JP3360139B2 - Gate insulating film forming method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an oxide thin film, and more particularly, to a method for forming a gate insulating film of a thin film transistor represented by a liquid crystal driving circuit.
The present invention relates to a method for forming an oxide thin film such as O.

[0002]

2. Description of the Related Art Conventionally, a simple matrix, a two-terminal element, and a three-terminal element have been used as a driving circuit for a liquid crystal. -SiTFTs are the most promising and some have been put to practical use. However, a-Si
TFT (amorphous silicon thin film transistor)
Since the electron mobility of a-Si is small, there is a limit to its application to large-screen, high-definition display required for high-definition television. Therefore, poly- with high electron mobility
It has been proposed that a poly-Si TFT using Si is used for a driving circuit, and a viewfinder and a CC have already been used.
D. Used in some LCD projectors. Since the poly-Si TFT is manufactured by using an LSI process, a process temperature of 1000 ° C. or more is applied when a gate insulating film is formed on a substrate by a thermal oxidation method, and therefore, the substrate has high heat resistance and high cost. Can use only quartz glass. Since large-screen poly-Si TFTs are used in OA equipment and consumer equipment, the demand for inexpensiveness must be satisfied. For this purpose, a poly-Si TFT is formed by a low-temperature process capable of forming a gate insulating film on an inexpensive glass substrate. -S
It is desired to manufacture an iTFT.

[0003] Thermal CVD, PE-CVD, and sputtering are known as methods for forming an SiO2 film at a temperature of 600 ° C or lower. In the case of sputtering, a normal RF magnetron sputtering method using an SiO2 target is used. . However, if the SiO2 target is sputtered with only Ar, oxygen in the film becomes deficient, resulting in a film having a large leak current, which is not suitable as a gate insulating film. Therefore, generally, an oxygen gas is introduced simultaneously with an Ar gas to form a film by sputtering.

[0004]

As a gate insulating film, it is required that a leak current is small, a withstand voltage is large,
A low fixed charge density and a low interface state density are required. In order to satisfy these requirements,
It is considered that a film having a dense stoichiometric composition may be formed without damaging the film. If only a film having a dense stoichiometric composition is to be obtained, the film can be formed at a low temperature by lowering the Ar gas pressure and introducing a small amount of oxygen. The density and the interface state density cannot be reduced sufficiently. The cause is presumed to be damage due to the use of plasma, and ions, electrons, X-rays, and heat can be considered as causes of the damage. Of these four causes, the one with the highest energy is the ion,
It was speculated that the damage could be reduced by reducing the energy of the ions. Note that defects in the gate insulating film are observed as an increase in fixed charge density,
Further, defects at the interface between the substrate and the gate insulating film can be observed as an increase in interface state density.

An object of the present invention is to provide a method for forming a gate insulating film at a low temperature by reducing the damage caused by the plasma.

[0006]

According to the present invention, in a method of forming a gate insulating film composed of a thin film of an oxide such as Si-O by a sputtering method, a parallel magnetic field on a target surface is set to 500 gauss or more. The above-mentioned object was achieved by performing sputtering while maintaining the above.

[0007]

When an oxide film of, for example, SiO2 is formed on a glass substrate by a sputtering method, plasma is generated in a vacuum chamber, and a negative electrode such as O-, O2-, or the like generated in the vicinity of the target prepared in the vacuum chamber or in the plasma. The ions are accelerated by the sputtering voltage or the bias voltage Vdc, and rush into the substrate at -400 eV if Vdc is -400V, for example. As a result, in the conventional case, ions may cause micro damage to the SiO 2 film on the silicon substrate or the interface between the silicon substrate and the SiO 2 film, and defects such as unpaired atoms may occur. In this case, since a magnetic field of 500 gauss or more is formed on the target surface, the plasma impedance is reduced, the sputtering voltage or the bias voltage Vdc is reduced, the damage due to negative ions is reduced, and a high quality gate insulating film is formed. The substrate can be formed at a low temperature, and a low-cost ordinary glass substrate can be used as the substrate.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows a gate insulating film forming apparatus used for carrying out the method of the present invention. In this figure, reference numeral 1 denotes a suitable vacuum pump. 1 shows a vacuum chamber provided with an exhaust port 2 connected via a valve and a gas inlet 3 for introducing a sputtering gas composed of a mixed gas of Ar gas and oxygen gas, each of which is independently controlled by, for example, a mass flow controller. In the vacuum chamber 1, a target 4 and a normal inexpensive substrate 5 made of, for example, a 5-inch p-type Si wafer are provided to face each other, and a tantalum heater 6 is provided behind the substrate 5 to 5 was heated to a predetermined temperature. The target 4 is bonded to a backing plate 7 with a brazing material.
Fixed to The cathode container 8 is hermetically attached to the vacuum chamber 1 via an insulating plate 11. Inside the cathode container 8, an iron yoke 10 having a magnetic generating coil 9 is provided behind the target 4. , The iron yoke 10
A magnetic circuit which returns to the iron yoke 10 via a magnetic field leaking from the target 4 to the surface of the target 4 is formed. The cathode container 8 is connected to an RF power source via a matching box 12. Reference numeral 13 denotes a meter for measuring the bias potential Vac. In this case, the DC potential of the target 4 is read through the resistor 14. The magnetic circuit is configured such that the parallel magnetic field generated on the surface of the target 4 can be adjusted in the range of 200 to 1600 gauss by controlling the energization of the magnetic generation coil 9.

In the vacuum chamber 1, an earth shield 15,
A shutter 16 and a deposition prevention plate 17 were provided, and the substrate 5 and the heater 6 were mounted on a top plate 18. Cooling water is circulated in the cathode vessel 8 to cool the target 4 and the iron yoke 10.

When a gate insulating film is formed on the substrate 5 by using the above-described apparatus, the inside of the vacuum chamber 1 is evacuated to a pressure of 1 × 10 −6 Torr or less by a vacuum pump. Is heated to a predetermined temperature by the heater 6. Thereafter, an Ar gas is introduced into the vacuum chamber 1 and a predetermined oxygen gas is introduced so as to be constant at 1.5 mTorr.
A plasma discharge was generated while forming a magnetic field near the surface of the substrate 5, and a film was formed on the substrate 5. Target 4 is composed of Si and Si
Sputtering was performed in exchange for O2, and film formation was performed with the magnetic field intensity of the target 4 changed. The result is shown in FIG.
As shown in the figure, it is found that the bias voltage Vdc decreases with an increase in the magnetic field strength up to 500 Gauss regardless of the material of the target 4, but the Vdc does not change much at 500 Gauss or more. Therefore, target 4
By setting the upper parallel magnetic field to 500 gauss or more, although the absolute value of the bias voltage Vdc changes depending on the material of the target 4, it can be seen that the value of Vdc can be sufficiently reduced. Instead of the RF magnetron sputtering method described above, when a DC power supply was connected to the cathode and the sputtering of the Si target was attempted by the DC magnetron sputtering method, the same result, that is, the voltage of the target 4 was sufficiently reduced.

Further, using the apparatus shown in FIG. 1, an Ar pressure of 1.5 mTorr and a substrate temperature of 20
At 0 ° C. and RF power of 1 Kw, a 2,000 angstrom thick SiO2 gate insulating film is formed on each of several substrates using a Si target and a SiO2 target at an optimum oxygen pressure while changing the parallel magnetic field on the surface of the target 4. did. In this case, as can be seen in FIG.
The sample using the target has a bias voltage Vdc of-
When the voltage was between 500 V and -300 V, a phenomenon in which the fixed charge density rapidly changed was observed. On the other hand, in the sample using the Si target, a phenomenon was observed in which the fixed charge density rapidly changed when Vdc was between -300 V and -250 V. Since these bias voltages Vdc correspond to 400 gauss and 500 gauss when converted to magnetic field strength, it has been found that setting the magnetic field strength to 500 gauss or more reduces the fixed charge density. When the relationship between the bias voltage Vdc and the interface state density of these samples was examined, the results shown in FIG. 4 were obtained. FIG. 4 shows exactly the same tendency as FIG. 3, and when the intensity of the parallel magnetic field on the target surface is set to 500 gauss or more, S
It was found that an iO2 gate insulating film was obtained.

From the above, it is assumed that 5
When an oxide gate insulating film is formed by a sputtering method while forming a parallel magnetic field of 00 Gauss, the film can be formed with a reduced target voltage or bias voltage, and the fixed charge density and the interface state density are small. A gate insulating film can be obtained, and an inexpensive ordinary glass substrate having a low heat-resistant temperature can be used as the substrate.

In the apparatus shown in FIG. 1, a rare earth magnet is replaced with a target 4 instead of a magnetic generating coil 9 constituting a magnetic circuit.
May be provided on the back surface of.

When forming an SiO 2 gate insulating film by RF magnetron sputtering using an SiO 2 target in the method of the present invention, a mixture of Ar gas and oxygen gas is introduced, and the total pressure is in the order of 10 −3 Torr. Generally, the ratio between the partial pressures of Ar gas and oxygen gas is about 1: 0.1 to 1: 1. In addition, reactive sputtering using a Si target
When the iO2 gate insulating film is formed, either the DC magnetron sputtering method or the RF magnetron sputtering method may be used, and when the iO2 gate insulating film is formed by the RF magnetron sputtering method,
The total pressure of Ar gas and oxygen gas is on the order of 10 −3 Torr, and there are cases where Ar gas and oxygen gas are mixed and introduced and cases where they are introduced separately by a nozzle. Is generally 1: 0.5 to irrespective of the gas introduction method.
It is about 1:10.

[0015]

As described above, according to the present invention, the gate insulating film of an oxide such as Si-O is formed by the sputtering method while maintaining the parallel magnetic field on the target surface at 500 gauss or more. The energy of the negative ions entering the substrate decreases, and the interface between the substrate and the gate insulating film and damage to the gate insulating film can be reduced. As a result, an excellent gate having a small interface state density and a fixed charge density can be obtained. Since an insulating film is obtained and the temperature of the substrate during film formation decreases,
There is an effect that a gate insulating film can be formed by a sputtering method using an inexpensive glass substrate.

[Brief description of the drawings]

FIG. 1 is a cutaway side view of the apparatus used to implement the method of the present invention.

FIG. 2 is a diagram showing a relationship between the intensity of a parallel magnetic field on a target surface and a bias voltage.

FIG. 3 is a diagram showing a relationship between a bias voltage and a fixed charge density.

FIG. 4 is a diagram showing a relationship between a bias voltage and an interface state density.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Vacuum chamber 3 Gas inlet 4 Target 5 Substrate 9 Magnet generating coil
10 Iron yoke

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masanori Hashimoto 523 Yokota, Yamatake-cho, Yamatake-gun, Chiba Prefecture Japan Vacuum Engineering Co., Ltd. Chiba Super Materials Research Laboratory (72) Inventor Kazuyuki Ito 523 Yokota, Yamatake-cho, Yamatake-gun, Chiba Japan Nippon Vacuum (72) Inventor Yonezaki, Takezaki, 523 Yamatake-cho, Yamatake-gun, Chiba Prefecture Japan Vacuum Technology Co., Ltd. Chiba Super-Materials Research Laboratory (56) References JP-A-1-304736 ( JP, A) JP-A-60-132319 (JP, A) JP-A-2-232358 (JP, A) JP-A-3-249171 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) C23C 14/35 H01L 21/316 H01L 21/336

Claims (2)

(57) [Claims] In a method of forming a gate insulating film composed of a thin film of Si—O by a sputtering method, an upper limit value of a fixed charge density is about 7 × 10 ¹⁰ cm ⁻² and an interface state The gate insulating film is formed by sputtering while maintaining the parallel magnetic field on the target surface at 500 gauss or more so that the upper limit of the density is about 2 × 10 ¹¹ ev ⁻¹ cm ^−2. A method for forming a gate insulating film. 2. A method for forming a gate insulating film composed of a thin film of an oxide of Si—O by a sputtering method, wherein a glass substrate and a target made of SiO ₂ or Si are provided to face each other and 1 × 10 ^−6. In a vacuum chamber evacuated to a pressure equal to or lower than Torr, the partial pressure ratio between Ar gas and oxygen gas is 1: 0.
A mixed gas of about 1 to 1: 1 Ar gas and oxygen gas is introduced, and the total pressure in the vacuum chamber is set to 10 ^-3 Torr level, and further R
By setting the F power to 1 Kw and maintaining the parallel magnetic field on the target surface at 500 gauss or more, sputtering is performed to form a gate insulating film having a low fixed charge density and a low interface state density. A gate insulating film forming method.