Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1999-03-30
2001-07-17
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
Reexamination Certificate
active
06261860
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of fabricating a solid-state image sensor.
2. Description of the Related Art
Contaminants such as heavy metals may penetrate a wafer while a solid-state image sensor is being fabricated. In order to remove such contaminants out of a device active region, there is usually carried out gettering.
In one of gettering techniques, bulk-micro-defect (BMD) is formed in a wafer in an amount required for removing contaminants. That is, BMD is formed in heat treatment carried out in a process of fabricating a solid-state image sensor, and contaminants are gettered into the thus formed BMD. Accordingly, it is a key in gettering to form BMD in a wafer.
One of methods of forming BMD in a wafer includes the steps of slicing Cz (Czochralski-Zone)-Si ingot into wafers, polishing Si wafer at a surface thereof, carrying out first heat treatment at a temperature in the range of 1150 to 1200 degrees centigrade to thereby melt oxygen cores having been precipitated at a surface of a wafer, and externally diffuse oxygen from a surface of a wafer, and carrying out second heat treatment at a temperature in the range of 500 to 800 degrees centigrade to thereby form oxygen precipitation cores in a wafer.
In accordance with the above-mentioned method, oxygen precipitation cores are not formed at a surface of a wafer in the second heat treatment, since oxygen cores having been precipitated at a surface of a wafer is molten, and oxygen is externally diffused from a surface of a wafer in the first heat treatment. As a result, a wafer can have a non-defective layer at a surface thereof, and can contain oxygen precipitation cores therein.
After oxygen precipitation cores have been formed in a wafer in the above-mentioned way, the wafer is subject to heat treatment at a temperature in the range of 1000 to 1200 degrees centigrade to thereby cause oxygen to be precipitated out of oxygen precipitation cores. Thus, there is formed BMD.
However, the above-mentioned method of forming BMD is accompanied with a problem that a solid-state image sensor including a wafer fabricated in accordance with the above-mentioned method would have illuminated or white defects at a high level. This is considered because contaminants such as heavy metals which penetrate a wafer while a solid-state image sensor is being fabricated are insufficiently gettered into BMD in a wafer, and in particular, because BMD is insufficiently grown in a wafer.
Apart from the above-mentioned method, various gettering techniques have been suggested as follows in order to remove contaminants such as heavy metals out of a device active region.
For instance, Japanese Unexamined Patent Publication No. 57-35329 has suggested a method including the steps of precipitating oxygen atoms in a bulk at a temperature in the range of 1150 to 1200 degrees centigrade to thereby remove defects having been formed at a surface of a wafer, and growing oxygen precipitation at a temperature in the range of 1000 to 1100 degrees centigrade to thereby enhance gettering effect.
Japanese Unexamined Patent Publication No. 1-242500 has suggested a method including the steps of externally diffusing oxygen from a surface of a wafer at 1200 degrees centigrade to thereby diffuse silicon existing at a surface of a wafer, into a wafer, and cooling the wafer down to a temperature in the range of 500 to 800 degrees centigrade at a cooling rate of 1 to 10 degrees per minute, to thereby form oxygen cores in a bulk.
Japanese Unexamined Patent Publication No. 4-43646 has suggested a method of growing an epitaxial layer on a silicon substrate fabricated in accordance with Cz process, and carrying out both heat treatment at a relatively low temperature for a shorter period of time and heat treatment at a relatively high temperature for a longer period of time to thereby facilitate precipitation of oxygen in the Cz substrate.
Japanese Unexamined Patent Publication No. 5-74782 has suggested a method of carrying out intrinsic gettering by applying heat treatment at three stages to a silicon substrate sliced out of silicon monocrystal having been formed in accordance with Cz process. The first heat treatment is carried out at a temperature in the range of 1100 to 1150 degrees centigrade, the second heat treatment is carried out at a temperature equal to or higher than 1200 degrees centigrade, and the third treatment is carried out at a temperature lower than a temperature at which the first heat treatment was carried out.
Japanese Unexamined Patent Publication No. 9-199379 has taught that heat treatment to be carried out at a temperature in the range of 1000 to 1200 degrees centigrade is optimal for obtaining a high grade epitaxial wafer.
However, the inventor has analyzed the methods suggested in the above-mentioned Publications to thereby find out that it would be quite difficult or almost impossible to sufficiently grow BMD in a wafer even in accordance with the above-mentioned methods, and that it is not always possible to obtain desired gettering effect.
SUMMARY OF THE INVENTION
In view of the foregoing problem of the conventional methods, it is an object of the present invention to provide a method of fabricating a solid-state image sensor, which method is capable of growing BMD in a wafer in a greater size than a size of BMD grown in accordance with conventional methods to thereby ensure reduction in a level of illuminated or white defect in a solid-state image sensor, with a non-defective layer being kept as it is at a surface of a wafer.
There is provided a method of fabricating a solid-state image sensor, including the step of carrying out heat treatment before formation of a gate of the solid-state image sensor, a maximum temperature in the heat treatment being in the range of 1000 to 1200 degrees centigrade both inclusive, the step of carrying out heat treatment further including the steps of (a) carrying out lamp-up at least twice, and (b) carrying out lamp-down at least twice.
There is further provided a method of fabricating a solid-state image sensor, including the steps of carrying out first heat treatment at a temperature in the range of 1150 to 1200 degrees centigrade both inclusive, and carrying out second heat treatment at a temperature in the range of 500 to 800 degrees centigrade both inclusive, the first and second heat treatment both being to be carried out before formation of a gate of the solid-state image sensor, at least two lamp-up steps and at least two lamp-down steps being carried out while the first and second heat treatment are carried out.
It is preferable that a temperature in the first lamp-up step is in the range of 1000 to 1200 degrees centigrade both inclusive, and the second or later lamp-up step has an upper limit temperature of 1200 degrees centigrade.
It is preferable that a lamp-down step to be carried out after a lamp-up step has a lower limit temperature of 1000 degrees centigrade.
Temperatures achieved in the lamp-up steps may be the same as each other, but it is preferable that temperatures achieved in the lamp-up steps are different from each other.
Likewise, temperatures achieved in the lamp-down steps may be the same as each other, but it is preferable that temperatures achieved in the lamp-down steps are different from each other.
For instance, a lamp-up rate in the lamp-up steps may be designed to be in the range of 8 to 200 degrees per minute. Likewise, a lamp-down rate in the lamp-down steps may be designed to be in the range of 3 to 100 degrees per minute.
The number of the lamp-up steps is not always necessary to be the same as the number of the lamp-down steps. The number of the lamp-up steps may be greater than the number of the lamp-down steps, and vice versa. However, it is preferable that the number of the lamp-up steps is the same as the number of the lamp-down steps.
There is further provided a method of fabricating a solid-state image sensor, including the steps of (a) forming an epitaxial layer on a substrate, and (b) fabricating a solid-state
Bowers Charles
Christianson Keith
McGinn & Gibb PLLC
NEC Corporation
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