1. Field of the Invention
The present invention relates to a method of producing a semiconductor device and to a semiconductor device, more particularly relates to a method of producing a semiconductor device having MIS transistors, capacitors, or other different types of elements with different power source voltages formed on a same substrate and to a semiconductor device having the same.
2. Description of the Related Art
Recently, in general, in LSI processes, circuits are being made higher in integration and elements are being miniaturized more. Along with this, it is required that MIS transistors, capacitors, resistance elements, and other different types of elements having different power source voltages is simultaneously formed on the same substrate as much as possible without increasing the number of steps.
Further, to improve the performance of MIS transistors, more miniaturization is required. Along with the drop in the power source voltage due to this, gate insulating films are also being made thinner. Even if the main MIS transistors are formed by thin films, the MIS transistors supplied with high voltages used for example for I/O parts, analog circuits, etc. require thicker insulating films commensurate with those power source voltages.
The general practice is to form such MIS transistors, for example, MOS transistors, having different voltages with separate gate insulating films. As an example of a method of formation, for example, a heat oxide film is formed on a semiconductor substrate, a photo resist or other mask is formed only on regions for forming a thick gate oxide film, and the exposed parts of the heat oxide film are removed by wet etching using a hydrofluoric acid solution etc. The resist is removed, then the semiconductor substrate is heat oxidized a second time to form a heat oxide film. As a result, gate oxide films having different thicknesses are formed.
With the above method, by repeating the photo resist patterning step, the wet-etching step, and the heat-oxidation step, it is possible to obtain as many different types of thicknesses of heat oxide films as desired.
Further, in the past, the technique of simultaneously forming capacitors when forming MOS transistors has been used. As the method of formation, for example, a semiconductor substrate is doped with a high concentration of ions through a suitable sacrificial oxide film to make the silicon substrate an n+ or p+ type and form capacitor electrodes. The sacrificial film is removed, then heat oxidation is used to form a capacitive oxide film of the capacitors with desired oxide film thickness at the same time as the gate oxide film of the MOS transistors. In this oxidation step, the oxidation proceeds faster than normal due to crystal defects formed in the substrate by the ions previously implanted at a high concentration (accelerated oxidation). As a result, the capacitive oxide film of the capacitors becomes thicker than the gate oxide film. As following steps, polycrystalline silicon is deposited and patterned so as to form other electrodes of the capacitors at the same time as gate electrodes of the MOS transistors.
With the above method, it is possible to efficiently form the capacitors on the same substrate at the same time as the MOS transistors without greatly increasing the number of steps.
On the other hand, it is known that by making the concentration of impurities in a gate electrode high, depletion under the gate electrodes is prevented (Japanese Unexamined Patent Publication (Kokai) No. 2000-277636).
The above methods for selective formation of oxide films, however, first had the problem that the number of steps increased along with the number of types of thicknesses of the oxide films. Further, at the time of wet etching, the hydrofluoric acid solution etc. infiltrated the edges of the masked oxide film and ends up damaging the oxide film supposed to remain. This may influence the elements and lower the reliability. Therefore, it is not preferable to use these methods frequently.
In case of the method of formation of capacitors, the silicon substrate is doped with ions to a high concentration to form the electrodes. To obtain the desired capacity, it may be to promote accelerated oxidation to obtain a thick oxide film able to withstand a high voltage. However, if implanting ions at a particularly high concentration to promote accelerated oxidation, the damage given to the silicon substrate becomes greater, therefore the quality of the oxide film is lowered and the reliability ends up being reduced. Further, the ratio of poor quality film due to the accelerated oxidation increases and becomes a cause of a drop in reliability.
As explained above, raising the performance of the main MOS transistors of an LSI requires that the thickness of the gate oxide film is made physically thin. This oxide film has to maintain the thin thickness of the electrical effective oxide film when operating the transistors as it is. Therefore, it is necessary to satisfy the requirements of all of the thin film transistors, transistors with different power sources, capacitors, and other elements requiring different oxide film thicknesses.