Etching a substrate: processes – Gas phase etching of substrate – With measuring – testing – or inspecting
Reexamination Certificate
1998-02-05
2001-04-03
Acquah, Samuel A. (Department: 1711)
Etching a substrate: processes
Gas phase etching of substrate
With measuring, testing, or inspecting
C216S002000, C216S058000, C216S059000, C216S067000, C216S072000, C438S009000, C438S014000, C250S305000, C250S307000
Reexamination Certificate
active
06210593
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an etching method whereby a target film, such as a metal alloy film, formed on a wafer is etched by using a plasma in the process of manufacturing a semiconductor device and to an etching apparatus used in the etching method.
A method of generating a plasma by utilizing RF discharge has found a wide range of applications in such fields as plasma etching for micro-fabrication and plasma CVD for thin-film formation, each conducted in a semiconductor process. In particular, the field of plasma-assisted dry etching has required the establishment of a dry-etching technique which promises a high-precision and stable etching process with improved reproducibility, productivity, and yield.
In the field of dry etching performed with respect to a target film such as a metal alloy film, a method of reactive ion etching (RIE) has been used predominantly.
A description will now be given to a conventional plasma-etching apparatus.
FIG. 8
shows the schematic structure of the conventional RIE plasma-etching apparatus. A chamber
1
maintained under vacuum is internally provided with a sample stage
3
for carrying a wafer
2
as a sample to be etched. For example, RF power of 13.56 MHz is applied from an RF power source
4
to the sample stage
3
, which also serves as a lower electrode. The chamber
1
is formed with a gas inlet
5
for introducing reactive gas into the chamber
1
and with a gas outlet
6
for exhausting the reactive gas from the chamber
1
. On the sample stage
3
, there is provided a focus ring (equalizer ring)
7
with a height of 10 to 20 mm, which is positioned around the wafer placed on the sample stage
3
to equalize the distribution of radicals composed of reactive gas and directed to a target film on the wafer
2
.
In the conventional process of etching the target film such as a metal alloy film, optimum etching conditions under which the uniformity of the etching rate is ±5% are predetermined so that the target film is etched under the optimum etching conditions.
To optimize the etching conditions, there have been adopted a method of replacing the focus ring
7
with another of a different height and a method of controlling the direction in which the reactive gas blows into the chamber
1
. Briefly, the level of the focus ring
7
and the direction of the blowing gas are varied during actual etching till the optimum level and direction that provide the optimum etching rate are achieved.
Table 1 shows an example of etching conditions for dry etching performed with respect to a metal alloy film.
TABLE 1
PARAMETER
VALUE
POWER
700 W
(13.56 MHz)
GAS SPECIES
100/100/20 sccm
(BCl
3
/Cl
2
/N
2
)
PRESSURE
150 mTorr
FIG.
9
(
a
) shows the cross-sectional structure of a sample before dry etching is performed. FIG.
9
(
b
) shows the cross-sectional view of the sample after dry etching was performed. As shown in FIG.
9
(
a
), a BPSG film
11
, a first TiN film
12
, an Al-1% Cu film
13
, and a second TiN film
14
are successively deposited on a semiconductor substrate
10
made of silicon. A resist pattern for forming a pattern with a 0.7 &mgr;m line width is formed on the second TiN film
14
. When dry etching is performed with respect to the second TiN film
14
and Al-1% Cu film
13
masked with the resist pattern
15
under the etching conditions shown in Table 1, a metal wire
16
with a 0.7 &mgr;m line width is formed as shown in FIG.
9
(
b
).
Table 2 shows the relationship between the number of processed wafers with a diameter of 8 inches and the etching rate.
TABLE 2
UNIFORMITY OF ETCHING RATE (%)
WHEN PROCESS IS
WHEN PROCESS IS
STABLE
UNSTABLE
1ST WAFER
±3.9
±4.0
5TH WAFER
±4.8
±8.0
10TH WAFER
±4.1
±5.3
15TH WAFER
±4.6
±10.9
20TH WAFER
±4.3
±8.7
25TH WAFER
±4.6
±4.5
As shown in Table 2, when the number of processed wafers is small and the etching process is stable, the uniformity of the etching rate remains within ±5% for each of 6 wafers selected from 25 wafers and examined for the etching rate. However, as the number of processed wafers increases and the conditions in the reaction chamber change, the etching process becomes unstable. When the etching process is unstable, the uniformity of the etching rate becomes ±5% or more, as indicated by the 5th, 15th, and 25th wafers, resulting in a reduced etching rate.
Although the foregoing data represents the relationship between the number of processed wafers with a diameter of 8 inches and the etching rate, the aforesaid tendency is more noticeable in the case of performing etching with respect to wafers with a diameter of 12 inches. In other words, the non-uniformity of the etching rate is more conspicuous with wafers larger in diameter.
However, even when the uniformity of the etching rate is degraded, the conventional etching method continues etching without changing etching conditions. It is not until the problem arises that the focus ring is replaced with another of a different height or the direction of the blowing reactive gas is changed after the pressure inside the reaction chamber is switched from the vacuum state to an atmospheric state.
As a result, the reliability of a semiconductor device becomes less stable and the yield lowers due to the degraded uniformity of the etching rate.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to prevent the lowering of the reliability and yield of a semiconductor device resulting from the non-uniformity of the etching rate.
To attain the above object, the present invention has been achieved by focusing attention on the fact that plasma etching is predominantly performed by radicals and the non-uniformity of the etching rate at a target film mostly results from different quantities of radicals supplied to the peripheral and central portions of the target film being etched. According to the present invention, the etching rate is measured at the peripheral and central portions of each target film and the quantities of radicals supplied thereto are controlled, whereby a uniform etching rate is achieved at each target film.
An etching method according to the present invention is for sequentially etching target films formed on respective wafers by using a plasma and comprises: an etching-rate measuring step of measuring an etching rate at each of peripheral and central portions of the first target film formed on the first wafer; and a radical-quantity controlling step of controlling a quantity of radicals by increasing the quantity of radicals arriving at a central portion of the second target film formed on the second wafer to be etched subsequently to the first wafer or decreasing the quantity of radicals arriving at a peripheral portion of the second target film when the etching rate is higher at the peripheral portion of the first target film than at the central portion thereof or controlling the quantity of radicals by decreasing the quantity of radicals arriving at the central portion of the second target film or increasing the quantity of radicals arriving at the peripheral portion of the second target film when the etching rate is lower at the peripheral portion of the first target film than at the central portion thereof.
In accordance with the etching method according to the present invention, the quantity of radicals is controlled by increasing radicals arriving at the central portion of the second target film being etched or decreasing the quantity of radicals arriving at the peripheral portion of the second target film being etched when the etching rate is higher at the peripheral portion of the first target film than at the central portion thereof. Consequently, the quantity of radicals arriving at the central portion of the second target film in the etching process is relatively increased compared with the quantity of radicals supplied to the central portion of the first target film in the etching process, so that the etching rate at the centra
Kubota Masafumi
Ohkuni Mitsuhiro
Acquah Samuel A.
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
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