-O thin film is amorphous.

  Thereafter, the a-IGZO film was patterned by wet etching.

  First, after the resist film was formed and dried, a pattern was opened by photolithography. Opening in this step is a region where the a-IGZO film is removed. Thereafter, wet etching was performed with dilute hydrochloric acid to remove the a-IGZO film other than the necessary region. Thereby, the a-IGZO film used as an active layer was patterned.

The maximum temperature up to this step was a drying temperature of 120 ° C.
Here, heat treatment in the atmosphere was performed as stabilization treatment of the a-IGZO film. The heat treatment time was 20 minutes. However, in order to obtain a difference in stabilization effect, the temperature was changed at four points of 150 ° C., 200 ° C., 250 ° C., and 300 ° C.

  Next, the source and drain electrodes 10 and 11 were formed by a lift-off method.

  First, after forming and drying the resist, patterns corresponding to the source and drain electrodes were opened by photolithography. Thereafter, a Ti layer 4 and an Au layer 5 were formed in this order by an electron beam vacuum deposition method. The film thickness of Ti was about 5 nm, and the film thickness of Au was about 40 nm. Thereafter, the resist was peeled off, and unnecessary Ti and Au films were removed to form source and drain electrodes.

  The maximum temperature in all steps except the stabilization treatment was the substrate temperature during film formation and the drying temperature, which was about 120 ° C.

  Through the above process, a bottom gate-top contact type TFT having an a-IGZO film as an active layer as shown in FIG. 1 was formed.

  The following electrical measurements were all performed at room temperature, in an air atmosphere and in the dark.

The initial characteristics of the TFT in this example (W / L = 180 μm / 30 μm) were a field effect mobility of 7.1 cm ² / Vs and a threshold voltage of 6.4 V when Vds = 12 V.

  An electrical stress was applied to the TFT and the resistance to the stress was examined. There are two conditions for electrical stress. Condition 1 was a condition in which 12V was applied to the gate electrode and 6V was applied to the drain electrode for 800 seconds, and condition 2 was a condition in which 20V was applied to the gate electrode and 20V was applied to the drain electrode for 800 seconds.

  The stress tolerance was examined from the change in the threshold voltage of the TFT before and after applying these two stresses. The result is shown in FIG. Here, (threshold voltage change) = (threshold voltage after stress) − (threshold voltage before stress).

As a result, irrespective of the conditions 1 and 2, in the heat treatment from 120 ° C. to 200 ° C. in the comparative example described later, the resistance to electrical stress gradually increases, and the heat treatment from 200 ° C. to 250 ° C. High tolerance. On the other hand, when the heat treatment at 300 ° C. was performed, the resistance decreased.
Therefore, by applying a heat treatment (stabilization treatment) at 200 ° C. to 250 ° C., it was possible to obtain a TFT having high resistance to electrical stress.

  Since the above temperature range is sufficiently lower than the heat resistant temperature of the glass substrate, it can be applied as a stabilization process for TFTs on the glass substrate. Further, even a plastic substrate can be used because it is lower than the heat-resistant temperature in a substrate using polyimide or the like. Furthermore, this process can be applied to the case where a top emission type OLED display is configured using a stainless steel substrate.

  Separately from the fabrication of the TFT, an a-IGZO-based oxide semiconductor was formed on a single crystal Si substrate having a thermal Si oxide film formed on the surface by an RF sputtering apparatus. The same film formation conditions as those for the active layer of the TFT were used. The film thickness was set to 20 nm by adjusting the film formation time.

  After film formation, heat treatment was performed in the atmosphere, and then temperature programmed desorption measurement was performed. The heating temperatures were 150 ° C., 200 ° C., 250 ° C., and 300 ° C., and each of the different substrates was heated for 20 minutes.

  Thermal desorption measurement was performed at a temperature of 60 ° C. per minute from about 50 ° C. to 700 ° C. at the temperature of a thermocouple brought into contact with the substrate surface.

  FIG. 7 shows a diagram relating to Zn among the desorbed gas species. For comparison, the results of a non-heat-treated film and a film heat-treated at 120 ° C. and 350 ° C. for 20 minutes were also added. Here, the horizontal axis represents the temperature of the thermocouple brought into contact with the substrate surface, and the vertical axis represents the ionic strength of mass number 64 measured by a quadrupole mass spectrometer.

Whether or not the desorbed gas species is Zn can be confirmed from the fact that the ion intensity of Zn ⁺ of mass numbers 64, 66 and 68 changes synchronously according to the isotope ratio.

  From FIG. 7, it was found that Zn started to desorb at about 260 ° C. and desorbed at about 600 ° C. Desorption was not observed at a temperature of 600 ° C. or higher. In addition, slight desorption was observed at 260 ° C. or lower.

  FIG. 8 shows a plot of the integrated ionic strength of the desorption amount of Zn with respect to the heat treatment temperature, assuming that the film not subjected to the stabilization treatment is 1. In addition, the amount of Zn desorbed from the film not subjected to the stabilization treatment is shown at the point where the heating temperature is 20 ° C.

  When the heating temperature was 150 ° C., the integrated ionic strength of Zn desorption was 1.05 times that of the film not subjected to the stabilization treatment.

  When the heating temperature was increased to 200 ° C., the amount of Zn desorbed increased rapidly and became 1.48 times.

  When the heating temperature was increased to 250 ° C., the amount of Zn desorbed further increased to 1.62 times.

  However, when the heating temperature increased to 300 ° C., the amount of Zn desorbed rapidly decreased to 0.64 times.

  When the heating temperature was raised to 350 ° C., the amount of Zn desorbed further decreased and became 0.42 times.

That is, it is understood that the heating temperature dependence of the amount of desorbed Zn increases rapidly from 150 ° C. to 200 ° C. and decreases rapidly from 250 ° C. to 300 ° C., similar to the heating temperature dependence of the conductivity. It was.
[Comparative Example 1]
Comparative Example 1 of the present invention will be described below.

  In this comparative example, a TFT manufactured by the same method as in Example 1 was used except that the stabilization treatment was not performed.

  The maximum temperature in all steps was the substrate temperature during film formation and the drying temperature, both of which were about 120 ° C.

  The following electrical measurements were all performed at room temperature, in an air atmosphere and in the dark.

The initial characteristics of the TFT (W / L = 180 μm / 30 μm) in this comparative example were a field effect mobility of 7.1 cm ² / Vs and a threshold voltage of 6.4 V at Vds = 12V.

In the same manner as in Example 1, an electrical stress was applied to the TFT, and the resistance to the stress was examined. The conditions for electrical stress are the same as in Example 1.
The stress tolerance was examined from the change in the threshold voltage of the TFT before and after applying these two stresses. The result is shown in FIG. However, since the maximum temperature of the TFT forming process is 120 ° C. in this comparative example, the data is plotted at a point of 120 ° C. The contents of condition 1 and condition 2 are the same as those in the first embodiment. The definition of the threshold voltage change is the same as in the first embodiment.
[Comparative Example 2]
In this comparative example, a TFT manufactured by the same method as in Comparative Example 1 was used. However, the film thickness of the a-IGZO film in this comparative example was 40 nm.

  All electrical measurements below were performed at room temperature in the dark.

The initial characteristics of the TFT (W / L = 300 μm / 50 μm) in the air atmosphere of this comparative example are as follows: when Vds = 12 V, the field-effect mobility is 8.5 cm ² / Vs, and the threshold voltage is 7.1 V. there were.

  In this comparative example, the electrical stress resistance was measured by the same method as in Comparative Example 1 after the TFT was created. The electrical stress applied in this comparative example was 6 V applied to the gate electrode and 6 V applied to the drain electrode for 800 seconds. Furthermore, in order to see the effect of the atmosphere, the measurement was performed in an air atmosphere and a vacuum atmosphere. The measurement of the electrical stress resistance in the air atmosphere was measured 1, and the measurement in the vacuum atmosphere was measured 2.

  The result of evaluating the electrical stress resistance is shown in FIG. The definition of the threshold voltage change is the same as in the first embodiment.

  As a result, it was found that the threshold voltage change due to stress is less than 1 V regardless of the atmosphere.

  In this example, the TFT of Comparative Example 2 was subjected to a stabilization treatment at 200 ° C. in a vacuum atmosphere for 1 hour.

  All electrical measurements below were performed at room temperature in the dark.

  As in Comparative Example 2, the effect of the stabilization treatment was evaluated by examining the change in TFT characteristics before and after applying electrical stress to the TFT. As in the comparative example 2, the electrical stress applied in this example was 6 V applied to the gate electrode and 6 V applied to the drain electrode for 800 seconds.

  Furthermore, in order to see the effect of the atmosphere, measurements were performed in a vacuum atmosphere and an air atmosphere. The measurement in the vacuum atmosphere after the stabilization treatment is taken as measurement 3, and the measurement in the air atmosphere after the stabilization treatment is taken as measurement 4.

  FIG. 9 shows changes in the threshold voltage in the two measurements. The definition of the threshold voltage change is the same as in the first embodiment.

  As a result, it was found that the threshold voltage change can be suppressed to 0.2 V or less after the stabilization process regardless of the atmosphere.

  Therefore, by applying a stabilization treatment at 200 ° C. in a vacuum atmosphere, the threshold voltage change can be suppressed from less than 1 V to 0.2 V or less, and resistance to electrical stress can be improved.

  Since the above temperature is sufficiently lower than the heat-resistant temperature of the glass substrate, it can be applied as a stabilization process for TFTs on the glass substrate. In addition, even a plastic substrate can be used because it has a heat resistant temperature or lower in the case of a substrate having high heat resistance such as polyimide. Furthermore, this process can be applied to the case where a top emission type OLED display is configured using a stainless steel substrate.

  The AM type OLED display of this embodiment will be described below.

  FIG. 10 shows an inverted staggered (bottom gate) TFT element on a glass substrate used in this example. A method for producing this TFT element will be described below.

  First, a transparent conductive film IZO having a thickness of 150 nm is formed on the glass substrate 11 by sputtering, and then the gate electrode 12 is formed by photolithography and wet etching using hydrochloric acid.

Subsequently, a first insulating film 13 made of a-SiO _x is formed to a thickness of 200 nm by sputtering. At that time, a SiO ₂ target is used as the sputtering target, and Ar gas is used as the sputtering gas.

  Next, an a-IGZO oxide semiconductor film used as the active layer 14 is formed by sputtering at room temperature to a thickness of 20 nm. Here, as the stabilization process of the a-IGZO film, the stabilization process is performed in the atmosphere at 250 ° C. for 20 minutes. Thereafter, the active layer is patterned by photolithography and wet etching using hydrochloric acid.

Next, for the protection of the active layer and the etching stop function, an insulating layer made of a-SiO _x is formed to a thickness of 100 nm as the second insulating film 15 by sputtering. At that time, an oxidizing atmosphere having an O ₂ / Ar mixed gas ratio of 50% is used as a sputtering gas. Thereafter, patterning of the second insulating film is performed using photolithography and dry etching with CF ₄ gas.

  Next, after forming a transparent conductive film ITO by a 150 nm sputtering method, a source electrode 16 and a drain electrode 17 are formed by a photolithography method and a wet etching method using a commercially available ITO etching solution.

  Thus, the inverted staggered (bottom gate) type transparent TFT element shown in FIG. 10 can be formed.

  The gate electrode, source electrode and drain electrode include not only transparent conductive oxide films such as IZO and ITO, but also metals such as Ni, Cr, Rh, Mo, Nd, Ti, W, Ta, Pb and Al, and the like. Alloys or silicides can also be used. In addition, the gate electrode, the source electrode, and the drain electrode can be formed using different materials, or a part of each electrode can be formed using different materials.

  A pixel which is a basic component of an AM type OLED display can be formed using the TFT shown in FIG. FIG. 11 shows a configuration example including the TFT and the OLED element shown in FIG. Constituent members that are the same as those shown in FIG.

  In FIG. 11, the anode electrode 18 is connected to the source electrode, and constitutes a through electrode and a transparent electrode. An organic light emitting layer 20 is provided between the transparent electrode portion of the anode electrode 18 and the cathode electrode 21. The cathode electrode 21 is made of, for example, cesium carbonate and aluminum. Reference numeral 19 denotes a planarizing film.

  The AM type OLED display of this example has high electrical stress resistance because the TFT is stabilized at 250 ° C. Therefore, an AM type OLED display that can be driven for a long time can be realized.

  In the present embodiment, it is formed on a glass substrate, but a plastic substrate having a heat resistance of about 250 ° C. can be similarly formed. In that case, a flexible display is also possible.

  Further, the TFT shown in FIG. 10 of the present embodiment can be used for an AM type LCD and other AM type displays such as an AM type electrophoretic element reflective display as well as the AM type OLED display.

  The present invention can be applied to a thin film transistor having good electrical stress resistance, and can be used for an AM type OLED display, an AM type LCD, and other AM type displays such as an AM type electrophoretic element reflective display.

It is a figure which shows the structure of the thin-film transistor concerning 1st Example of this invention and a 1st comparative example. It is a figure which shows the thermal desorption measurement result of mass number 64 (Zn ^<+> ) of the a-IGZO film heat-processed in the vacuum. It is a figure which shows the integral ion intensity | strength of mass number 64 (Zn ^<+> ) with respect to the heat processing temperature in a vacuum in a thermal desorption analysis. It is a figure which shows the heat processing temperature dependence of the electrical conductivity of an a-IGZO film. from a-IGZO film is a diagram showing a temperature-programmed desorption measurement results of the mass number 18 _(H 2 ^{O +).} It is a figure which shows the stabilization process temperature dependence of the threshold voltage change by the electrical stress concerning the 1st Example of this invention and a 1st comparative example. It is a figure which shows the thermal desorption measurement result of mass number 64 (Zn ^<+> ) of the a-IGZO film heat-processed in air | atmosphere concerning the 1st Example of this invention. It is a figure which shows the integral ion intensity | strength of mass number 64 (Zn ^<+> ) with respect to the heat processing temperature in air | atmosphere in the thermal desorption analysis concerning the 1st Example of this invention. It is a figure which shows the threshold voltage change by the electrical stress before and behind the stabilization process concerning the 2nd comparative example and 2nd Example of this invention. It is a figure which shows the structure of the thin-film transistor concerning the 3rd Example of this invention. It is a figure which shows the structure of the thin-film transistor and organic electroluminescent element concerning the 3rd Example of this invention.

Explanation of symbols

12 Gate electrode
13 First insulating layer
14 a-IGZO active layer
15 Second insulation layer
16 Source electrode
17 Drain electrode
18 Anode electrode (transparent electrode and through electrode)
19 Planarization film
20 Organic light emitting layer
21 Cathode electrode (cesium carbonate + aluminum)

Claims (13)

A thin film transistor whose active layer is an amorphous oxide composed of In and Zn,
Providing the active layer with energy for stabilizing electrical characteristics after the formation of the active layer;
The active layer imparted with energy desorbs more than 1.05 times Zn in a temperature range of 200 ° C. to 700 ° C. by temperature-programmed desorption analysis with respect to the active layer not imparted with energy. A thin film transistor.   The thin film transistor according to claim 1, wherein the energy is applied by heat treatment.   The thin film transistor according to claim 2, wherein the heat treatment is performed in the atmosphere, and the heating temperature is higher than 150 ° C and lower than 281 ° C.   The thin film transistor according to claim 2, wherein the heat treatment is performed in a vacuum, and the heating temperature is higher than 150 ° C. and lower than 300 ° C. 4.   5. The thin film transistor according to claim 1, wherein the active layer is an amorphous oxide containing In and Zn and Ga. 6.   6. The active layer is an amorphous oxide formed by a sputtering apparatus using a target having a cation ratio of In: Zn: Ga = 1: 1: 1. 2. A thin film transistor according to item 1.   A display device, wherein the source electrode or the drain electrode of the thin film transistor according to any one of claims 1 to 6 is connected to an electrode of the display element.   A display device according to claim 7, wherein the display element is an organic light emitting diode.   A method of manufacturing a thin film transistor in which an active layer is composed of an amorphous oxide containing In and Zn, and energy is applied to the active layer to stabilize electrical characteristics after the step of forming the active layer A process for producing a thin film transistor, comprising the step of:   10. The method for manufacturing a thin film transistor according to claim 9, wherein the step of applying energy is performed after the step of forming an insulating layer in contact with the active layer.   11. The method of manufacturing a thin film transistor according to claim 9, wherein the energy is applied by a heat treatment, and a heating temperature in the heat treatment is higher than a heat treatment temperature in a step other than the step of applying the energy.   The method for manufacturing a thin film transistor according to any one of claims 9 to 11, wherein the heat treatment is performed in the atmosphere, and the heating temperature is higher than 150 ° C and lower than 281 ° C.   The method for manufacturing a thin film transistor according to any one of claims 9 to 11, wherein the heat treatment is performed in a vacuum, and the heating temperature is higher than 150 ° C and lower than 300 ° C.