Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-03-06
2002-12-10
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S349000, C257S639000
Reexamination Certificate
active
06492681
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to as TFT) formed on a substrate and to an electronic device using the TFT. In particular, the present invention relates to an insulating film that is provided between a substrate and a semiconductor layer serving as an active layer of a TFT. Such an insulating layer, which is referred to as a blocking layer or a base film, is employed for the purpose of preventing the active layer from being contaminated by an impurity such as an alkali metallic element that is in the substrate. Together with preventing the reduction in the reliability and the deterioration of the TFT that are caused by the contamination of the active layer, the present invention relates to a structure of the insulating film that is suitable for obtaining a TFT having good characteristics and small fluctuations within the substrate.
Typically, a liquid crystal display device may be cited as the electronic device of the present invention. It is to be noted that a semiconductor device as used herein throughout the specification refers to any device which functions by utilizing semiconductor characteristics, and the above-mentioned TFT, the electronic device and electronic equipment having the electronic device mounted therein as a display portion are included in semiconductor devices.
2. Description of the Related Art
In recent years, an active matrix type display device that utilizes a TFT, which has an active layer formed of a crystalline semiconductor layer, as switching elements of pixels and driver circuits is attracting much attention as a means of realizing an incredibly fine and high quality image display. A crystalline silicon layer formed of an amorphous silicon layer that is crystallized by a known method such as laser annealing or thermal annealing may be suitably used as a material of the crystalline semiconductor layer. In the TFTs using the crystalline silicon layer, an electric field effect mobility is high and because a high current driving ability can be attained even if fine processing is performed, it is possible to increase an aperture ratio of a pixel portion.
A quartz glass substrate that does not contain alkali oxide therein and a low alkali glass substrate that contains alkali oxide therein can be used as the substrate of such an active matrix type display device. However, it is preferable that an inexpensive low alkali glass substrate is used rather than the quartz glass substrate in order to realize a low price active matrix type display device. However, in the case of using the low alkali glass substrate as the substrate, the highest temperature in the manufacturing process thereof must be at between 600° C. and 700° C. in terms of the heat resistance of the glass substrate.
Further, it was necessary to at least form a blocking layer that is made of a silicon oxide film or a silicon nitride film on the side of the surface of the glass substrate on which the TFT will be formed such that a small amount of alkali metal such as sodium (Na) that is contained in the substrate will not mix into the active layer of the TFT. Known structures of TFTs formed on the glass substrate are the top gate type and the bottom gate type (or inverted stagger type). The top gate type has a structure in which at least a gate insulating film and a gate electrode are provided on a surface that is the opposite side of the substrate side of the active layer. In this top gate type TFT, among the alkali metallic elements in the glass substrate, the ones that have been ionized are drawn to the side of the active layer depending on the polarity of the gate electrode when a voltage is applied thereto. Therefore, the blocking layer as mentioned above is formed on a surface that is opposite from the surface where the active layer is in contact with the gate insulating film (hereinafter referred to as back channel side throughout the present specification for the sake of convenience). If the quality of this blocking layer is poor, the alkali metallic elements in the glass substrate will easily mix into the active layer, whereby the electrical characteristic of the TFTs will change. Hence, reliability cannot be secured. In addition, if the blocking layer is provided and an amorphous semiconductor layer is formed thereon to thereby form a crystalline semiconductor layer by laser annealing or thermal annealing, then an internal stress of the blocking layer will change, causing the crystalline semiconductor layer to distort. Even if the TFT is completed under such conditions, the electrical characteristics of the TFT such as a threshold voltage (hereinafter abbreviated as Vth) and a subthreshold constant (hereinafter abbreviated as S-value and referred to as such) will vary from a target value.
Therefore, it is disclosed in Japanese Patent Application No. Hei 11-125392 that a blocking layer made of a lamination of a silicon oxynitride film (A) and a silicon oxynitride film (B) is provided on the back channel side of the TFT to thereby prevent contaminations caused by impurities such as the alkali metallic elements from the substrate. In addition, an appropriate range of the composition and the film thickness of the first layer silicon oxynitride film (A) and the second layer silicon oxynitride film (B) such that the internal stress becomes small before and after the crystallization process of the amorphous semiconductor layer, that is, influences to the crystalline semiconductor layer will be small, is disclosed in the above mentioned application.
The oxygen concentration contained in the silicon oxynitride film (A) is set to between 20 atomic % and 30 atomic %, and the nitrogen concentration is set to between 20 atomic and 30 atomic %, or the composition ratio of nitrogen to oxygen is set to between 0.6 and 1.5. Furthermore, the oxygen concentration contained in the silicon oxynitride film (B) is set to between 55 atomic % and 65 atomic %, and the nitrogen concentration is set to between 1 atomic % and 20 atomic %, or the composition ratio of nitrogen to oxygen is set to between 0.01 and 0.4. The hydrogen concentration of the silicon oxynitride film (A) is set to between 10 atomic % and 20 atomic %, or the composition ratio of hydrogen to oxygen is set to between 0.3 and 1.5, and the hydrogen concentration of the silicon oxynitride film (B) is set to between 0.1 atomic % and 10 atomic %, or the composition ratio of hydrogen to oxygen is set to between 0.001 and 0.15.
Further, the density of the silicon oxynitride film (A) is set to between 8×10
22
atoms/cm
3
and 2×10
23
atoms/cm
3
, and the density of the silicon oxynitride film (B) is set to between 6×10
22
atoms/cm
3
and 9×10
22
atoms/cm
3
. The etching rate of a mixed aqueous solution containing 7.13% of ammonium hydrogen fluoride (NH
4
HF
2
) and 15.4% of ammonium fluoride (NH
4
F) of such a silicon oxynitride film (A) at 20° C. is between 60 nm/min and 70 nm/min (after heat treatment at 500° C. for 1 hour, and heat treatment at 550° C. for 4 hours, the etching rate is between 40 nm/min and 50 nm/min). The etching rate of the silicon oxynitride film (B) is between 110 nm/min and 130 nm/min (after heat treatment at 500° C. for 1 hour, and heat treatment at 550° C. for 4 hours, the etching rate is between 90 nm/min and 100 nm/min). The etching rate defined here is a value obtained from performing etching at 20° C. with an aqueous solution containing 7.13% of NH
4
HF
2
and 15.4% of NH
4
F as the etching solution.
By providing the silicon oxynitride film (A) in contact with the substrate at a thickness of between 10 nm and 150 nm, preferably between 20 nm and 60 nm and providing the silicon oxynitride film (B) thereon at a thickness of between 10 nm and 250 nm, preferably between 20 nm and 100 nm, the contamination of the active layer by impurities such as alkali metallic elements in the substrate can be prevented.
Furthermore, because the blocking layer is formed by laminating the silicon oxynitride film (A) a
Itoh Masataka
Kitakado Hidehito
Koyama Jun
Ogawa Hiroyuki
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Prenty Mark V.
Semiconductor Energy Laboratory Co,. Ltd.
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