Etch-ending point measuring method for vapor etch process

A method for determining an etch-ending point using a vapor etch apparatus having a chamber is disclosed including the steps of providing a vapor-state etchant in the chamber, inserting a material to be etched in the chamber and etching the material by the etchant, measuring an ion current intensity of a by-product generated during the vapor etch process, calculating a thickness variation value of the material by using the ion current intensity value, and stopping the vapor etch process when the thickness variation value reaches a preset value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for measuring an etch-ending 
point, and more particularly, to a method for measuring an etch-ending 
point during a vapor etch process. 
2. Discussion of the Related Art 
An etching process is usually carried out for forming a variety of patterns 
during a semiconductor device manufacturing process. It can be used to 
etch a semiconductor layer, an insulating layer, or a conductive layer. 
Conventionally, etching processes are classified into processes using 
either a dry etch method or a wet etch method. Recently, a vapor etch 
method has been introduced where a material is etched in a chamber filled 
with a vapor-state etchant. 
For the dry etch, the wet etch, and the vapor etch methods, one important 
factor is the ability to decide when a layer subjected to etching has been 
etched to the desired thickness. In order to assure the success of the 
semiconductor device manufacturing process which requires minute patterns, 
it is essential to be able to decide accurately when each of the layers 
subjected to etching has been etched to the desired thickness during the 
etch process for forming the respective patterns. In other words, it is 
essential to accurately measure an etch-ending point during an etch 
process. 
The conventional methods for measuring an etch-ending point will now be 
described for both the dry etch process and the wet etch process. 
For the dry etch process, the widely adopted etch-ending point measuring 
method is a light wavelength method. When the dry etch of a silicon oxide 
layer (SiO.sub.2) is performed by using an etchant CF.sub.4, the etchant 
CF.sub.4 reacts to the silicon oxide layer to produce a by-product such as 
CO.sub.y F.sub.x. A light beam, such as laser, is then projected upon the 
silicon oxide layer which is reacting with the etchant CF.sub.4. When the 
light is reflected from the silicon oxide layer and the by-product 
resulting from the reaction, the intensity of the reflected light at 
specific wavelengths will vary in accordance with the degree of etching 
completed of the silicon oxide layer. The intensity of the reflected light 
at a specific wavelength is measured during the etch process and compared 
to a certain pre-determined intensity value set by prior experiments. 
Thus, when the measured intensity of the light is equal to the 
pre-determined intensity value, the etch-ending point has been reached. 
The above described light wavelength method for measuring the etch-ending 
point is also used in a conventional chemical-mechanical polishing (CMP) 
process. The CMP process etches a layer to a desired thickness by milling 
the layer subjected to the etch process. 
For the wet etch process, three conventional methods for measuring the 
etch-ending point are often used. For the purpose of discussion, it is 
assumed that an insulating layer is wet-etched to form a desired pattern 
in a structure having the insulating layer on a semiconductor layer. 
The first method is a color method. After placing this structure into a 
bath filled with an etchant and conducting the wet etch for a period of 
time, the resultant structure is taken out of the bath to determine the 
etch-ending point by examining the surface color of the resultant 
structure. If the insulating layer is etched to the desired thickness, the 
surface color of the structure would differ from that before conducting 
the etch. This is because the color of the underlying semiconductor layer 
is different from that of the insulating layer. 
This color method is mainly used in a partial etch process. For example, it 
is used when the insulating layer of 5000.ANG. is etched by 4000.ANG. 
only. 
The second method is a surface tension method. A structure comprised of a 
semiconductor layer, such as silicon, and an insulating layer is placed in 
a bath filled with a wet-etchant and subjected to the wet etch. Then, the 
resultant structure is taken out of the bath and the surface is sprayed 
with water. If the insulating layer is fully etched, the surface of the 
silicon would be exposed. Accordingly, water will stay on the silicon 
surface due to the surface tension of the silicon. If the insulating layer 
is not fully etched, the sprayed water does not adhere to the surface. 
Instead, it flows over the surface since the insulating layer has almost 
no surface tension. Thus, the etch-ending point can be measured. 
The third method for determining the etch-ending point uses a thickness 
meter. Initially, a structure comprised of a semiconductor layer and an 
insulating layer is placed into the bath and subjected to the wet etch for 
a period of time. Then, the resultant structure is taken out of the bath, 
and the thickness of the structure is measured by using a thickness meter. 
If the insulating layer is fully etched, the measured thickness of the 
structure equals the thickness of the semiconductor layer. If the 
insulating layer is not fully etched, the measured thickness of the 
structure would be larger than the thickness of the semiconductor layer. 
For the vapor etch process, a material is etched using a vapor-state 
etchant, instead of a gas-state etchant. In this sense, the vapor etch 
process is more similar to the wet etch process than to the dry etch 
process. Therefore, the conventional etch-ending point measuring methods 
for the wet etch process are also used for the vapor etch process. 
All three etch-ending point measuring methods for the wet etch process are 
visual methods, which can be difficult to apply when the structure has 
patterns. Since the etch process employed for manufacturing semiconductor 
devices is frequently used to form patterns, and the patterns are often of 
a minute nature, it is almost impossible to visually measure the 
etch-ending point under those circumstances. 
Conventionally, in order to actually apply the above-described methods 
during the manufacturing process of the semiconductor devices, a test 
pattern with an enlarged configuration is utilized. In other words, the 
same manufacturing process is simultaneously performed upon the test 
pattern when the process is used to manufacture the semiconductor devices. 
The test pattern is a considerable enlargement of the actual structure, so 
that the above three etch-ending point measuring methods can be applied. 
The etch-ending point of the actual structure can be only estimated by 
using the color, surface tension, and thickness of the test pattern. 
This technique of using the etch-ending point on the test pattern to 
provide an estimation for the etch-ending point of the actual structure 
has several problems. First, it has an inherent risk of inaccuracy of the 
etch-ending point for the actual structure, and such inaccuracy usually 
causes failures of the manufacturing process. 
In addition, due to the large size of the test pattern involved and the 
large amount of the chemical material (such as the etchants) required, the 
whole manufacturing process becomes very complicated and expensive. 
Furthermore, the structure is taken out of the etching bath or chamber one 
by one to either determine the color and surface tension, or measure the 
thickness with the meter, thereby increasing the processing time. Usually, 
the processing time using the conventional methods is similar to that 
required for etching a material with a thickness corresponding to 150-200% 
of the thickness of a target material. 
Therefore, the conventional etch-ending point measuring methods for the wet 
etch and the vapor etch processes are not as effective in various aspects 
as the measuring method for the dry etch process. Consequently, an 
effective etch-ending point measuring method for the vapor etch and the 
wet etch processes is needed 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to an etch-ending point 
measuring method for a vapor etch process that substantially obviates one 
or more of the problems, limitations, and disadvantages of the related 
art. 
An object of the present invention is to provide an etch-ending point 
measuring method for a vapor etch process with reduced risk of failure and 
lower cost. 
Another object of the present invention is to provide an etch-ending point 
measuring method that can be performed within a shorter period of time 
compared to the conventional method. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To achieve these and other advantages and in accordance with the purpose of 
the invention, as embodied and broadly described, the etch-ending point 
measuring method of the present invention includes the steps of providing 
a vapor-state etchant in a chamber, performing a vapor etch process by 
inserting a material to be etched in the chamber, measuring an ion current 
intensity value of a by-product generated during the vapor etch process, 
calculating a thickness variation value of the material by using the ion 
current intensity value, and stopping the vapor etch process when the 
thickness variation value reaches a preset value. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
A preferred embodiment of the present invention will now be explained with 
reference to FIG. 1, which shows the sequence of an etching-ending point 
measuring method for a vapor etch process in accordance with the present 
invention. 
As shown in FIG. 1, a vapor-state etchant is first introduced into an 
etching chamber. Then, a material that is to be subjected to the etch 
process is inserted into the etching chamber. During the vapor etching 
process, the ion current intensity value of a specific by-product of the 
etching process is measured. 
Different ion current intensity values of the specific by-product 
correspond to different etched thickness values of the material, and these 
corresponding (e.g. one-to-one) relationships are measured and recorded in 
a table according to previous experiments. Thus, the etched thickness 
value AT (also referred to as the thickness variation) of the material is 
determined by using the measured ion current intensity value. This etched 
thickness value AT is then displayed continuously or discretely by a 
display device. When the AT value displayed on the display device matches 
a predetermined value corresponding to the etch-ending point, the vapor 
etch process is completed. 
FIG. 2 is a schematic diagram showing a vapor etching apparatus for 
embodying the etch-ending point measuring method in accordance with the 
present invention. In FIG. 2, a reference numeral 10 denotes an etch 
chamber, and 10a is a stage for placing a material that is to be subjected 
to the etch process thereon. Numeral 20 is a measuring device for 
measuring the ion current intensity of a specific by-product generated 
during the vapor etch process, and 20a is a valve for controlling the 
amount of the specific by-product that is introduced into the measuring 
device 20. Numeral 30 is a .DELTA.T calculator for calculating the etched 
thickness value .DELTA.T (thickness variation) of the material 
corresponding to the measured ion current intensity value. Numeral 40 is a 
.DELTA.T display device for displaying the calculated thickness variation 
.DELTA.T on a monitor. Numeral 50 is another valve for controlling the 
amount of vapor-state etchant that is introduced into the chamber 10. In 
addition, reference numeral 60 denotes a silicon substrate, and 70 is an 
oxide layer such as silicon dioxide subjected to the etch process that is 
formed on the silicon substrate 60. 
The process for vapor etching the silicon oxide layer will now be explained 
with reference to FIG. 2. First, anhydrous-state (i.e., vapor-state) HF is 
introduced into the etch chamber as an etchant through the valve 50. Then, 
a proper amount of vapor-state H.sub.2 O is introduced into the chamber 
10. Next, the silicon oxide layer 70 (formed on the silicon substrate 60) 
is placed on the stage 10a located at a predetermined portion of the etch 
chamber 10. 
In the preferred embodiment, anhydrous-state HF is used as the etchant. 
Other anhydrous state etchants may also be used such as H.sub.3 PO.sub.4 
and NH.sub.4 OH, for etching the silicon oxide layer. Anhydrous H.sub.2 
SO.sub.4 may be used to etch a silicon semiconductor layer. One common 
characteristic of all the etchants listed above is that they all contain 
hydrogen ions. 
In the preferred embodiment, the silicon oxide layer 70 may be a thermal 
oxide layer formed on the silicon substrate 60. A certain amount of 
vapor-state H.sub.2 O is added to the vapor-state etchant HF during the 
vapor etching of the silicon oxide layer 70. Consequently, the etch rate 
of the silicon oxide layer 70 varies with the amount of H.sub.2 O added. 
FIGS. 3a through 3d are cross sectional views showing the vapor etching 
process of the silicon oxide layer 70. As shown in FIG. 3a, when the 
vapor-state HF and H.sub.2 O are initially introduced into the etch 
chamber 10, they react with each other according to the following chemical 
formula: 
EQU 2HF+H.sub.2 O.fwdarw.H.sub.3 O.sup.+ +HF.sub.2.sup.- (1) 
where (H.sub.3 O.sup.+ +HF.sub.2.sup.-) is in a liquid state and exists on 
the surface of the silicon oxide layer 70, as shown in FIG. 3b. Then, 
(H.sub.3 O.sup.+ +HF.sub.2.sup.-) reacts with the silicon oxide layer 70, 
thus etching the silicon oxide layer 70. As shown in FIG. 3c, during the 
etching of the silicon oxide layer 70, (H.sub.3 O.sup.+ +H.sub.2 SiF.sub.6 
+H.sub.2 O+HF.sub.2.sup.-) in liquid state exists on the surface of the 
silicon oxide layer 70. 
FIG. 3d shows a fully etched state of the silicon oxide layer 70. When the 
silicon oxide layer 70 is fully etched, (HF+H.sub.2 O+SiF.sub.4) in vapor 
state is generated, and (H.sub.2 SiF.sub.6 +H.sub.2 O+HF.sub.2.sup.-) in 
liquid state exists on the surface of the silicon substrate 60. 
The reaction in FIGS. 3b through 3d can be shown in the following chemical 
formula: 
EQU SiO.sub.2 +2H.sub.3 O.sup.+ +2HF.sub.2.sup.- .fwdarw.SiF.sub.4 +4H.sub.2 
O(2) 
The above formula (2) shows that the by-product SiF.sub.4 is generated by 
the etching reaction between the silicon oxide layer 70 and the vapor 
mixture of HF/H.sub.2 O. 
FIG. 4 shows the ion current intensity values of the respective by-products 
generated during the etching process of the thermal silicon oxide layer 70 
at room temperature. As shown in FIG. 4, with the lapse of a predetermined 
time (about 500 seconds in FIG. 4) after starting the vapor etch, the 
by-products are generated and the ion current intensity values of the 
respective by-products are measured. This measurement is done in a manner 
such that the measuring device 20 (see FIG. 2) continuously or discretely 
captures only a specific by-product such as SiF.sub.4 and measures its ion 
current intensity. 
As shown in FIG. 4, with a lapse of about 500 seconds after starting the 
vapor etch, the ion current intensity value of the by-product SiF.sub.4 
continuously increases to a maximum point, and it then slowly decreases 
with a lapse of about 300 seconds. This maximum point corresponds to a 
state where the silicon oxide layer 70 is fully etched. This fact is 
ascertained through prior experiments. The variation value of the ion 
current intensity (.DELTA.ICI) of the by-product SiF.sub.4, and the 
thickness variation value (.DELTA.T) of the silicon oxide layer 70 can be 
written in a table with a one-to-one corresponding relationship by using 
the results from such prior experiments. This table is then stored in a 
memory (not shown) in the .DELTA.T calculator 30 (see FIG. 2) for 
determining the thickness variation value (.DELTA.T) of the silicon oxide 
layer 70 based on the measured .DELTA.ICI value. 
Thus, when the measuring device 20 measures the ion current intensity of 
SiF.sub.4 either continuously or discretely, the .DELTA.T calculator 30 
calculates the corresponding thickness variation value .DELTA.T (i.e., the 
etched thickness) of the silicon oxide layer 70. The calculated .DELTA.T 
value is then displayed by the .DELTA.T display device 40. 
Accordingly, the .DELTA.T display device 40 displays the .DELTA.T value 
continuously or discretely on the monitor during the vapor etching of the 
silicon oxide layer 70. When the .DELTA.T value displayed by the .DELTA.T 
display device 40 reaches a predetermined value corresponding to the 
etch-ending point, the vapor etch process is completed. 
In the present preferred embodiment, the material subjected to the etch 
process is a silicon oxide layer. Similarly, the etch-ending point 
measuring method of the present invention can be applied to a vapor etch 
process of a nitride insulating layer, a polysilicon semiconductor layer, 
or a metal layer. When a different material is subjected to the vapor 
etching, only the etchant and the specific by-product (selected to measure 
the ion current intensity) are different from that of the above-described 
embodiment. For example, if a polysilicon layer is etched, a different 
by-product is generated because anhydrous H.sub.2 SO.sub.4 is used as an 
etchant. 
The present invention has the following advantages. First, since the 
present invention is an automatic etch-ending point measuring method, 
instead of a visual measuring method, it is more convenient and enables 
the etching process to be performed within a shorter period of time than 
the conventional methods. 
Second, the present invention is cost effective because an enlarged test 
pattern is not required. This prevents unnecessary consumption of the 
chemical etchant and the material to be etched. 
Third, the etch-ending point is more accurately measured using the method 
in the present invention than the conventional methods, thus preventing 
possible failures during the vapor etch process. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the etch-ending point measuring method of 
the present invention without departing from the spirit or scope of the 
invention. Thus, it is intended that the present invention cover the 
modifications and variations of this invention provided they come within 
the scope of the appended claims and their equivalents.