In recent years, in order to realize further higher packing densities, higher performances, and lower power consumptions of semiconductor devices, various researches have been carried out on improvements of characteristics of insulating films used for semiconductor devices. Examples of insulating films used for semiconductor devices include gate insulating films of transistors, element isolation films, capacitor insulating films, interlayer insulating films, and passivation films, and researches have been carried out on insulating film materials in accordance with respective places for uses.
With respect to the insulating film, attempts have been made to reduce the film thickness tox in terms of silicon oxide film, while the leakage current has been maintained. The film thickness tox in terms of silicon oxide film is defined as tox=εSiOt/ε with respect to a thin film having a relative dielectric constant of ε and an actual film thickness of t.
For example, a gate insulating film of a 0.05 μm-gate-length-generation semiconductor transistor is required to have such an insulating property that the gate leakage current density is 1 A/cm2 or less at a gate voltage of 1.0 V when the film thickness is 1 nm or less in terms of silicon oxide.
Silicon oxide films have been previously used as gate insulating films of transistors. However, when a voltage of 1 V is applied to a silicon oxide film of 1 nm in thickness, even a direct tunneling current alone exceeds 10 A/cm2 and, therefore, the silicon oxide film cannot be used.
Consequently, an attempt has been made to apply a metal oxide having a high dielectric constant to the above-described insulating film. If no deterioration occurs in the mobility of electron in a channel due to the application of the metal oxide to the gate insulating film, a reduction in voltage and low power consumption can be realized with no decrease in speed of the transistor.
Besides the above-described insulating property, the following characteristics are also required of the metal insulating material applied to the gate insulating film.
First, the interface to a silicon substrate (or a silicon substrate covered with any one of an extremely thin silicon oxide film, a silicon nitride film, and a silicon oxynitride film) is thermodynamically stable in order to prevent deterioration of the gate capacity due to a heat treatment in a transistor production process.
Second, no fixed charge is included in the film in order to suppress a threshold shift and a decrease in channel mobility of the transistor.
Third, no impurity diffusion in the film occurs in order to suppress a threshold shift and variations of the transistor.
From the viewpoint of the insulating property and the stability of an interface to a silicon substrate, researches have been carried out on applications of ZrO2, HfO2, silicates of them, lanthanoid oxides, and silicates thereof to insulating films until now. ZrO2, HfO2, and lanthanoid oxides have high dielectric constants of at least 20 and excellent insulating properties, but have the following problems in the use as gate insulating films.
The crystallization temperatures are low, and are 400 to 600 degrees. Consequently, when a transistor is produced, noticeable crystallization of an insulating film occurs due to a heat treatment in the forming process therefor. The flatness of the interface to silicon is thereby deteriorated, and the mobility in a channel is reduced. Furthermore, grain boundaries are randomly generated in the insulating film, and may cause variations in characteristics. If a material of an upper electrode reaches the silicon substrate through grain boundaries of crystals, the mobility in a channel is reduced and variations occur in the threshold shift during production of the transistor and, thereby, the transistor has high possibility of deterioration in the performance.
When crystallization occurs randomly in a surface, etching cannot be uniformly carried out in etching of the gate insulating film. As a result, some portions may not be etched and may remain in a source-drain region. In addition, since zirconium, hafnium, and lanthanoid are metal materials and are not present previously in processes of silicon semiconductor devices, many studies on contamination are necessary for introduction of a silicon semiconductor production line. Consequently, ZrO2, HfO2, and lanthanoid oxides are not used readily as gate insulating films of silicon semiconductor transistors as of now.
Silicates of ZrO2, HfO2, and lanthanoid oxides are most promising gate insulating films because the band gap is large although the dielectric constant is in the order of 10, and a phase separation-crystallization temperature is high and is at least 800 degrees. However, silicates cannot be formed into films by the use of vapor phase atomic layer growth, while the vapor phase atomic layer growth is a most promising film-forming process for gate insulating films from the viewpoint of the uniformity in film thickness and the like.
Many studies on contamination are also necessary for introduction of a production line because ZrO2, HfO2, and lanthanoid oxides are contained. Consequently, silicates of ZrO2, HfO2, and lanthanoid oxides are not readily used as gate insulating films of silicon semiconductor transistors as of now.
Researches have also been carried out on application of aluminum oxide (Al2O3) to the gate insulating film since the crystallization temperature is high and Al has already been present in the silicon semiconductor process. Al2O3 has a relatively high relative dielectric constant (about 8 to 10 with respect to amorphous, and about 12 with respect to single crystal) and a high insulating property, and the interface to silicon is thermodynamically stable. Al2O3 has a crystallization temperature of at least 800 degrees. An element Al has already been commonly used for the silicon semiconductor process. Furthermore, Al2O3 can be formed into a film by the use of vapor phase growth atomic layer growth, while the atomic layer growth is a most promising film-forming process for a gate insulating film. For the above-described reasons, Al2O3 has been actively researched as the above-described gate insulating film in recent years.
A prototype of a fine transistor of 0.08 μm in gate length is reported in International Electron Device Meeting Technical Digest 2000 P.223, wherein an aluminum oxide film is used as a gate insulating film, and polycrystalline silicon is used as a gate electrode. However, this includes the following problems.
First, a negative charge is present in the aluminum oxide (Al2O3) film. A negatively electrified fixed charge is believed to be generated when Al vacancy or interstitial oxygen is present in aluminum oxide. Although it is not clear which is the origin of this negative charge as of now, the mobility of electron in the channel is reduced due to this negative fixed charge when the aluminum oxide (Al2O3) film is applied to the gate insulating film. In addition, the threshold of the transistor is shifted as well. In this report, the mobility of electron actually becomes one-third of that in the case where a silicon oxide film is used, while this reduction is due to the negative fixed charge in the film. Therefore, no advantage is found with respect to the use of the aluminum oxide film as the gate insulating film.
Second, the aluminum oxide thin film has no resistance to diffusion of boron. Consequently, the threshold of the transistor is shifted when boron-doped polycrystalline silicon is used for the gate electrode as in a previous manner.
According to the description in Appl. Phys. Lett., Vol. 77 (2000), P.2207, when an annealing temperature was controlled at 800 degrees to 850 degrees in a boron-doped polycrystalline silicon electrode/Al2O3 (8 nm)/n-Si system, 8.8×1012 B ions/cm2 of boron diffused from the polycrystalline silicon electrode to a silicon substrate through Al2O3, and the flat band voltage was significantly shifted and was 1.54 V.
Since a heat treatment is carried out at about 1,000° C. in the formation of a transistor, the threshold of the transistor shifts and varies significantly due to diffusion of boron. Consequently, in this report, an extremely thin silicon oxynitride film of 0.5 nm or less in thickness was provided between the Al2O3 film and Si so as to block the diffusion of boron and, thereby, the flat band voltage difference was able to be controlled at about 90 mV even after the annealing was carried out at 800° C. to 850° C. However, as described above, the mobility of electron in the channel is reduced due to the negative fixed charge when Al2O3 is applied to the gate insulating film. Furthermore, when a silicon nitride film is used at the interface, the nitrogen concentration becomes large at the interface to silicon and, thereby, the mobility of electron in the channel is reduced due to a positive fixed charge.
In Japanese Unexamined Patent Application Publication No. 7-193147, Al is introduced in a laminated film of SiO2 and Si3N4, SIALON (Si3N4—AlN—Al2O3-based solid solution) is applied to a gate insulating film and, thereby, the attempt is made to improve the insulating property and the dielectric constant. However, the dielectric constant is reduced because large amounts of Si is contained.
As described above, in order that Al2O3 can be used as the gate insulating film, the fixed charge must be reduced and the impurity diffusion must be reduced in the film while the insulating property and the stability of the interface to silicon are maintained. However, simultaneous realization of them is difficult as of now and, in particular, no solution is known to reduce the fixed charge of Al2O3.
The present invention was made in consideration of these problems, and overcomes problems in the case where aluminum oxide is used as a gate insulating material of a semiconductor transistor. It is an object of the present invention to provide a structure and a production method for a device including a metal insulating material thin film as a gate insulating film, wherein the thin film satisfies characteristics required of a gate insulating film of a 0.05 μm-gate-length-generation semiconductor transistor, in particular, no fixed charge is included in the film, and the impurity diffusion is reduced in the film.