JPS63172469A - Thin film transistor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Summary] As a method for preventing an increase in off-state current and a decrease in field effect mobility of a thin film transistor, a titanium or chromium metal thin film is placed between a transparent conductive film and an amorphous silicon film. Interposed thin film transistor.

[Industrial application field]

The present invention relates to a structure of a thin film transistor with improved electrical characteristics.

Thin film transistors (abbreviated as TPT) are made by depositing amorphous silicon (
A semiconductor film made of silicon nitride (S
It is made by forming an insulating film or a metal film such as silicon dioxide (SiO□) or silicon dioxide (SiO□), and combining this with photolithography to form a fine pattern in layers.

Since a transistor array can be formed over a wide area using this technique, TPT is used as a switching element in an active matrix type liquid crystal display panel.

In such applications, TPT is required to have high switching speed and high field effect mobility (hereinafter simply referred to as mobility).

[Conventional technology]

There are two types of TPTs: a staggered type and an inverted staggered type, depending on the arrangement of source and drain electrodes and gate electrodes.

FIG. 3 is a cross-sectional diagram of a conventional staggered TPT, in which tin oxide (tin oxide) is deposited on a glass substrate l by sputtering or the like.
A low-resistance transparent conductive film (hereinafter commonly referred to as ITO film) made of a solid solution of indium oxide (InzOa) and amorphous silicon film doped with phosphorus (P) (hereinafter abbreviated as n)
After forming the "a-5t film) 3, selective etching is performed using photolithography to pattern the source electrode S and the drain electrode.

Next, an amorphous silicon film (hereinafter abbreviated as a-Si) is placed on top of this.
After forming Dj 4, a silicon nitride film (
A gate insulating film 5 made of any one of a silicon oxide film (Stow film), a silicon oxynitride film (SiON film), and an electrode film 6 made of a metal such as chromium (Cr) or nichrome (Ni-Cr). After forming a layer, a gate electrode G is patterned by selective etching using a photolithography technique, and elements are separated to form a staggered TPT.

On the other hand, an inverted staggered TPT is formed with a reverse configuration in which a gate electrode G is provided on a glass substrate 1.

TPTs with such a configuration undergo a heat treatment (annealing) process at 200 to 300°C to stabilize the electrical characteristics after the element is completed, and are also subjected to various heat treatments when forming a liquid crystal display panel. S
In and Sn atoms of the ITO film 2 constituting the drain electrode pass through the n''-a-5i film 3 to the a-Si
There is a problem in that the a-St film 4 is diffused into the film 4, thereby increasing the number of defects in the a-St film 4, resulting in an increase in off-state current and a decrease in mobility.

[Problem that the invention seeks to solve]

The ITO film 2 is used for the source electrode S and drain electrode of the TPT used to drive the liquid crystal display panel because they are patterned in the same process as the transparent electrode of the liquid crystal element. Initially, ITO is processed through heat treatment associated with the formation of active matrix type liquid crystal display panels
A-
The problem is that it diffuses into the St film 4, thereby increasing the number of defects in the a-St film 4, resulting in an increase in off-state current and a decrease in mobility.

[Means for solving problems]

The above problem can be solved by interposing a metal thin film between the transparent conductive film forming the source and drain electrodes of the TFT and the n+ amorphous silicon peritoneum.

[Effect]

In the present invention, In which constitutes the source electrode S and the drain electrode
As a method for preventing the diffusion of Sn atoms into the a-3il14, titanium (Ti) or chromium (Cr) is used between the ITO film 2 and the n''a-5i film 3 provided for making an ohmic connection. A high melting point metal film is provided.

FIG. 1 is a cross-sectional diagram of a staggered TPT according to the present invention, in which a metal film 7 is interposed between an ITO film 2 and an n'' a-Si film 3 that form a source electrode S and a drain electrode. ing.

Here, a necessary condition for the metal film is that it is difficult to diffuse even when heat is applied, and from this point on, T
i, Cr, etc. are suitable.

The present invention prevents the diffusion of In atoms and Sn atoms from the source electrode S and the drain electrode by providing such a metal film with a thickness of about 500 nm.

In this case, the names of the source electrode S and the drain electrode include both the ITO film 2 and the metal film 7, but the metal film 7 is almost the same as the a-5t film 4 in order not to impair the transparency of the ITO film 2. It needs to be shaped to size.

[Example] FIG. 4 is a sectional view showing the manufacturing process of a staggered TPT according to the present invention, and an example is shown as follows.

10 ITO films 2 are deposited on a glass substrate 1 by sputtering.
After forming the film to a thickness of 0.00 mm, T
A metal film 7 was formed by forming a metal film 7 with a thickness of 500 mm, and an n0a-5i film 3 was formed thereon with a thickness of 300 mm using the P-CVD method (see A in the same figure).

Next, reactive ion etching (abbreviated as RIB) is performed to form a source electrode S and a drain electrode.

Here, a mixed gas of carbon tetrafluoride (CF4) and oxygen (0□) was used as a reactive gas for etching the n'a-5i film.
For Ti etching, carbon tetrachloride (CC14) and 0! A mixed gas of carbon difluorochloride (CCj!zh) and 0□ was used for etching ITO.
Figure B).

Next, an a-Si film 4 is formed on the substrate by the p-cvo method.
The gate insulating film 5 was formed by forming a SiNx film to a thickness of 2,000 wafers and a SiNx film to a thickness of 3,000 ni.
).

Next, NiCr was deposited to a thickness of 800 nm by electron beam evaporation to form an electrode film 6, and then chemically etched to form a gate electrode G (see D in the figure).

Next, element isolation is performed using RIB, and staggered T
The PT has been completed (see E in the same figure).

FIG. 2 is a drain current (■.) vs. gate voltage (VG) characteristic diagram of the staggered TPT according to the present invention, and compared to the conventional TPT indicated by the broken line 8, the TPT according to the present invention indicated by the solid line 9 is The off-state current is about two orders of magnitude lower, and the start-up characteristics are also excellent.

Note that similar results were obtained when Cr was used as the metal film 7 instead of Ti0.

〔Effect of the invention〕

As described above, by implementing the present invention, it is possible to reduce the OFF current and increase the degree of mobility, thereby improving the electrical characteristics.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional configuration diagram of a staggered TPT according to the present invention, FIG. 2 is an Io-Vc characteristic diagram of a staggered TPT according to an embodiment, FIG. 3 is a cross-sectional configuration diagram of a conventional staggered TPT, and FIG.
Figures (A) to (E) are cross-sectional views showing the manufacturing process of the staggered TPT according to the present invention. In the figure, 1 is a glass substrate, 2 is an ITO film, and 3 is n″a
-5i film, 4 is a-Si film, 5 is gate insulating film,
6 is an electrode film, and 7 is a metal film. Part 5 Figure

Claims (2)

[Claims] (1) A thin film transistor characterized in that a metal thin film is interposed between a transparent conductive film and an n^+ amorphous silicon film forming a source electrode and a drain electrode of the thin film transistor. (2) The thin film transistor according to claim 1, wherein the metal thin film is titanium or chromium.