The CVD (chemical vapor deposition) method is one of methods of forming an insulating film in the process of LSI manufacture. According to the CVD method, various insulating films can be formed on a semiconductor substrate by changing the kind of material gas.
Table 1 shows the relationship between material gases and insulating films (hereinafter referred to as CVD films) formed by means of the CVD method using the material gases.
TABLE 1 ______________________________________ RELATION OF CVD FILM AND MATERIAL GASES CVD FILM MATERIAL GASES ______________________________________ SiO.sub.2 ##STR1## PSG MATERIAL OF SiO.sub.2 + PH.sub.3 BSG MATERIAL OF SiO.sub.2 + B.sub.2 H.sub.6 ASG MATERIAL OF SiO.sub.2 + AsH.sub.3 BPSG MATERIAL OF BSG AND PSG PO(OCH.sub.3).sub.3 GSG MATERIAL OF SiO.sub.2 + GeH.sub.4 Si.sub.3 N.sub.4 ##STR2## SiO.sub.x N.sub.y MATERIAL OF SiO.sub.2 + MATERIAL OF Si.sub.3 N.sub.4 Al.sub.2 O.sub.3 ##STR3## TiO.sub.2 ##STR4## Ta.sub.2 O.sub.5 ##STR5## ______________________________________
For LSI, SiO.sub.2, PSG, ASG, BPSG, GSG, Si.sub.3 N.sub.4 and SiOxNy are often used as interlayer insulating films and passivation films.
When a gas having Si--H combination (for example, SiH.sub.4, SiH.sub.2 Cl.sub.2, or the like) is used as a material gas in forming one of those films, the Si--H combination is formed partly in the resulting CVD film (SiO.sub.2, SiOxNy, or the like).
H (hydrogen) contained in the CVD film is trapped in the gate oxide of transistors, the interface between the gate oxide film and the semiconductor substrate, and so on, causing variations in the threshold of the transistors and reducing the life of hot carriers.
It is therefore required that the CVD film contain as little hydrogen as possible.
As the CVD methods, there are known atmospheric pressure CVD, low pressure CVD, plasma CVD, and so on.
In the atmospheric pressure CVD and the low pressure CVD, since thermal energy causes a chemical reaction to take place, CVD film deposition is generally performed at a high temperature of 700.degree. C. or more. In contrast, in the plasma CVD, CVD film deposition is performed by producing radicals of particles (atoms, molecules) in a plasma and causing a chemical reaction to take place between active particles, thus allowing the CVD deposition to take place at a low temperature of 250 to 400.degree. C.
That is to say, insulating films formed by the plasma CVD (hereinafter referred to as P-CVD films) are often used as insulating films (interlayer insulating films and passivation films) on a metal interconnection layer made of aluminum or the like.
However, the CVD films (the P-CVD film in particular) have the following drawbacks:
a. Electron trap PA1 b. Moisture resistance
H in CVD film is considered to reach the gate oxide film and tunnel oxide film (SiO.sub.2) of a transistor and cut the combination between Si and O in the gate oxide film and tunnel oxide film during the formation of that film or in the process subsequent to the film formation.
And it is supposed that electrons are trapped at the combination cut sites and the trapped electrons have an adverse effect, such as variations in threshold, on transistors forming memory cells of a semiconductor memory (DRAM, SRAM, nonvolatile memory, or the like) and transistors making up logic.
Insulating films (passivation films in particular) for use with LSI are required to have a property of blocking moisture. This is because, when the moisture reaches metal interconnection lines made of metal, such as aluminum, it will corrode them.
When the plasma CVD is performed using, for example, SiH.sub.4, N.sub.2 O, N.sub.2 as material gases, an SiO.sub.2 film is formed. However, it is known that the SiO.sub.2 film is inferior in moisture resistance to an SiN film.
On the other hand, when the plasma CVD is performed using SiH.sub.4, NH.sub.3, N.sub.2 as material gases, an SiN film is formed. The SiN film is very good in moisture resistance. However, since the SiN film contains a large amount of H (Si--H combination), it is known that a large number of electron traps are produced in the gate insulating film and the like.
The present invention is made to solve the drawbacks and its object is, first, to find such an amount of Si--H combination in a CVD film fabricated by material gases containing a gas having Si--H combination as permits variations in the threshold of transistors due to electron traps in the gate oxide film (or the tunnel oxide film) to remain unchanged from those in the case where there is no CVD film fabricated by material gases including a gas having Si--H combination and apply the result to semiconductor devices, and, second, to eliminate threshold variations due to electron traps in a CVD film fabricated by material gases including a gas having Si--H combination and improve the moisture resistance.