Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
1999-03-30
2001-11-13
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S773000, C438S774000, C438S775000, C438S778000, C438S782000, C438S787000
Reexamination Certificate
active
06316371
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method for the chemical treatment of a semiconductor substrate.
BACKGROUND OF THE INVENTION
Such a similar method is known from the American patent specification U.S. Pat. No. 5,633,212 and is used to apply oxide layers to semiconductor substrates such as silicon wafers. One or more wafers are placed in a wafer rack and are then transferred to an oven. During the insertion, the atmosphere is inert and will consist mainly of nitrogen.
In this American patent specification U.S. Pat. No. 5,633,212, the so called ‘wet oxidation’ is described. Water vapor must be made and this is done by fitting a combustion chamber upstream of the reactor. There, oxygen and hydrogen are mixed with each other and combusted at a raised temperature into water vapor.
To control this combustion, the American patent specification in question suggests that after inserting the wafers into the oven, an oxygen atmosphere should first be introduced into the combustion chamber. To that end, the nitrogen supply is completely cut off, from one moment to the next, and the supply of oxygen is gradually started until the intended working maximum is reached. The supply of hydrogen is then gradually started up and, after a certain hydrogen flow is reached, the combustion takes place. During the increase of the oxygen concentration before the introduction of hydrogen, no oxidizing occurs due to the comparatively high concentration of nitrogen in the oven. Only after the abrupt cutting off of the nitrogen supply does the oxidation reaction actually start in the oven. The combustion is then already taking place in the desired manner in the pre-combustion chamber.
In the state of the art there is on the one hand the tendency to decrease the thickness of the oxide layer and on the other hand there is the tendency to increase the dimensions of every wafer. It has been shown that, with these two steps, the application of oil layers becomes critical. By increasing the dimensions of the wafers, increased non-uniform oxidation will occur in the radial direction. In general this is not critical except when a thin layer of oxide has to be applied. In such a case, control of the process and of the oxidized thickness is of great importance. It has also been observed that with a vertical oven where several wafers are inserted that lie vertically behind each other, differences occur in the oxide growth of the uppermost and lowermost wafers. Such differences are also undesirable.
This problem does not only hold for oxidation but for many other processes wherein changes in the process conditions take place, that is, when one process gas is switched over to another process gas.
SUMMARY OF THE INVENTION
The object of the present invention is to improve the treatment of wafers such that oxidation takes place in a more uniform manner. A first proposal to solve this problem is to introduce a vertical temperature profile in a vertical oven into which a number of vertically spaced wafers are introduced. In this way, the reaction speed of the gas can be influenced and a constant reaction in the vertical direction can be realized. It appears, however, that this is not a solution for the phenomena noted above where the thickness of the oxide layer is not constant in the radial direction of the wafer.
It is the object of the present invention to remove this disadvantage and also to provide as treatment conditions as uniform as possible in the radial direction.
This object is realized for a method described above in that second gas for the preparation of the second gaseous atmosphere being introduced into the oven in such a way that with the gradual increase of the flow of that second gas in said oven, the flow of said first gas, that provides that first gaseous atmosphere, decreases supplied to the oven by a substantially equal amount, wherein both the first and second gaseous atmospheres are in that oven for at least 1 minute. In contrast to the American patent specification U.S. Pat. No. 5,633,212, the replacement of the first gaseous atmosphere by the second gaseous atmosphere takes place gradually, so that the supply of the first gas decreases by approximately the same amount as the supply of the second gas increases. In the above-mentioned American patent specification U.S. Pat. No. 5,633,212, the supply of nitrogen is abruptly cut off from one moment to the next without a simultaneous and equally large extra supply of other gasses. According to the invention, in contrast to the prior art, the supply of the first gaseous atmosphere such as nitrogen is not directly switched off by the switching on of the supply of the gas which takes part in the reaction, such as oxygen, but this transition is brought about gradually. Even though the process gas concentration profile as a function of time is still subject to spreading, it appears that if this transition is carried out in this way, the negative effects of the spread on the uniformity of the layer thickness are significantly smaller and the uniformity in layer thickness both in the radial and axial directions are considerably better than when the process gas is switched on according to a step function. It is therefore also possible to grow comparatively thin oxide layers on larger wafers, for example 300 mm wafers.
This also holds for the end of the process. After oxidation, the oxidant present, such as oxygen, is forced out of the tube by switching over to an inert gas, such as nitrogen. The standard method is to switch over in one go, whereas a gradual switching over also results in a considerable improvement in the uniformity of the layer thickness in this case.
In the oxidation reaction described above, the wafers are placed into the oven at approximately 600° C. and, once in the oven, the temperature is raised to between 650 and 1200° C. where the actual oxidation takes place. Then cooling down occurs, possibly preceded by a phase at a raised temperature without the addition of substantial amounts of reactant gas such as oxygen.
The invention can be used both with the direct introduction of the gasses into the oven and with indirect introduction, that is, with further apparatuses being connected in front of the oven, such as a combustion chamber.
According to an advantageous embodiment of the invention, this can be used in particular during oxidation of silicon with oxygen, so-called ‘dry’ oxidation, wherein the reaction rate is proportional to the oxygen concentration to the power N, where N=0.7. The more that N deviates from 1, the more harmful the effect of spreading on the uniformity. Indeed, when N=1, the effect of the lower concentration by spreading is completely compensated for by the longer exposure time to the reaction gas and there is no harmful effect on the uniformity of the layer thickness. With wet oxidation, N=1 and the application of the present invention does not yield any advantage.
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Copy of International Search Report.
Berg Remco Van Der
Huussen Frank
Oosterlaken Theodorus Gerardus Maria
ASM International N.V.
Jr. Carl Whitehead
Knobbe Martens & Olson Bear LLP.
Thomas Toniae M.
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