JPH0239571A - Thin film transistor - Google Patents

Abstract

PURPOSE:To raise the driving frequency of a shift register and to reduce power consumption by forming an active layer for forming the channel of a thin film transistor of nondoped polysilicon, and setting the threshold voltages of N- and P-channel transistors to a predetermined value. CONSTITUTION:An active layer for forming the channel of a thin film transistor is formed of nondoped polysilicon, the threshold voltage of an N-channel transistor is 2V or more, and the threshold voltage of a P-channel transistor is -5V or less. Thus, N-channel and P-channel are composed. The polysilicon active layer is not channel doped, and enhancement-driven even if its n<-> conductivity type remains as it is. If the threshold voltage of the N-channel transistor is less than 2V or the threshold voltage of the P-channel transistor is higher than -5V, the balance of the N-channel and P-channel transistors is broken, and a normal CMOS operation is not conducted. The threshold voltages of the transistors are controlled by limiting the manufacturing process conditions of the transistors.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thin film transistor (hereinafter referred to as TPT) used in a line sensor drive circuit, an active matrix type 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 CCD (charge-coupled device) using single-crystal Si.

Recently, 1x sensors that do not require this reduction optical system have become mainstream. A-3i:H or the like is used for the photoconductive layer of the sensor array of this same-size sensor, and it is formed on an insulating substrate.

Methods for mounting a circuit that reads signals from the sensor array include a method using an external LSI and a method of fabricating a TPT 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 TPT channels are formed include a-Si and ρoly.
-Si is used, and when high switching speed is required, poly-5L, which has higher carrier mobility than a-3i, 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 TPT has a much smaller device size when the screen size is the same; The display quality is also excellent.

FIG. 2 shows an example of a TPT conventionally used in these equal-magnification sensors and LCD drive circuits. In the TPT shown in FIG. 2, 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, an interlayer insulating film 7 is deposited, and a contact is made to the glabella 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 ly-Si has an n- conductivity type, it becomes a depression drive type when an N-channel transistor is manufactured. 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.

[Problem to be solved by the invention]

The present invention is based on poly-Si deposited by LPVCD method.
When using a CMO, the poly-Si of the N-channel transistor which is depletion driven due to its n- conductivity type is left undoped for enhancement drive.
It is an object of the present invention to provide a TPT that can increase the ratio of on and off current values, increase the drive frequency of the shift register, and reduce current consumption when configured as an S shift register.

[Means to solve the problem]

The present invention provides a thin film transistor fabricated on an insulating substrate, in which the active layer forming the channel of the thin film transistor is made of non-doped poly-Si, and the threshold voltage of the N-channel transistor is 2V or more, and the threshold voltage of the P-channel transistor is 2V or more. is -5V or less.

In the present invention, the threshold voltage of the N-channel transistor is 2V or more, and the threshold voltage of the P-channel transistor is -5V or less.

poly-3L forming N-channel and P-channel
This was completed based on the finding that enhancement drive can be performed even when the active layer is not channel-doped and remains of n- conductivity type.

In the present invention, if the threshold voltage of the N-channel transistor is less than 2V or the threshold voltage of the P-channel transistor exceeds -5V, the balance between the N-channel and P-channel transistors will be lost, and normal CM
O3 stops working.

Note that the threshold voltage vth of the transistor is v”r'D
-Vg (I. Nidorain current, vg: gate voltage) curve can be measured and determined from the intersection of the straight line portion and the Vg axis.

Furthermore, the threshold voltage of each channel transistor can be easily controlled to a desired value by limiting the transistor manufacturing process conditions.

Here, an example of manufacturing the TPT according to the present invention will be explained based on the process diagram shown in FIG.

(1) Transparent quartz glass with a sufficiently polished surface (5゜mm
After thoroughly cleaning 9 (250 mm x 1.6 mm), remove the non-doped poly-Si thin film 17 that will become the active layer.
A film with a thickness of 0.00 mm is formed on transparent quartz glass by the LPCVD method. The conditions are as follows.

Substrate temperature: 629° C. SiH, flow rate: 145 SCCM pressure: 0.13 Torr (2) Poly-3L active layer 10 is formed by patterning (see FIG. 1(a)).

(3) Thermal oxidation of the poly-3i active layer to 1300
A gate insulating film 11 with a human thickness is formed. The oxidation conditions are as follows.

Insertion/removal temperature: 600℃ thermal oxidation temperature
1050℃ heating rate 3℃/+n1n (4) LPCVD poly-Si gate electrode 12
It is deposited to a thickness of about 4,000 by the method.

(5) A PSG film is formed on the poly-3i gate electrode by a coating method, and the resistance of the poly-Si gate electrode is reduced by thermal diffusion (1000° C., 30 minutes). After that, the PSG film that served as a diffusion source is removed.

(6) Patterning the poly-3i gate electrode 12 and gate insulating film 11 with a predetermined channel length (
(See Figure 1(b)).

(7) PSG (for N-channel TPT farming)
After depositing a film or a BSG (in the case of P-channel TPT production) film by a coating method, the source 13 and drain 14 regions are formed by self-alignment by thermal diffusion, and then the PSG film or BSG film that has become a diffusion source is removed ( Figure 1 (c
)reference).

(8) Deposit a PSG film 15 with a thickness of 1 μm using the LPCVD method to form an interlayer insulating film (see Figure 1(d)).
.

The film forming conditions are as follows.

Substrate temperature 430℃SiH4 flow rate
88 SCCMO, 200 PH, 8 Pressure 0.20 Torr (9) Open the contact hole 16 and take out the AI2 electrode wiring 17 from the source and drain (Fig. 1(e))
reference).

(10) Perform plasma hydrogen treatment.

The conditions are as follows.

Substrate temperature 350℃H2 flow t
100 SCCM pressure CO
Torr RF Power 2401 (13,56
MHz) time 35
minHole mobility measurement of the TPT active layer produced by the process described above revealed that the conductivity type was n-. Further, when the threshold voltage was measured, it was 2V for the N channel and -5V for the P channel. Furthermore, when a CMO3 was made by combining these N-channel TPTs and P-channel TPTs and a shift register was constructed, the maximum driving frequency was 2 MHz or more.

[Action/effect of the invention]

As described above, in the present invention, the threshold voltage of the N-channel TPT fabricated on an insulating substrate is set to 2V or more, and the threshold voltage of the P-channel TPT is
Since the threshold voltage of PT is set to -5V or less, the active layer forming the channel is made of non-doped poly-Si.
Even if it has an n- conductivity type, it is driven by enhancement, so if a CMOS shift register is constructed by combining these N-channel TPTs and P-channel TPTs, the driving frequency is high and the current consumption can be reduced. can.

[Brief explanation of the drawing]

FIG. 1 is a process explanatory diagram showing an example of manufacturing a TPT according to the present invention. FIG. 2 is a cross-sectional view of a conventional TPT. 1... Insulating substrate 2, 13... Source 3.14... Drain 4, 11... Gate insulating film 5... Active layer 6... Gate electrode 7.16... Contact hole 8 ...Metal electrode wiring 9...Transparent quartz glass 10...Non-doped poly-Si active layer 12...p
oly-3i gate electrode 15...PSG film 17...AQ electrode wiring patent applicant Ricoh Co., Ltd. and one other person Figure 2

Claims (1)

[Claims] 1. In a thin film transistor fabricated on an insulating substrate,
A thin film transistor characterized in that an active layer forming a channel of the thin film transistor is made of non-doped poly-Si, and a threshold voltage of an N-channel transistor is 2V or more, and a threshold voltage of a P-channel transistor is -5V or less. .