JPH0258336A - Manufacture of thin film semiconductor device - Google Patents

Abstract

PURPOSE:To form a gate oxide film at a low temperature by implanting predetermined boron into the whole face of polycrystalline silicon, then activating it to oxidize the whole face by oxygen plasma, removing the unnecessary parts of an oxide film and the silicon, and forming a channel. CONSTITUTION:Polysilicon is deposited 1000Angstrom on a glass board in a reduced pressure CVD furnace. Then, after boron is implanted 2X10<13>cm into the whole face at 40keV by an ion implanting device, it is activated at 580 deg.C for 3 hours. Thereafter, oxygen plasma is generated by induction coupling type RF discharge, and it is anode-oxidized in the plasma. Subsequently, a channel is formed by a photoetching method. After it is patterned with negative resist, an oxide film is etched with aqueous solution of fluoric acid and a polysilicon film is etched with mixture solution of fluoric acid and ammonium fluoride. Then, a gate electrode is formed, boron is implanted 1X10<15>cm on a whole face at 40keV by the ion implanting device, and a P-channel is formed. After the P- channel is covered with photoresist, phosphorus is implanted 3X10<15>cm at 80keV, and an N-channel is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of forming a gate oxide film by plasma anodic oxidation.

[Conventional technology]

In recent years, development and commercialization of thin film semiconductors have been progressing mainly for liquid crystal display devices.

There are two main types of NM semiconductors: a type using polysilicon and a type using amorphous silicon.

The characteristic of thin film semiconductors using polysilicon is 1000°C.
It is possible to obtain Si◯ and insulating films thermally oxidized at the above-mentioned high temperatures, and it also has the advantage of being easy to apply MO8FET manufacturing technology, making it easy to obtain stable characteristics.

Furthermore, the conductivity type and threshold voltage of the channel can be selected by implanting impurities.

Furthermore, since the carrier mobility is as high as about 10 cm"/V-s, it is possible to integrate the drive circuit on the same substrate as the thin film transistor. Additionally, polysilicon is generally insensitive to light. , there is no need to provide a light shielding layer.

Taking advantage of these characteristics of polysilicon thin film semiconductors, liquid crystal display devices with built-in drive circuits and close-contact line sensors with on-chip amorphous silicon photoelectric conversion elements have been developed and commercialized.

Amorphous silicon can be made into SiH using glow discharge, etc.
Formed from +. Silicon oxide and silicon nitride are often used for gate insulating films, and both are often made using glow discharge.

Amorphous silicon is chemically stable and photolithography can be used to fabricate thin film transistors. Since the dark resistance is high, the off-resistance of the thin film transistor can be increased. Carrier mobility is 0.1-1cm'
/V-s, which is small, but can be used for active matrix LCD switches.

The manufacturing process of amorphous silicon transistors is
The entire process is carried out at a low temperature of 300° C. or less, and an inexpensive glass substrate can be used.

Amorphous silicon thin film semiconductors have been put to practical use mainly in liquid crystal color displays, mainly due to their manufacturing advantages, such as the ability to form thin films at low temperatures and use large, inexpensive substrates.

[Problem to be solved by the invention]

However, since the gate oxide film of polysilicon thin film semiconductors is formed at high temperatures as described above, an expensive material such as quartz must be used for the substrate.

Therefore, there was a problem that it was difficult to increase the size of the substrate and the cost increased.

On the other hand, since the gate of amorphous silicon thin film semiconductor is formed by glow discharge, it does not have the stable characteristics of a thermal oxide film, and has the problem of poor long-term reliability.

The present invention is intended to solve these problems, and its purpose is to provide a manufacturing method for forming a gate oxide film of a polysilicon thin film semiconductor at a low temperature.

[Means to solve the problem]

The method for manufacturing a thin film semiconductor of the present invention includes (1) a step of forming polycrystalline silicon on an insulating substrate, and (2) a step of implanting boron of 5xlO"cm or less into the entire surface of the polycrystalline silicon and then activating it. (3) oxidizing the entire surface of the polycrystalline silicon with oxygen plasma while applying a positive bias; and (4) removing unnecessary portions of the oxide film and polycrystalline silicon using a photoetch method. It is characterized by the formation of

〔Example〕

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Below, the present invention will be described in detail according to an embodiment in which the present invention is applied to a contact image sensor in which a sensor and a driving circuit are integrated on a chip.

1,000 layers of polysilicon are deposited on a glass substrate in a low-pressure CVD furnace. As the glass substrate, borosilicate-based heat-resistant glass was used. Next, boron was implanted into the entire surface using an ion implantation device to a depth of 2×10” at 40 K e V, and then an activation treatment was performed at 580° C. for 3 hours.

Next, oxygen plasma was generated using inductively coupled RFTI, and anode oxidation was performed in this oxygen plasma. The method of anodic oxidation is as follows. The reactor was a quartz glass tube with an inner diameter of 60 cm and a soaking length of 80 cm. After evacuating the reactor to 0.01 Torr or less using a rotary pump, 99.9% oxygen was flowed in and the pressure was maintained at 0.3 Torr. The substrate was made of polysilicon and was heated to 250°C by applying +20V from a constant voltage power supply. In this state, a high frequency power source of 13.56 MHz was used in the reactor, and the output was set to 1°2 KW to generate plasma. Oxidation increases almost linearly up to about 5000 people. In this example, the gate oxide film is 12008.

The rate of formation of the oxide film increases as the positive bias voltage increases, the plasma output increases, and the heating temperature of the substrate increases.

Regarding oxygen pressure, unlike the above items, the maximum rate of oxide film formation can be obtained at a certain pressure.

In particular, the quality of the gate oxide film is greatly influenced by the heating temperature of the substrate. If the temperature is below 150°C, the dielectric strength will drop significantly, and if it is above 300°C, the stability of the plasma reactor will deteriorate, which is not preferable.

Desirably, it is 230°C to 280°C.

The thickness of the oxide film was within ±5% within a 20 cm square glass substrate.

Next, a channel portion is formed by photoetching. After patterning using a negative resist, the oxide film was etched with an aqueous solution of hydrofluoric acid, and the polysilicon film was etched with a mixed solution of hydrofluoric acid and ammonium fluoride.

Next, a gate electrode was formed. The gate electrode used was made by depositing 3,500 layers of polysilicon and then thermally diffusing phosphorus to make it low in resistance.

? The photo-etching method was used for the tti pattern. Polysilicon etching creates a tapered step, so C
This was done using a plasma of a mixed gas of F and ○.

Next, boron was irradiated with lX at 40 KeV using an ion implantation device.
A P channel was formed by implanting 10" cm over the entire surface.
Next, after covering the P channel part with photoresist, 3x10''
'cot implant N channel was formed.

Next, remove the photoresist and fill it at 600°C.
Time activation treatment was performed. Next, as an interlayer insulating film, 8000 SiO2 (N S G ) was formed in a normal pressure CVD furnace, and 5000 SiO2 (N S G ) was formed in a forming gas (Ar + 5% H2).
Annealing was performed at 0°C for 1 hour. This annealing process can remove strain from the NSC film and at the same time diffuse hydrogen into the polysilicon to improve transistor characteristics.

Next, ITO (InzOs doped with SnOx) was formed as a transparent conductive film by sputtering, and a lower electrode was formed by photoetching. Next, an amorphous silicon film was formed as a light receiving sensor by plasma CVD. This amorphous silicon film is ITO
From the side, 500 P-type amorphous SiC workers, 800
The structure is made of 0 amorphous Si and 500 N-type amorphous SiC. Here, X is from 0 to 1
However, in this example, X was approximately 0.3. After photo-etching the amorphous silicon, the interlayer insulating film was removed by photo-etching in order to establish conduction with the transistor. Next, a film of Al was formed by sputtering to conduct the transistor and form an upper electrode of the sensor, and an electrode was formed by photoetching.

Next, a passivation layer is formed to ensure reliability.The passivation layer is made of SiC, boimide, and S.
It was formed with three layers of iO2. SiO2 is made by sputtering method,
Each polyimide was formed by a spin cord method.

In this way, we were able to create an on-chip contact image sensor in which the drive circuit was formed using polysilicon transistors and a portion of the sensor was formed using amorphous silicon.

The characteristics of a transistor are determined by the quality of the polysilicon film and the quality of the gate film, but since polysilicon is formed at a relatively low temperature even with conventional methods, the film quality does not pose any practical problem. On the other hand, although the gate film was formed at a low temperature, which is completely different from the conventional high-temperature oxidation method, no change in transistor characteristics was observed. Since the gate oxide film according to the present invention is formed by an anodic oxidation method, if non-uniformity occurs in the oxide film, a potential difference distribution will occur between the substrate and the surface layer, and thinner portions will be preferentially oxidized. Therefore, it has the characteristic that the uniformity of the oxide film automatically improves.

〔Effect of the invention〕

The effects of the present invention are shown below.

(1) Since the gate oxide film can be formed at a low temperature, inexpensive glass can be used for the substrate, and the cost can be reduced by increasing the size.

(2) The thickness distribution of the gate oxide film within the substrate is uniform;
Device characteristics are improved.

(3) Since the oxide film is grown from the substrate rather than being deposited, the plasma equipment is not contaminated and mass production is stable.

(4) Since the channel portion of the transistor is doped with boron, it is possible to prevent shifts caused by damage to the transistor, which is particularly likely to occur in the N channel.

As described above, the present invention is extremely excellent in practical use, and can be used in devices to which thin film transistors are applied, such as liquid crystal televisions and liquid crystal displays with built-in drive circuits.

that's all

Claims (1)

[Claims] (1) A process of forming polycrystalline silicon on an insulating substrate, a process of implanting boron of 5 x 10^1^3 cm or less over the entire surface of the polycrystalline silicon and activating it, and biasing the polycrystalline silicon in a positive direction. A method for manufacturing a thin film semiconductor device, characterized in that a channel portion is formed by the steps of: oxidizing the entire surface using oxygen plasma; and removing unnecessary portions of the oxide film and polycrystalline silicon using a photo-etching method.