1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter referred to as TFTs) and a method of producing the same. More specifically, the invention relates to an electro-optical device as represented by an active matrix-type liquid crystal display device having, for example, a pixel portion and a driver circuit formed on the same substrate, and to an electronic device mounting the above electro-optical device as a part.
In this specification, the semiconductor device stands for devices that work utilizing the semiconductor characteristics, in general. Electro-optical devices as represented by the active matrix-type liquid crystal display devices formed by using thin-film transistors, electronic devices mounting such electro-optical device as a part, and semiconductor circuits, are all semiconductor devices.
2. Prior Arts
There has been developed a thin-film transistor (TFT) by using a semiconductor thin film formed on an insulating substrate such as a glass. As a material of the semiconductor thin film, there can be used an amorphous silicon film or a crystalline silicon film formed by crystallizing the amorphous silicon film by a laser annealing method or a heat annealing method. Among them, the TFT using the polycrystalline silicon film as an active layer features a high current drivability owing to its high degree of electric field mobility, can be finely machined, and makes it possible to increase the numerical aperture in the pixel portion.
Such a TFT has been widely applied to electronic devices such as ICs and electro-optical devices, and is used, for example, as a pixel switching element or driver circuit in the active matrix-type display device. As the TFT substrate, there is typically used a glass substrate such as of barium borosilicate glass or alumino-borosilicate glass. The glass substrate has a heat resistance inferior to that of a quartz substrate, but offers such an advantage that the substrate of a large area can be easily and cheaply produced to cheaply realize a display device of a large screen.
The glass substrate such as barium borosilicate glass or alumino-borosilicate glass contains trace amounts of impurities such as alkali metal elements like sodium (Na), and the electric characteristics of the TFT vary as the impurity ions infiltrate into the active layer from the glass substrate. To prevent this variation in the electric characteristics, a base film (blocking layer) comprising a silicon oxide film, a silicon nitride film or a silicon oxynitride film has been formed on the surface of the glass substrate on where TFTs are formed.
It has been known that the TFT characteristics can be effectively stabilized if there is used, as a base film, a laminated film comprising a silicon nitride film on the side of the substrate and a silicon oxide film on the side of the active layer.
In the case of a top-gate TFT, the TFT characteristics are greatly affected by the quality of the base film since the channel-forming region is in contact with the base film.
When a voltage is applied to the gate electrode of TFT, an electric field is formed in the active layer and impurity ions in the glass substrate are pulled toward the side of the active layer. Depending upon the quality of the base film, therefore, the impurity ions infiltrate into the active layer or into the gate-insulating layer through the base film, and the electric characteristics of the TFTs undergo a change accompanying thereto to deteriorate the reliability.
A silicon nitride film as the base film exhibits a merit in that it exhibits a large effect for blocking impurity ions but has a defect in that it exhibits many large trap levels to affect the TFT characteristics. Besides, the silicon nitride film produces a large internal stress and causes a distortion on the interface relative to the active layer, resulting in the deterioration in the TFT characteristics such as negative shift of Vth (threshold voltage) and an increase in the S-value (subthreshold constant). Further, the silicon oxide film as the base film exhibits merits such as a wider band gap than that of the silicon nitride film, a high insulating property and a low trap level accompanied, however, by such defects as easy absorption of water and small effect for blocking impurity ions. The silicon oxynitride film exhibits properties that differ depending upon the nitrogen concentration and the oxygen concentration in the film. To enhance the effect for blocking impurity ions, therefore, the film must be formed dense by increasing the nitrogen concentration in the film. However, an increase in the nitrogen concentration in the silicon oxynitride film develops the same defect as that of the silicon nitride film.
Further, when a film containing large amounts of fixed electric charge is brought, as a base film, into contact with the active layer, a trap level is formed on the interface between the base film and the active layer causing the TFT characteristics to be deteriorated. In general, the fixed electric charge increases with an increase in the nitrogen concentration in the film.
When the silicon nitride film and the silicon oxide film are laminated one upon the other, special film-forming chambers are required since they are constituted by different elements. In this case, the temperature of the film-forming chambers drops since the substrate is cooled as it is being conveyed. Accordingly, preheating is required so as not to drop the temperature of the substrate. The preheating time accounts for an increase in the treating time.
The present invention is concerned with a technique for solving the above-mentioned problems, and provides a base film which helps stabilize TFT characteristics, improves reliability, and can be excellently produced, and a method of forming the same.
In order to solve the above-mentioned problems according to the present invention, a silicon oxynitride film is formed by a plasma CVD method by using SiH4, N2O and H2, and this film is used as a base film for a TFT. The properties of the silicon oxynitride film that is formed is controlled by, chiefly, varying the flow rates of N2O and H2. The hydrogen concentration and the nitrogen concentration are increased in the film upon increasing the flow rate of H2. Upon increasing the flow rate of N2O, further, the hydrogen concentration and the nitrogen concentration decrease in the film, and the oxygen concentration increases. On the other hand, the silicon concentration does not almost change even if only a ratio of N2 and N2O gas flows is changed. This makes it possible to form a silicon oxynitride film on the side of the substrate, the silicon oxynitride film having a composition exhibiting properties which are merits of the silicon nitride film, and to form a silicon oxynitride film on the side of the active layer, the silicon oxynitride film having a composition exhibiting properties which are merits of the silicon oxide film, while continuously changing the compositions thereof, thereby to form a base film of good quality picking up merits of both the silicon nitride film and the silicon oxide film. The silicon oxynitride films exhibiting the above-mentioned properties are formed by the same film-forming method by simply changing the gas flow rate ratios, and can be formed in the same film-forming chamber contributing to enhancing the productivity.
Concretely speaking, there are formed a silicon oxynitride film formed over SiH4, N2O and H2 flow rate ratios of Xh=0.5 to 5 (Xh=H2/(SiH4+N2O)), Xg=0.94 to 0.97 (Xg=N2O/(SiH4+N2O)), and a silicon oxynitride film formed over flow rate ratios of Xh=0 (Xh=H2/(SiH4+N2O)), Xg=0.97 to 0.99 (Xg=N2O/(SiH4+N2O)). these silicon oxynitride films being separately used.
In forming the silicon oxynitride film by the plasma CVD method, H2 is added to a mixture gas of SiH4 and N2O to prevent radicals formed by the decomposition of SiH4 from being polymerized in the gaseous phase (in the reaction space) and to eliminate the formation of particles. On the surface on where the film grows, further, an excess of hydrogen is not taken in by the film, that is caused by the reaction of pulling hydrogen adsorbed on the surfaces by hydrogen radicals. This action is intimately related to the substrate temperature at the time when the film is deposited; i.e., the action is obtained by setting the substrate temperature to be from 300xc2x0 C. to 400xc2x0 C. and, preferably, 400xc2x0 C. As a result, it is allowed to form a dense film having less defect density, and trace amounts of hydrogen in the film effectively works to relax the lattice strain. To highly densely generate hydrogen radicals by decomposing hydrogen, a glow discharge is generated at a high power-source frequency of 13.56 MHz to 120 MHz and, preferably, 27 MHz to 60 MHz, with a discharge power density of 0.1 W/cm2 to 1 W/cm2.
Upon employing the above preparation conditions, the silicon oxynitride film to which the invention is applied possesses a nitrogen concentration of from 0.5 atomic % to 10 atomic %, a hydrogen concentration of from 0.5 atomic % to 5 atomic % and an oxygen concentration of from 50 atomic % to 70 atomic %.
The feature of the invention resides in that in forming a base film of TFT by the silicon oxynitride films, the composition is differed in the base film depending on the side of the substrate and on the side of the active layer, and the nitrogen concentration and the hydrogen concentration in the film are set to be relatively higher and the oxygen concentration is set to be relatively lower for the former side.
For example, the first layer of the base film in contact with the substrate is formed of the silicon oxynitride film having a nitrogen concentration of 7 to 10 atomic %, a hydrogen concentration of 2 to 3 atomic % and an oxygen concentration of 52 to 55 atomic %, and the second layer in contact with the active layer is formed of the silicon oxynitride film having a nitrogen concentration of 1 to 2 atomic %, a hydrogen concentration of 0.5 to 2 atomic % and an oxygen concentration of 62 to 65 atomic % to form a step-like gradient of concentration. Or, there may not exist a clear distinction between the first layer and the second layer like the one described above but, instead, the composition may be continuously changed.
The silicon oxynitride film is formed by using a plasma device which is constituted by parallel flat plates of the capacitor-coupled type. It is also allowable to use the one of the induction coupled type or a plasma CVD device utilizing the energy of magnetic field such as of electron cyclotron resonance. The composition of the silicon oxynitride film is varied by using SiH4 and N2O gases and by adding H2 thereto. The plasma is formed under a pressure of 10 Pa to 133 Pa (desirably, 20 Pa to 40 Pa), with a high-frequency power density of 0.2 W/cm2 to 1 W/cm2 (preferably, 0.3 W/cm2 to 0.5 W/cm2), at a substrate temperature of 200xc2x0 C. to 450xc2x0 C. (preferably, 300xc2x0 C. to 400xc2x0 C.), and an oscillation frequency of high-frequency power source of 10 MHz to 120 MHz (preferably, 27 MHz to 60 MHz).
Table 1 shows three kinds of preparation conditions. The condition #210 is for forming the silicon oxynitride film from SiH4 and N2O. The conditions #211 and #212 are when H2 is added to SiH4 and N2O, and in which the flow rate of H2 being added is varied. In this specification, the silicon oxynitride film formed from SiH4 and N2O is expressed as silicon oxynitride film (A), and the silicon oxynitride film formed from SiH4, N2O and H2 is expressed as silicon oxynitride film (B). The silicon oxynitride film (A) is formed with SiH4, N2O and H2 flow rate ratios of Xh=0 (Xh=H2/(SiH4+N2O)) and Xg=0.97 to 0.99 (Xg=N2O/(SiH4+N2O)), and the silicon oxynitride film (B) is formed with SiH4, N2O and H2 flow rate ratios of Xh=0.5 to 5 (Xh=H2/(SiH4+N2O)) and Xg=0.94 to 0.97 (Xg=N2O/(SiH4+N2O)).
Table 1 further shows pretreatment conditions executed prior to forming the silicon oxynitride film. The pretreatment is not absolutely necessary but is effective in enhancing the reproducibility of silicon oxynitride film properties or in enhancing the reproducibility of properties when the invention is applied to the TFTs.
Referring to Table 1, the pretreatment conditions are such that the treatment is effected for 2 minutes by producing a plasma while introducing 200 SCCM of hydrogen, under a pressure of 20 Pa and with a high-frequency electric power of 0.2 W/cm2. The pretreatment may further be conducted by producing a plasma in the same manner as above but introducing 100 SCCM of hydrogen and 100 SCCM of oxygen. Though not shown in Table, the pretreatment may be conducted for several minutes by introducing N2O and hydrogen under a pressure of 10 Pa to 70 Pa and with a high-frequency power density of 0.1 W/cm2 to 0.5 W/cm2. In the pretreatment, it is desired that the substrate is maintained at a temperature of 300xc2x0 C. to 450xc2x0 C. and, preferably, at 400xc2x0 C. The pretreatment works to clean the to-be-deposited surface of the substrate and to stabilize the interfacial properties of the silicon oxynitride film that is deposited later by temporarily inactivating the to-be-deposited surface by the adsorption of hydrogen. The pretreatment further favorably works to lower the interfacial level density by oxidizing the to-be-deposited surface and the vicinities thereof by simultaneously introducing oxygen and N2O.
Concretely speaking, the silicon oxynitride film (B) is formed under the condition #211 by introducing 5 SCCM of SiH4, 120 SCCM of N2O and 500 SCCM of hydrogen with a high-frequency power density of 0.4 W/cm2 at a substrate temperature of 400xc2x0 C. The high frequency of electric power source is 10 MHz to 120 MHz and, preferably, 27 MHz to 60 MHz, but is here set to be 60 MHz. Under the condition #212, the film is formed by introducing hydrogen at a flow rate of 125 SCCM in contrast with the condition #211. The flow rates of the gases are not to specify their absolute values, but a meaning resides in the flow rate ratios. When Xh=[H2]/([SiH4]+[N2O]), then, Xh may be from 0.1 to 7. When Xg=[N2O]/([SiH4]+[N2O]), then, Xg may be from 0.90 to 0.99. The silicon oxynitride (A) is formed under the condition #210.
Table 2 shows representative properties of the silicon oxynitride films obtained under the above conditions, as well as compositions and densities of hydrogen (H), nitrogen (N), oxygen (O) and silicon (Si) found from Rutherford Backscattering Spectrometry (hereinafter abbreviated as RBS, systems used: 3S-R10 accelerator, NEC 3SDH pelletron end station; CE and ARBS-400).
From the results of Table 2, the concentration of hydrogen in the film increases as H2 is added during the formation of film. This also causes a change in the contents of oxygen and nitrogen. In the silicon oxynitride film (A), the ratio of O to Si is 1.9 (allowable range is 1.7 to 2), and the ratio of N to Si is 0.04 (allowable range is 0.02 to 0.06). On the other hand, the composition of the silicon oxynitride film (B) varies depending upon the flow rate of H2 added during the formation of film, and in which the ratio of O to Si is 1.6 (allowable range is 1.4 to 1.8) and the ratio of N to Si is 0.14 to 0.18 (allowable range is 0.05 to 0.5), the ratio of O to Si being decreased and the ratio of N to Si being increased.
The density of the film increases with an increase in the content of nitrogen, i.e., increases from 6.5 atoms/cm3 of the silicon oxynitride film (A) to 7.1 atoms/cm3 of the silicon oxynitride film (B). It is considered that the film becomes dense with an increase in the content of nitrogen since the etching rate is 120 nm/min for the silicon oxynitride film (A) and 63 to 105 nm/min for the silicon oxynitride film (B) with a mixture solution (trade name: LAL500, manufactured by Sterachemifa Co.) containing 7.13% of ammonium hydrogenfluoride (NH4HF2) and 15.4% of ammonium fluoride (NH4F) at 20xc2x0 C. It can therefore be said that the film is becoming dense with an increase in the nitrogen content. This fact indicates that a dense and hard film efficiently blocks impurities from the substrate, and it is meaningful to provide the silicon oxynitride film (B) on the side of the substrate.
When compared in terms of the internal stress, the compressive stress of xe2x88x924.26xc3x97108 Pa of the silicon oxynitride film (A) greatly changes to xe2x88x927.29xc3x97106 Pa through the heat treatment (500xc2x0 C. for one hour+550xc2x0 C. for 4 hours, equivalent to the treating conditions in the step of crystallization). The silicon oxynitride film (B), on the other hand, has a tensile stress of 2.31xc3x97108 Pa, which does not almost change even through the heat treatment. The phenomenon of change in the internal stress due to the heat treatment can be considered to be related to a change in the structure or a change in the composition of the film, and it can be said that the silicon oxynitride film (B) has a better thermal stability.
In the foregoing were described typical properties of the silicon oxynitride film. The silicon oxynitride film which is an insulating film used in the present invention is in no way limited to those shown in Tables 1 and 2. The silicon oxynitride film (A) has a composition, i.e., the insulating film suited as a base film of the semiconductor device as represented by TFT, has a composition that contains 1 to 2 atomic % of nitrogen, 0.5 to 2 atomic % of hydrogen and 62 to 65 atomic % of oxygen. The silicon oxynitride film (B) has a composition that contains 7 to 10 atomic % of nitrogen, 2 to 3 atomic % of hydrogen and 52 to 55 atomic % of oxygen. Further, the silicon oxynitride film (A) has a density of not smaller than 6xc3x971022 but smaller than 7xc3x971022 atoms/cm3 and the silicon oxynitride film (B) has a density of not smaller than 7xc3x971022 but smaller than 8xc3x971022 atoms/cm3. The etching rate with a mixture solution containing the above ammonium hydrogenfluoride (NH4HF2) and ammonium fluoride (NH4F) is 110 to 130 nm/min for the silicon oxynitride film (A) and 60 to 110 nm/min for the silicon oxynitride film (B).