Jewelry
Reexamination Certificate
2002-04-19
2003-12-16
Ghyka, Alexander (Department: 2812)
Jewelry
63, 63
Reexamination Certificate
active
06663674
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method of handling a silicon wafer which is employed as a dummy wafer during a silicon nitride deposition process.
Usually, during a silicon nitride deposition process such as low-pressure chemical vapor deposition, a certain number (usually at least 100) of silicon wafers to be coated with the silicon nitride layer are held vertically in a slotted quartz boat and brought into the chemical vapor deposition reactor. At the ends of this reactor normally inhomogenous reaction conditions will prevail.
Therefore, the end positions of the slotted quartz boat are occupied by dummy wafers which are coated with the silicon nitride layer but which are not used for chip production. These dummy wafers are used for several silicon nitride deposition cycles and then they are recycled by etching the silicon nitride layer with hydrofluoric acid.
The recycling becomes necessary because the stability of the dummy wafers greatly decreases due to stress, slip lines etc. when the layer thickness has reached a certain value. As a consequence, wafer breaking becomes very likely to occur. However, wafer breaking is clearly undesirable because the particles generated contaminate the depostion tool and the production wafers. Broken wafers will largely inhibit the silicon nitride deposition on those wafers which will be used for chip production.
At present, dummy wafers are recycled by etching in concentrated hydrofluoric acid when the silicon nitride layer has reached a thickness of at least 700 nm. If required, a cleaning step for particles and metal contamination can be performed before releasing the wafers back to the furnace. Then, the dummy wafers are again used for silicon nitride deposition. This cycle is repeated until the entire number of cycles has been performed 7 times. Thereafter, the dummy wafer quality will be greatly deteriorated so that the probability of wafer breakage increases.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method of handling a silicon wafer and in particular a recycling process for a dummy wafer that increases the lifetime of the dummy wafer.
As stated above, the invention relates to a method of handling a silicon wafer which is employed as a dummy wafer during a silicon nitride deposition process. More specifically, the invention relates to a recycling procedure for 300 mm nitride dummy wafers which have been previously provided with a special stabilization layer made of silicon dioxide. The recycling procedure is essentially based on selectively wet etching the deposited silicon nitride with respect to the silicon dioxide stabilization layer, preferably with hot phosphoric acid at 160° C.
In the following, reference will be made to a silicon nitride deposition process as well as a silicon nitride layer. However, as is to be clearly understood, the deposition of silicon oxinitride as well as a silicon oxinitride layer are likewise to be encompassed by these terms. Moreover, the present invention not only refers to dummy wafers used during a low pressure chemical vapor deposition process but to dummy wafers during any kind of silicon nitride deposition process, especially plasma enhanced chemical vapor deposition.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of handling a silicon wafer, the method includes the steps of:
depositing a silicon dioxide layer on a surface of the silicon wafer;
performing a nitride deposition process on the silicon wafer employed as a dummy wafer during the nitride deposition process for depositing a nitride compound selected from the group of silicon nitride and silicon oxinitride on the surface of the silicon wafer until a nitride compound layer of a given layer thickness is achieved; and
etching the nitride compound layer selectively with respect to the silicon dioxide layer.
In other words, according to the invention, the object of the invention is achieved by a method of handling a silicon wafer which is employed as a dummy wafer during a nitride deposition process, including the steps of depositing a silicon dioxide layer on the wafer surface, performing the nitride deposition process on the wafer to deposit silicon nitride or silicon oxinitride on the wafer surface until a predetermined layer thickness is reached, and etching the silicon nitride or silicon oxinitride layer selectively with respect to the silicon dioxide layer.
This new procedure allows to nearly double the number of cycles of nitride dummies in comparison to the hitherto used recycling process. Since due to the high wafer cost the increase of dummy wafer cycles is an important contribution for reducing cost in 300 mm manufacturing, the present invention is particularly useful for reducing the cost when 300 mm wafers are processed instead of 200 mm wafers.
As the inventors of the present invention surprisingly found out, the lifetime of a dummy wafer can be largely increased when the dummy wafer is coated with a silicon dioxide layer before depositing the silicon nitride layers. This is because a silicon dioxide layer will induce a kind of stress which will cause a bending of the wafer which is reverse to that caused by the silicon nitride layer. Both the stress of the silicon dioxide layer and the stress of the silicon nitride layer or the oxinitride layer preferably compensate each other.
The thickness of the silicon dioxide layer has to be appropriately chosen because the effect thereof will be too weak if the layer is too thin. More specifically, the thickness of the silicon nitride layer of different wafers may vary due to inhomogenous deposition conditions. Consequently, the silicon dioxide layer will be also etched, when the silicon nitride has been completely etched, since usually the etching agent used also etches the silicon dioxide layer, but at a much slower etching rate. Accordingly, if the silicon dioxide layer is chosen too thin, the silicon dioxide layer of some of the dummy wafers may be attacked and, thus, the stability of the dummy wafer may be deteriorated.
On the other hand, if the layer is too thick, the effect of the silicon dioxide stabilization layer will be too strong and, thus, cause an undesired amount of stress in the reverse direction. Accordingly, it is preferred that the silicon dioxide and the silicon nitride layers have a thickness ratio of approximately 1:5 to 1:10. The inventors found a layer thickness of the silicon dioxide layer of 70 to 150 nm to be appropriate.
According to the present invention, when a predetermined layer thickness of the silicon nitride layer is reached, the silicon nitride will be etched selectively with respect to the silicon dioxide layer. At the end of the etching step, the silicon nitride layer will be completely removed, while the silicon dioxide layer will not be etched or only to a small degree.
According to another mode of the invention, the etching step includes etching the nitride compound layer in a hot phosphoric acid bath. Hence, according to a preferred embodiment of the present invention, hot phosphoric acid (H
3
PO
4
, 85 wt-%) at approximately 160° C. is used for selectively etching the silicon nitride layer.
According to an especially preferred embodiment, the dummy wafers are etched in hydrofluoric acid before etching in hot phosphoric acid. In this case, since hydrofluoric acid does not selectively etch silicon nitride with respect to silicon dioxide, the etching time in hydrofluoric acid has to be appropriately chosen so that only the predominant part of the silicon nitride layer is etched with hydrofluoric acid. Accordingly, the etching time is chosen so that a silicon nitride layer thickness of about 100 nm or a thickness equal to that of the silicon dioxide layer will remain after the hydrofluoric etching step. By adding the etching step in hydrofluoric acid, the etching time of the silicon nitride layer can be remarkably reduced with respect to the embodiment according to which the silicon nitride layer is e
Breeden Terry
Köstler Wolfram
Ottow Stefan
Tucker Michael Thomas
Wissel Dan
Ghyka Alexander
Infineon Technologies SC300 GmbH & Co. KG
Mayback Gregory L.
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