JPH0223624A - thin film transistor - Google Patents

Abstract

PURPOSE:To change a conductivity type from an N<-> type to a P<-> type, and to obtain an enhancement-driven TFT by a simple constitution by forming a BSG film between a poly-Si active layer and an insulating substrate in the N channel TFT and thermally diffusing boron in the BSG film into the poly-Si active layer in a heating process. CONSTITUTION:A BSG film 10 is shaped onto an insulating substrate 9, a poly-Si thin film is deposited onto the BSG film, and a poly-Si active layer 11 is formed through patterning. A thermal oxide film 12 is formed by thermally oxidizing the poly-Si active layer. Since a thermal oxidation process is executed normally at a temperature of 1000 deg.C, boron in the BSG film is thermally diffused into the poly-Si active layer, and a P<-> poly-Si active layer 13 is shaped. Source-drain regions, a gate electrode, a contact electrode, etc., are formed through a normal process, thus manufacturing an N channel TFT. The N channel TFT manufactured in this manner functions as an enhancement type because the active layer is shaped in a P<-> type.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thin film transistor (hereinafter referred to as TFT) used in a line sensor drive circuit, an active matrix LCD drive circuit, and the like.

[Conventional technology and its problems]

Currently, office automation (OA) equipment is rapidly becoming lighter, thinner, and smaller. For example, an image reading device, which is important as a sending device in a facsimile, has been miniaturized by combining a reduction optical system and a CCO (charge coupled device) using single crystal Si.

Recently, 1x sensors that do not require this reduction optical system have become mainstream. The photoconductive layer of the sensor array of this same-size sensor uses a-Si:l (etc.), and is formed on an insulating substrate.

Methods for mounting a circuit that reads signals from the sensor array include a method using an external LLSI and a method of fabricating TFTs on the same substrate as the sensor array. Considering the miniaturization of devices, the latter method is far more advantageous than the former. Materials for the active layer in which the TFT channel is formed include a-Si and pol.
y-Si is used, and when high switching speed is required, poly-5i, which has higher carrier mobility than a-Si, is used.

The same thing can be said about LCDs (liquid crystal displays) as another example. In other words, compared to a simple matrix display driven by an external LSI, an active matrix display incorporating TFTs has a much smaller device size when the screen size is the same; The display quality is also excellent.

FIG. 1 shows an example of a TFT conventionally used in these equal-magnification sensors and LCD drive circuits. In the TFT shown in FIG. 1, after forming an active layer 5, a gate insulating film 4, a gate electrode 6, a source 2, and a drain 3 on an insulating substrate 1, a glabellar insulating film 7 is deposited, and a contact is made to an interlayer insulating film 7. After the hole is opened, the metal electrode wiring 8 is fabricated.

In such a TFT, when a poly-Si thin film deposited by LPCVD is used as the active layer, this poly-Si thin film is deposited by LPCVD.
Since the conductivity type of ly-8i is n-, it becomes a depression drive type when an N-channel transistor is fabricated. On the other hand, the P-channel transistor is driven by enhancement. Therefore, if a CMOS shift register is constructed by combining it with a P-channel transistor,
As mentioned above, since the ratio of the on/off current values of the N-channel transistor is small, problems such as a low driving frequency of the shift register and a large current consumption have arisen.

Conventionally, in order to solve these problems, boron was implanted into the active layer of an N-channel transistor by ion implantation to change the conductivity type of the active layer from n- to p-, thereby realizing enhancement drive. However, this method has problems such as high equipment cost and low throughput.

[Problems Imaki is trying to solve]

In the case of using poly-Si deposited by LPVCD method, the present invention changes the conductivity type from n- to p- by a simple configuration of the poly-Si of the N-channel transistor which is depletion driven due to the n- conductivity type. , enhancement drive, and when a CMOS shift register is configured by incorporating this N-channel transistor, a TFT is provided that can increase the ratio of on and off current values, increase the drive frequency of the shift register, and reduce current consumption. The purpose is to

[Means to solve the problem]

The present invention provides an N-channel thin film transistor manufactured on an insulating substrate, in which the active layer forming the channel of the thin film transistor is a poly-Si thin film, and a BSG film is provided between the insulating substrate and the active layer, and the TFT manufacturing process In the heating process, the boron in this BSG film becomes poly-
It is characterized by thermal diffusion into the Si active layer.

The present invention will be specifically explained below with reference to FIG. 2 showing one embodiment of the present invention. Figure 2 shows enhancement type N.
This is a part of the process for manufacturing a channel transistor.

In FIG. 2(a), a BSG film 10 is formed on an insulating substrate 9. The BSG film can be deposited by LPCVD, coating, or the like. On this BSG film, poly-Si
A thin film is deposited and patterned to form a poly-Si active layer 11. Poly-Si is formed into a film by LPCVD, sputtering, or the like.

Next, in FIG. 2(b), the poly-Si active layer is thermally oxidized to form a thermal oxide film 12. This oxide film will later be used as a gate insulating film. Since the thermal oxidation process is usually carried out at a temperature of 1000°C, the B.S.
Boron in the G film is thermally diffused into the poly-Si active layer to form a p-poly-5i active layer 13.

After this, the source and drain regions are
Gate electrodes, contact electrodes, etc. are formed to produce an N-channel TFT. N-channel TFT made in this way
has an enhancement type operation because the active layer is p-.

An example of manufacturing such a TFT will be explained with reference to FIG.

(1) After thoroughly cleaning the transparent quartz glass 14 whose surface has been sufficiently polished, a 50% BSG film is applied by LPCVD.
Deposited with a thickness of 0 people. The deposition conditions are as follows.

Substrate temperature 430℃ SiH4 flow rate 88 SCCM02
200 B21. n 5 n Pressure 0.20 Torr (2) Patterning is performed so that the BSG film 15 remains only in the portion that is to become the active layer of the N-channel TFT (see FIG. 3(a)).

(3) A poly-Si thin film (with a thickness of 1700 mm) is formed by the LPCVD method. The conditions are as follows.

Substrate temperature 629℃ SiH4 flow rate 145 SCCM pressure
0.13 Torr (4) Poly-Si active layer 16 of N-channel TFT
and poly-Si active layer 17 of P-channel TFT
(See FIG. 3(b)).

(5) Thermal oxidation of the poly-Si active layer to 1400
A thermal oxide film 18 with a human thickness is formed. The oxidation conditions are as follows.

Insertion/removal temperature: 600℃ thermal oxidation temperature
1050°C heating rate 3°C/min At this time, the lower B layer is added to the active layer of the N-channel TFT.
Boron in the SG film is thermally diffused and becomes a p-po'1y-Si active layer 19 (see FIG. 3(C)).

(6) Deposit a poly-Si gate electrode to a thickness of 4000 nm by LPCVD.

(7) A PSG film is formed on the poly-Si gate electrode by a coating method, and the resistance of the poly-Si gate electrode is reduced by thermal diffusion. Thereafter, the psG film that served as a diffusion source is removed.

(8) Pattern the n poly-Si gate electrode 20 and gate insulating film 21 to a predetermined channel length (see FIG. 3(d)).

(9) PSG (for N-channel TFT fabrication)
After depositing a film or a BSG (in the case of P-channel TFT fabrication) film by a coating method, a source 22 and a drain 23 are formed by self-alignment by thermal diffusion, and then the PSG film or BSG film that has become a diffusion source is removed (step 3). (See Figure 3(e)).

(10) Form a thin oxide film 24 (~200 layers) by thermal oxidation. The oxidation temperature is 925°C (see Figure 3(f)).

(11) 6,000 yen of PSG film 25 by LPCVD method
It is deposited to a thickness of 0 μm to form an insulating film between the eyebrows (Figure 3 (g)).
reference).

The film forming conditions are as follows.

Substrate temperature 430°CSiH4 flow rate 88 SCCMO□ u
200 /'PH%+
8# Pressure 0.20 Torr (12) Open the contact hole 26 and take out the AQ electrode wiring 27 from the source and drain (see FIG. 3(h)).

(13) Perform plasma hydrogen treatment.

The conditions are as follows.

Substrate temperature 350℃H2 flow rate
100 SCCM pressure 1.0
Torr RF Power 2401j (
13,56MHz) time
When a shift register was constructed using CMO8 manufactured by the process described above, the maximum drive frequency was 2 MHz or more and the current consumption was sufficiently small.

[Action/effect of the invention]

In the present invention as described above, a BSG film is provided between the poly-Si active layer and the insulating base in the N-channel TFT, and the TFT
During the heating process during the FT fabrication process, boron in this BSG film is thermally diffused into the poly-Si active layer, so the conductivity type changes to n.
TF that changes from - to p- and performs enhancement drive
T can be obtained by a simple construction.

Therefore, by incorporating this N-channel transistor, C
When a MOS shift register is configured, the ratio of on and off current values becomes high, the drive frequency of the shift register becomes high, and the current consumption is reduced.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of a conventional TFT. FIG. 2 is an explanatory diagram showing a part of the manufacturing process of the TFT of the present invention. FIG. 3 is an explanatory diagram showing an example of the process of manufacturing the TFT of the present invention.

Claims (1)

[Claims] 1. In an N-channel thin film transistor fabricated on an insulating substrate, the active layer forming the channel of the thin film transistor is a poly-Si thin film, and a BSG (boron silicate glass) film is provided between the insulating substrate and the active layer. , an N-channel thin film transistor characterized in that boron in the BSG film is thermally diffused into the poly-Si active layer during the heating step of the TFT manufacturing process.