Method for preparing a dummy wafer

The method of the invention starts with forming a mask on a blank wafer, wherein the mask contains a number of openings that expose a portion of the wafer. By performing a wet oxidation process, field oxide is formed on the exposed surface of the wafer. The wafer surface is then become ragged after the mask and the field oxide are removed. In order to further increase the surface area of a dummy wafer, an etching process is performed on the ragged surface after a hemispherical grained layer is formed on the ragged surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for preparing a dummy wafer, and more 
particularly, to a method for preparing a dummy wafer used in a chemical 
vapor deposition (CVD) process. 
2. Description of Related Art 
Chemical vapor deposition (CVD) is a common process used for forming a 
semiconductor thin film device that is theoretically based on controlling 
the reaction of gasiform reactant occurring with in a reacting furnace or 
tube. Generally, a CVD system basically consists of a reactor, gas-pumping 
system, an exhaust system, and a process control system. 
For a tubular-type low-pressure chemical vapor deposition (LPCVD) system 
shown in FIG. 1, the most commonly used batch-type CVD system, the system 
contains a hot-wall reactor 10 made of annealed quartz. The reactor 10 is 
further surrounded by a three-zone heater 12 used for heating the reactor 
10. While a LPCVD process is performed, the gasiform reactant is pumped 
into the reactor 10 from a door 14, and a boat 20 that carries a number of 
wafers 18 is then transported into the reactor 10 and placed at a 
pre-determined position. The exhaust gas of the LPCVD process is released 
by an exhaust system through a releasing end 16. 
The boat 20 further contains a wafer stage for holding a certain number of 
wafers 18. When a CVD process is proceeded, it is required that the wafer 
stage is fully loaded in order to ensure the uniformity of the thickness 
of deposited materials. In case that a insufficient number of wafers 18 
are placed on the wafer stage of the boat 20, the vacancies on the wafer 
stage need to be occupied with dummy wafers to ensure the uniformity of 
deposition. 
Conventionally, black wafers are usually placed on the wafer stage of a CVD 
system to serve as dummy wafers while a CVD process is performed. Since 
the profile of a dummy wafer, which has a smooth surface, is very 
different from what of a productive wafer that has an accidented surface 
constructed by devices formed on the wafer, the amount of consumed 
reactant deposited on a dummy wafer is different from what of a productive 
wafer. Therefore, loading effect, that is, the thickness of material 
deposited on the surface of wafers is not uniform, occurs. The loading 
effect is more distinct when a CVD process of silicon nitride layer is 
performed. 
SUMMARY OF THE INVENTION 
It is therefore an objective of the present invention to provide a method 
for preparing a dummy wafer to prevent the occurrence of loading effect. 
It is another an objective of the present invention to provide a method for 
preparing a dummy wafer to ensure that a uniform thickness of deposited 
material can be obtained on the surfaces of wafers. 
In accordance with the foregoing and other objectives of the present 
invention, the method of the invention processes the smooth surface of a 
dummy wafer into a grained surface in order to increase the surface area, 
hence, to reduce the difference between a productive wafer and a dummy 
wafer. 
The method of the invention starts with forming a mask on a blank wafer, 
wherein the mask contains a number of openings that expose a portion of 
the wafer. By performing a wet oxidation process, field oxide is formed on 
the exposed surface of the wafer. The wafer surface is then become ragged 
after the mask and the field oxide are removed. In order to further 
increase the surface area of a dummy wafer, an etching process is 
performed on the ragged surface after a hemispherical grained layer is 
formed on the ragged surface. 
The material deposited on the dummy wafer can be removed by performing an 
etching process, therefore, the dummy wafer of the invention is reusable. 
For example, a silicon nitride layer formed on the dummy wafer can be 
removed by performing an etching process with hydrofluoric acid (HF) as an 
etchant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention provides a new method for preparing a dummy wafer used in a 
chemical vapor deposition system, as shown in FIGS. 2A through 2E. The 
processed dummy wafer contains a ragged grained surface, which has larger 
surface area than a planar smooth surface. 
Referring to FIG. 2A, a pad oxide layer (not shown in figure) is formed on 
a provided substrate 100, such as a semiconductor substrate, by performing 
a process like thermal oxidation. A patterned mask 102, such as silicon 
nitride, is formed on the pad oxide layer over the substrate 100, wherein 
the mask contains opening 104 exposing the structure underneath. 
Referring next to FIG. 2B, a field oxide layer 106 is formed on the exposed 
region over the substrate. The field oxide layer 106 can be formed by 
performing a wet oxidation process, or other oxidation processes capable 
of achieving the same goal. Since the oxidation process is performed on 
the surface of the wafer 100 with the substrate 100, for example, a 
silicon substrate, as a reactant, part of substrate 100 is consumed by the 
oxidation reaction and oxidized into oxide of a certain thickness 
according to the reaction period. The remains of the substrate 100 now are 
turned into a substrate 100a that has a number of dished regions beneath 
the field oxide layer 106. 
Referring to FIG. 2C, the mask layer 102 and the field oxide layer 106 are 
removed by performing a process like wet etching to expose the surface of 
the substrate 100a, wherein the etchant used in a wet etching process can 
be hydrofluoric (HF) acid of a concentration of about 40% to 60%. Because 
part of the substrate 100a contains dished regions formed by the oxidation 
process, the total surface area of the substrate 100a is larger than the 
total surface area of the original substrate 100. 
Referring next to FIG. 2D, in order to further increase the total surface 
area of the substrate 100a, a hemispherical grained polysilicon (HSG-Si) 
layer 108 is formed on the substrate 100a. The method of forming the 
HSG-Si layer 108 can be a low-temperature chemical vapor deposition 
process at a pre-determined reaction temperature with SiH.sub.4 or 
Si.sub.2 H.sub.6 as a reactant. The pre-determined reaction temperature is 
between the generation temperatures of amorphous silicon and polysilicon. 
Other processes under proper conditions can be used to form the HSG-Si 
layer 108 as well. 
Referring to FIG. 2E, an etching process is performed on the substrate 100a 
to enhance the junction between the substrate 100a and the HSG-Si layer 
for remaining the roughness of the surface of the substrate 100a. The 
surface area of the enhanced dummy wafer 100b is about 2 to 4 times over 
the surface area of a conventional dummy wafer. 
Since the surface area of the dummy wafer according to the invention can be 
roughly determined by the thickness of the field oxide layer 106, it can 
be made to be close to the surface area of a productive wafer to prevent 
the occurrence of loading effect by controlling the oxidation time. 
The surface area of a dummy wafer of the invention is controlled by the 
thickness of the field oxide layer, that is, the oxidation time. Hence, by 
properly controlling the oxidation time, the surface area of a dummy wafer 
is made close to the surface area of a productive wafer. Therefore, by 
occupying the vacancies on the wafer stage of a boat 20 shown in FIG. 1 
with the dummy wafers of the invention, the thickness of deposited film 
formed by a CVD process, especially a silicon nitride layer formed by a 
LPCVD process, can be more uniform. 
The deposited material on the dummy wafer of the invention can be removed 
by applying solution of hydrofluoric acid without damaging the surface of 
the dummy wafer, wherein the concentration of the solution is about 40% to 
60%. So, the dummy wafer of the invention is reusable. 
In accordance with the foregoing, it is a specificity of the invention to 
obtain a dummy wafer containing a larger surface area through simple 
semiconductor fabrication processes. The surface area of the dummy wafer 
according to the invention is about 2 to 4 times over the surface area of 
a convention dummy wafer. 
It is another specificity of the invention to simulate the surface area of 
a dummy wafer by controlling the thickness of the field oxide layer, that 
is, controlling the oxidation reaction time, to prevent the occurrence of 
loading effect. 
It is also a specificity of the invention that the dummy wafer can be 
reused by simply applying solution of hydrofluoric acid to remove the 
deposited materials such as a LPCVD silicon nitride film, wherein the 
concentration of the solution is about 40% to 60%. 
The invention has been described using exemplary preferred embodiments. 
However, it is to be understood that the scope of the invention is not 
limited to the disclosed embodiments. On the contrary, it is intended to 
cover various modifications and similar arrangements. The scope of the 
claims, therefore, should be accorded the broadest interpretation so as to 
encompass all such modifications and similar arrangements.