US20120135562A1

US20120135562A1 - Methods of forming hydrophobic silicon dioxide layer and forming organic thin film transistor

Info

Publication number: US20120135562A1
Application number: US13/006,424
Authority: US
Inventors: Ching-Lin Fan; Ping-Cheng Chiu; Chang-Chih Lin
Original assignee: National Taiwan University of Science and Technology NTUST
Current assignee: National Taiwan University of Science and Technology NTUST
Priority date: 2010-11-29
Filing date: 2011-01-14
Publication date: 2012-05-31
Anticipated expiration: 2031-01-14
Also published as: TW201222667A; US8178447B1; TWI474400B

Abstract

A method of forming a hydrophobic silicon dioxide layer is provided. A substrate is provided. Thereafter, a hydrophobic silicon dioxide layer is formed on the substrate by using a plasma chemical vapour deposition (CVD) system, in which tetraethyl orthosilicate (TEOS) and an oxygen-containing gas are introduced at a reactive temperature between 25° C. and 150° C. A method of forming an organic thin film transistor (OTFT) including the hydrophobic silicon dioxide layer as a gate insulating layer is also provided. In the present invention, the hydrophobic silicon dioxide layer can be directly formed at low temperature without using the conventional surface modification treatment. Accordingly, the process is simplified and the cost is reduced.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 99141265, filed on Nov. 29, 2010. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.

BACKGROUND OF THE INVENTION

1. Field of Invention
The present invention relates to methods of forming an insulating layer and forming a device, and more generally to methods of forming a hydrophobic silicon dioxide (SiO₂) layer and forming an organic thin film transistor (OTFT) including the hydrophobic SiO₂layer.
2. Description of Related Art
OTFTs have recently attracted much attention for their applications in radio-frequency identification (RFID) tags, large-area sensors and active matrix flat panel display (AMPD) backplane driving circuits.
Generally, a SiO₂layer is one of the most widely used gate insulating layers in OTFTs. When an OTFT is formed on a flexible substrate, the process temperature of the SiO₂gate insulating layer must be lowered for compatibility, otherwise the flexible substrate may be damaged. However, the SiO₂gate insulating layer formed at low temperature usually includes a significant amount of hydroxyl (OH) groups, which makes the surface of the SiO₂gate insulating layer more polar and hydrophilic. This OH-terminated SiO₂surface may cause poor crystalline growth of an organic semiconductor active layer (e.g. pentacene), and thus the performance of the OTFT is affected.
It is known that the OTFT performance can be greatly improved by modifying the SiO₂surface with a hydrophobic self-assembled monolayer (SAM), such as hexamethyldisilazane (HMDS) or octadecyltrichlorosilane (OTS). HMDS or OTS can form a hydrophobic methyl SAM (CH₃-SAM) on the SiO₂surface, so as to reduce the SiO₂surface energy and improve the crystalline growth quality of the organic semiconductor active layer (e.g. pentacene). However, due to the surface modification treatment, the process becomes more complicated and time-consuming and the cost is increased.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of directly forming a hydrophobic SiO₂layer, in which the conventional surface modification treatment is not required, so that the process is simplified and the cost is reduced.
The present invention further provides a method of forming an OTFT, in which a hydrophobic SiO₂gate insulating layer is formed by a simple process to enhance the performance of the device.
The present invention provides a method of forming a hydrophobic silicon dioxide layer. A substrate is provided. A hydrophobic silicon dioxide layer is formed on the substrate by using a plasma chemical vapour deposition (plasma CVD) system, in which tetraethyl orthosilicate (TEOS) and an oxygen-containing gas are introduced at a reaction temperature between 25° C. and 150° C.
According to an embodiment of the present invention, the oxygen-containing gas includes oxygen or ozone.
According to an embodiment of the present invention, the reaction temperature is between 50° C. and 125° C., for example.
According to an embodiment of the present invention, a flow rate ratio of TEOS to the oxygen-containing gas is between 1:3 and 1:5, for example.
According to an embodiment of the present invention, the substrate includes a hard substrate or a flexible substrate.
According to an embodiment of the present invention, the plasma CVD system includes a plasma enhanced CVD (PE CVD) system, a high density plasma CVD (HDP CVD) system, a hollow cathode CVD (HC CVD) system, an electro cyclone resonance CVD (ECR CVD) system or an inductively coupled plasma CVD (ICP CVD) system.
The present invention further provides a method of forming an organic thin film transistor. A substrate is provided. A gate insulating layer is formed on the substrate by using a plasma CVD system, in which TEOS and an oxygen-containing gas are introduced at a reaction temperature between 25° C. and 150° C. A source electrode and a drain electrode are formed on the gate insulating layer beside the substrate. An organic semiconductor active layer is formed on the gate insulating layer and between the source electrode and the drain electrode.
According to an embodiment of the present invention, a flow ratio of TEOS to the oxygen-containing gas is between 1:3 and 1:5, for example.
According to an embodiment of the present invention, the method further includes forming a gate electrode between the substrate and the gate insulating layer after the step of providing the substrate and before the step of forming the gate insulating layer on the substrate.
According to an embodiment of the present invention, the plasma CVD system includes a PE CVD system, an HDP CVD system, an HC CVD system, an ECR CVD system or an ICP CVD system.
In view of the above, in the present invention, a hydrophobic SiO₂gate insulating layer of an OTFT can be formed at low temperature. This hydrophobic SiO₂surface favours the crystalline growth of an organic semiconductor active layer with a larger grain size, and thus, the performance of the device is greatly improved. In addition, the hydrophobic SiO₂layer can be directly formed at low temperature without using the conventional surface modification treatment. Accordingly, the process is simplified and the cost is reduced.
In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIGS. 1A to 1C schematically illustrate cross-sectional views of a method of fanning an OTFT according to a first embodiment of the present invention.

FIGS. 2A to 2B schematically illustrate cross-sectional views of a method of forming an OTFT according to a second embodiment of the present invention.

FIG. 3 illustrates the comparison of Fourier transform infrared (FTIR) spectra between Example 1 and Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

First Embodiment

FIGS. 1A to 1C schematically illustrate cross-sectional views of a method of forming an OTFT according to a first embodiment of the present invention.
Referring to FIG. 1A, a substrate 110 is provided. The substrate 110 can be a hard substrate (e.g. a glass substrate, a quartz substrate or a silicon substrate) or a flexible substrate (e.g. a plastic substrate or a metal thin film).
Thereafter, a gate electrode 120 is formed on the substrate 110. The material of the gate electrode 120 includes Ag, Au, Cu, Al or an alloy thereof. The gate electrode 120 is formed by performing a physical vapour deposition (PVD) process or a thermal evaporation process.
Referring to FIG. 1B, a gate insulating layer 130 is formed on the gate electrode 120. The gate insulating layer 130 is formed by performing a plasma CVD process system, in which tetraethyl orthosilicate (TEOS) and an oxygen-containing gas are introduced at a low reaction temperature, so as to form a hydrophobic SiO₂layer on the gate electrode 120. The oxygen-containing gas includes oxygen or ozone.
The reaction temperature is between 25° C. (room temperature) and 150° C., for example. In an embodiment, the reaction temperature is between 50° C. and 125° C. In another embodiment, the reaction temperature is between 60° C. and 100° C. In yet another embodiment, the reaction temperature is between 70° C. and 90° C.
In addition, the flow rate ratio of TEOS to the oxygen-containing gas is between 1:3 and 1:5, for example. For example, the flow rate of TEOS is about 50-80 sccm, and the flow rate of the oxygen-containing gas is about 150-400 sccm. Besides, the radio frequency (RF) power is about 300-500 W, and the pressure is about 10-50 mtorr.
The plasma CVD system may include, but not limited to, a plasma enhanced CVD (PE CVD) system, a high density plasma CVD (HDP CVD) system, a hollow cathode CVD (HC CVD) system, an electro cyclone resonance CVD (ECR CVD) system or an inductively coupled plasma CVD (ICP CVD) system.
Referring to FIG. 1C, a source electrode 1405 and a drain electrode 140D are formed on the gate insulating layer 130 beside the gate electrode 120. The material of the source electrode 140S and the drain electrode 140D includes Ag, Au, Cu, Al, Pt, Cr or an alloy thereof. The method of forming the source electrode 140S and the drain electrode 140D includes forming a metal layer by a PVD process or a thermal evaporation process, and then patterning the metal layer by a lift-off process.
Thereafter, an organic semiconductor active layer 150 is formed on the gate insulating layer 130 and between the source electrode 140S and the drain electrode 140D. The material of the organic semiconductor active layer 150 includes organic small molecules, organic polymers, or organic/inorganic mixture layers, such as pentacene. The organic semiconductor active layer 150 is formed by performing a thermal evaporation process, a vacuum evaporation process, a spin coating process or an inkjet printing process, for example. The OTFT of the first embodiment is thus completed.
It is noted that in the present invention, the reaction temperature is lower, so that residual hydrophobic methyl (CH₃) groups are generated due to incomplete decomposition of TEOS in a low temperature environment. The hydrophobic methyl groups are simultaneously deposited during the process of forming the SiO₂layer, and thus, the surface of the resulting SiO₂layer exhibits a hydrophobic character.
This hydrophobic SiO₂layer (i.e. the gate insulating layer 130) favours the crystalline growth of the organic semiconductor active layer 150. In details, since the material used for the organic semiconductor active layer usually exhibits a hydrophobic character, the organic semiconductor active layer is required to growth on a substrate having the same surface property (e.g. hydrophobicity) so as to enhance the crystallinity of the organic semiconductor active layer and simultaneously form a larger grain size. Accordingly, the performance of the device is improved.

Second Embodiment

FIGS. 2A to 2B schematically illustrate cross-sectional views of a method of forming an OTFT according to a second embodiment of the present invention. The second embodiment is similar to the first embodiment. The differences between them are described in detail hereafter, while similarities thereof are omitted.
Referring to FIG. 2A, a substrate 110 is provided. It is noted that when the substrate 110 is a heavily doped silicon substrate, it can serve as a gate electrode. In this case, the step of forming the gate electrode can be omitted, and a gate insulating layer 130 can be directly formed on the substrate 110. The gate insulating layer 130 is a hydrophobic SiO₂layer, the forming method thereof has been described in the first embodiment, and the details are not iterated herein.
Referring to FIG. 2B, a source electrode 1405 and a drain electrode 140D are formed on the gate insulating layer 130 beside the substrate 110. Thereafter, an organic semiconductor active layer 150 is formed on the gate insulating layer 130 and between the source electrode 140S and the drain electrode 140D. The materials and forming methods of the source electrode 140S, the drain electrode 140D and the organic semiconductor active layer 150 have been described in the first embodiment, and the details are not iterated herein. The OTFT of the second embodiment is thus completed.
The above embodiments in which the hydrophobic SiO₂layer serves as a gate insulating layer in an OTFT are provided for illustration purposes, and are not construed as limiting the present invention. It is appreciated by persons skilled in the art that the hydrophobic SiO₂layer can be applied to any member or device including a hydrophobic insulating layer. For example, the hydrophobic SiO₂layer can serve as a protection layer upon the requirement.
Several examples and comparative examples are numerated below to prove the performance of the present invention. Since the SiO₂gate insulating layer formed through a thermal oxidation process exhibits excellent desired characteristics and is regarded as a golden sample, it is compared with the hydrophobic SiO₂layer formed through a low temperature plasma CVD process in the following.

Example 1

An N-type heavily doped silicon substrate is provided. Thereafter, a plasma CVD process in which TEOS and oxygen are introduced at 80° C. is used to form a hydrophobic SiO₂layer as a gate insulating layer on the silicon substrate. The TEOS flow rate is 65 sccm, and the oxygen flow rate is 200 sccm. Besides, the RF power is 300 W, and the pressure is 50 mtorr. The hydrophobic SiO₂layer has a thickness of 100 nm.

Comparative Example 1

An N-type heavily doped silicon substrate is provided. Thereafter, a SiO₂layer as a gate insulating layer is formed on the silicon substrate by a thermal oxidation process at 980° C. The SiO₂layer has a thickness of 100 nm.

Example 2

The N-type heavily doped silicon substrate with the hydrophobic SiO₂layer thereon of Example 1 is provided. Thereafter, a pentacene layer of 50 nm thick is formed on the surface of the hydrophobic SiO₂layer.

Comparative Example 2

The N-type heavily doped silicon substrate with the hydrophilic SiO₂layer thereon of Comparative Example 1 is provided. Thereafter, a pentacene layer of 50 nm thick is formed on the surface of the hydrophilic SiO₂layer.
FIG. 3 illustrates the comparison of Fourier transform infrared (FTIR) spectra between Example 1 and Comparative Example 1, wherein peak (A) presents Si—O—Si rocking, peak (B) presents Si—O—Si bending, peak (C) presents Si—OH stretching, peak (D) presents Si—O—Si stretching, and peak (E) presents Si—CH₃bending. Table 1 shows the testing results of Examples 1-2 and Comparative Examples 1-2.

TABLE 1

				Surface	Penta-
				rough-	cene
Water	Surface			ness	grain
contact	energy	Si—CH₃	Surface	R_rms	size
angle	(mJ/m²)	bending	property	(nm)	(μm)

Example 1	51.48°	47.93	Yes	hydro-	0.315	—
				pho-
				bicity
Comparative	31.81°	62.28	No	hydro-	0.216	—
Example 1				phil-
				icity
Example 2	—	—	—	—	—	4.6
Comparative	—	—	—	—	—	~2.5
Example 2

As shown in FIG. 3, as compared with the SiO₂layer of Comparative Example 1, Si—CH₃bending (i.e. peak (E)) is obviously present in the SiO₂layer of Example 1. That is, the SiO₂layer of Example 1 has Si—CH₃hydrophobic groups so as to exhibit surface hydrophobicity. In addition, Table 1 shows that the SiO₂layer of Example 1 exhibits a hydrophobic character while the SiO₂layer of Comparative Example 1 exhibits a hydrophilic character, so that the SiO₂layer of Example 1 has a higher water contact angle than that of Comparative Example 1.
Besides, the surface energy can be estimated from the water contact angle by Girifalco-Good-Fowkes-Young (GGFY) model. The surface energy of the pentacene layer is about 38-48 mJ/m². Table 1 shows that the surface energy in Example 1 (47.93 mJ/m²) is closer to that of the pentacene layer as compared with that in Comparative Example 1 (62.28 mJ/m²). The surface energy is one of the main factors known to affect the crystalline growth of the pentacene layer. Since the SiO₂layer of the present invention has a similar surface energy with the pentacene layer and exhibits surface hydrophobicity, the crystalline growth quality of the pentacene layer is greatly improved. As shown in Table 1, the pentacene layer of Example 2 has a larger grain size than that of Comparative Example 2. In the channel region of an OTFT, larger pentacene grains can significantly reduce the number of grain boundaries. The grain boundaries act as potential trap sites so as to impede the carrier movement. Therefore, the less the number of grain boundaries, the higher the device performance.

Example 3

The N-type heavily doped silicon substrate with the hydrophobic SiO₂layer thereon of Example 1 is provided. Thereafter, a Cr/Pt alloy serving as a source electrode and a drain electrode is formed on the hydrophobic SiO₂layer by a thermal evaporation system. The Cr/Pt alloy has a thickness of 2 nm/70 nm. A lift-off process is then performed to pattern the Cr/Pt alloy, so as to form the source electrode and the drain electrode on the hydrophobic SiO₂layer beside the silicon substrate. Afterwards, a pentacene layer as an organic semiconductor active layer is formed on the hydrophobic SiO₂layer and between the source electrode and the drain electrode by a thermal evaporation system with the substrate temperature maintained at room temperature (25° C.). The pentacene layer has a thickness (i.e. channel thickness) of 50 nm. In addition, the channel width and length of the OTFT are 500 μm and 10 μm respectively.

Comparative Example 3

The N-type heavily doped silicon substrate with the hydrophilic SiO₂layer thereon of Comparative Example 1 is provided. Thereafter, the Cr/Pt alloy as the source electrode and the drain electrode and the pentacene layer are formed in accordance with the same procedure as in Example 3.
Table 2 shows the electrical testing results of Example 3 and Comparative Example 3. An HP 4145B semiconductor parameter analyzer is used to perform the testing in a dark environment, wherein the electrical parameters include threshold voltage (V_TH), field effect mobility (μ_FE), sub-threshold swing (SS) and on/off current ratio (I_ON/I_OFF).

	TABLE 2

	Example 3	Comparative Example 3

Deposition temperature of SiO₂	80	980
layer (° C.)
Thickness of SiO₂layer (nm)	100	100
Channel thickness (nm)	50	50
V_TH(V)	−3	−5
μ_FE(cm²/V s)	2.3 × 10⁻²	3.7 × 10⁻²
SS (V/decade)	0.6	1.01
I_ON/I_OFF	1.27 × 10⁵	1 × 10⁶

Referring to Table 2, the OTFT including the hydrophobic SiO₂layer as a gate insulating layer (Example 3) has comparable electrical performance with that including the hydrophilic SiO₂layer as a gate insulating layer (Comparative Example 3). Specifically, Example 3 and Comparative Example 3 provide similar μ_FEand V_TH. Moreover, Example 3 provides a lower SS than Comparative Example 3 so as to achieve a better electrical performance.
It is noted that the hydrophobic SiO₂layer of the present invention is fabricated in a low temperature environment, without using a high temperature apparatus or performing a conventional surface modification treatment. Accordingly, the method in accordance with the present invention is very competitive.
In summary, in the present invention, a SiO₂layer with surface hydrophobicity is formed by using a plasma CVD system, in which TEOS and an oxygen-containing gas are introduced at low temperature. This SiO₂layer with surface hydrophobicity can be used as a gate insulating layer of an OTFT device.
The hydrophobic SiO₂layer of the present invention has similar surface properties (e.g. hydrophobicity, surface energy, etc.) with the organic semiconductor active layer, so as to favour the crystalline growth of the organic semiconductor active layer with a larger grain size, thereby greatly improving the performance of the device. In the present invention, the hydrophobic SiO₂layer can be directly formed at low temperature without using the conventional surface modification treatment. Accordingly, the process is simplified and the cost is reduced. Further, the hydrophobic SiO₂layer of the present invention is fabricated at low temperature, so that a flexible substrate is not damaged from the low process temperature. Therefore, the hydrophobic SiO₂layer of the present invention can be widely applied in flexible products.
The present invention has been disclosed above in the preferred embodiments, but is not limited to those. It is known to persons skilled in the art that some modifications and innovations may be made without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be defined by the following claims.

Claims

1. A method of forming a hydrophobic silicon dioxide layer, comprising:

providing a substrate; and

forming a hydrophobic silicon dioxide layer on the substrate by using a plasma chemical vapour deposition (plasma CVD) system, in which tetraethyl orthosilicate (TEOS) and an oxygen-containing gas are introduced at a reaction temperature between 25° C. and 150° C.,

wherein a radio frequency power of the plasma CVD system is 300-500 W, and

wherein the hydrophobic silicon dioxide layer has a Si—CH₃hydrophobic group.

2. The method of claim 1, wherein the oxygen-containing gas comprises oxygen or ozone.

3. The method of claim 1, wherein the reaction temperature is between 50° C. and 125° C.

4. The method of claim 1, wherein a flow rate ratio of TEOS to the oxygen-containing gas is between 1:3 and 1:5.

5. The method of claim 1, wherein the substrate comprises a hard substrate or a flexible substrate.

6. The method of claim 1, wherein the plasma CVD system comprises a plasma enhanced CVD (PE CVD) system, a high density plasma CVD (HDP CVD) system, a hollow cathode CVD (HC CVD) system, an electro cyclone resonance CVD (ECR CVD) system or an inductively coupled plasma CVD (ICP CVD) system.

7. A method of forming an organic thin film transistor, comprising:

providing a substrate;

forming a gate insulating layer on the substrate by using a plasma chemical vapour deposition (plasma CVD) system, in which tetraethyl orthosilicate (TEOS) and an oxygen-containing gas are introduced at a reaction temperature between 25° C. and 150° C.;

forming a source electrode and a drain electrode on the gate insulating layer beside the substrate; and

forming an organic semiconductor active layer on the gate insulating layer and between the source electrode and the drain electrode,

wherein a radio frequency power of the plasma CVD system is 300-500 W, and

wherein the gate insulating layer has a Si—CH₃hydrophobic group.

8. The method of claim 7, wherein a flow ratio of TEOS to the oxygen-containing gas is between 1:3 and 1:5.

9. The method of claim 7, further comprising forming a gate electrode between the substrate and the gate insulating layer after the step of providing the substrate and before the step of forming the gate insulating layer on the substrate.

10. The method of claim 7, wherein the plasma CVD system comprises a plasma enhanced CVD (PE CVD) system, a high density plasma CVD (HDP CVD) system, a hollow cathode CVD (HC CVD) system, an electro cyclone resonance CVD (ECR CVD) system or an inductively coupled plasma CVD (ICP CVD) system.