This invention pertains to a process wherein silicon wafers are processed in a processing chamber, which is pressurized sequentially with different atmospheres, particularly but not exclusively in oxidation of silicon wafers at high temperatures and high pressures.
It is known for silicon wafers to be oxidized by a process known as thermal oxidation and practiced at high temperatures, which typically may range from 700.degree. to 1,000.degree. C., and at high pressures, which typically may range from 5 to 25 atmospheres, in a tubular vessel, which has a removable closure. Typically, the tubular vessel is flooded preliminarily with a purging atmosphere as silicon wafers, which are disposed edgewise and uprightly on a boat, are loaded into the tubular vessel, whereupon the tubular vessel is pressurized to a processing pressure with a series of different processing atmospheres, each processing atmosphere displacing a preceding atmosphere except for a residual portion remaining with such processing atmosphere and diminishing in concentration with time.
In a typical case, the purging atmosphere is or comprises nitrogen, which may be regarded as inert under these conditions although there are some indications of its reactivity under these conditions, one of the processing atmospheres is or comprises dry oxygen, which is a slow oxidizer, and another of the processing atmospheres is or comprises steam, which is a fast oxidizer. In the same case, dry oxygen can be introduced initially at a fast rate, so as to promote displacement of the purging atmosphere of nitrogen, and subsequently at a slow rate once the purging atmosphere essentially has been displaced, so as to conserve dry oxygen. Dry oxygen effects slow formation of a thin film of silicon dioxide on the wafers. Next, steam may be introduced at a slow rate, which is limited by physical constraints of available apparatus, so as to effect fast formation of the thin film of silicon dioxide. Also, steam may be followed by dry oxygen, which may be followed by nitrogen, whereupon the tubular vessel may be depressurized.
One type of known apparatus, in which the process described above for thermal oxidation of silicon wafers can be practiced, is disclosed in U.S. Pat. No. 4,253,417, wherein the tubular vessel is enclosed within an outer vessel, which is pressurized outside the tubular vessel, so as to equalize the pressures outside and inside the tubular vessel except for a small differential so that leakage of the tubular vessel is leakage out, whereby the tubular vessel may be made of quartz, silicon, or other fragile material. Other types of known apparatus, in which similar processes can be practiced, are disclosed in U.S. Pat. No. 4,018,184 and U.S. Pat. No. 4,167,915.
In the process described above for thermal oxidation of silicon wafers, as practiced before this invention, nonuniform oxidation of the wafers has been a problem of serious magnitude. It has been found that the thin films of silicon dioxide formed on the wafers when the wafers are disposed edgewise and uprightly in the oxidizing atmosphere have observable gradients of thickness from thicker portions at upper edges of the wafers to thinner portions at lower edges of the wafers. Also, the problem is exacerbated at higher pressures (20 to 25 atmospheres) among the high pressures (5 to 25 atmospheres) discussed above, and at lower temperatures (700.degree. to 800.degree. C.) among the high temperatures (700.degree. to 1000.degree. C.) discussed above.
Because the process is thermally activated, whereupon the process proceeds more rapidly at higher temperatures and less rapidly at lower temperatures, prior efforts to deal with the problem have been directed to improved insulating, baffling, and heating measures, by which it has been found that nonuniformity of the temperature of the oxidizing atmosphere enveloping the wafers can be reduced, controlled, or eliminated, so as to alleviate the problem. The temperature of the oxidizing atmosphere at the upper and lower reaches of the tubular vessel can be measured nonintrusively by thermocouples arrayed on the tubular vessel.
However, it has been found that the problem can be alleviated but cannot be eliminated solely by such measures, as it has been found that the thin films of silicon dioxide formed on the wafers are thicker at upper edges of the wafers and thinner at lower edges of the wafers, even if the temperature of the oxidizing atmosphere enveloping the wafers is uniform at the upper and lower edges of the wafers, and even if the temperature of the oxidizing atmosphere is hotter at lower edges of the wafers and cooler at upper edges of the wafers.
Other efforts to deal with the problem have been directed to means to promote turbulent flow of the oxidizing atmosphere. However, turbulent flow of the oxidizing atmosphere may be difficult to accomplish effectively, particularly if the oxidizing atmosphere is or comprises steam at high pressures.
For a constant volume of an ideal gas, the density of the gas is proportional to the number of molecules of the gas and to the molecular weight of the gas. For constant pressure, volume, and temperature, the number of molecules of the gas is independent of the species of the gas. Thus, the density of the gas is proportional to the molecular weight of the gas at constant pressure, volume, and temperature. Thus, the density of a mixture of two, three, or more gases at constant pressure, volume, and temperature is proportional to the sum of the respective products of the molecular weights and the mole fractions of its components.
Herein, all molecular weights are stated in round numbers, whenever stated. Herein, the molecular weight of a particular atmosphere being one gas refers to its molecular weight, and the molecular weight of a particular atmosphere being a mixture of gases refers to mean molecular weight of the mixture.