Wafer etching method

Details Wafer etching method Wafer etching method

2000-02-28

2002-09-17

Alanko, Anita (Department: 1746)

Etching a substrate: processes

Gas phase etching of substrate

Application of energy to the gaseous etchant or to the...

C216S069000, C216S079000, C438S715000, C438S719000, C438S727000

Reexamination Certificate

active

06451217

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wafer etching method for locally etching relatively thick portions present on the surface of a wafer.
2. Description of the Related Art
In recent years, a wafer etching method has been devised which locally etches relatively thick portions on the surface of a silicon wafer etc. to thin the wafer and flatten the surface and thereby reduce the variation in shape, that is, the total thickness variation (TTV) or local thickness variation (LTV).
FIG. 7
is a schematic view of a wafer etching method of the related art.
In
FIG. 7
, reference numeral
100
represents a plasma generator. The plasma generator
100
causes plasma discharge of sulfur hexalluoride (SF
6
) to generate an activated species gas C containing fluorine (F) ions and radicals. This activated species gas C is sprayed onto the surface of a silicon wafer W from a nozzle
101
.
The silicon wafer W is fixed on a stage
120
. The stage
120
is made to move in the horizontal direction and the nozzle
101
made to scan the entire surface of the silicon wafer W, whereby portions Wa relatively thicker than the prescribed thickness on the surface of the silicon wafer W (hereinafter referred to as the “relatively thick portions”) are led directly under the nozzle
101
.
Due to this, the activated species gas G is sprayed on the protruding relatively thick portions Wa from the nozzle
101
and the relatively thick portions Wa are locally etched, whereby the surface of the silicon wafer W is flattened.
In this wafer etching method using SF
6
gas, however, as shown by the hatching in
FIG. 8
, white turbidity B occurs at the surface of the silicon wafer W along the line of scanning A of the nozzle
101
at the time of etching. Not only is the surface of the silicon wafer W contaminated, but also the white turbidity inhibits uniform etching and can cause the TTV and LTV to become worse than desired.
As opposed to this, there is a wafer etching method which causes plasma discharge of carbon tetrafluoride (CF
4
). If this method is used, no white turbidity B is formed at the surface of the silicon wafer W, but the etching rate is much slower than the method using SF
6
gas.
Therefore, a wafer etching method using SF
6
which does not cause white turbidity has been desired.
As one example of such a technique, there is the technique shown in FIG.
9
.
In this technique, the silicon wafer W is placed in a low atmospheric pressure environment of 1 Torr. A small discharge chamber
200
serving also as an electrode and filled with SF
6
gas is brought close to a relatively thick portion Wa. In that state, plasma discharge of the SF
6
gas is caused at a high frequency of 13.56 MHZ, whereby the relatively thick portion Wa is locally etched. At his time, since the plasma in the discharge chamber
200
is close to the relatively tick portion Wa, at the same time as the activated species gas G is etching the relatively thick portion Wa, the ions in the activated species gas G strike the relatively thick portion Wa. Therefore, it is believed the white turbidity is eliminated by the impact of the various types of ions and no white turbidity remains on the surface of the silicon wafer Wa.
In this wafer etching method of the related art shown in
FIG. 9
, however, since the ions in the activated species gas G strike the surface of the silicon wafer W, the crystal structure of the silicon wafer W is disturbed, impurities caused by the collisions of the various types of ions enter into the silicon wafer W, and a high quality mirror polish of the silicon wafer W might not be able to be achieved. Further, with this method, while it is not possible to discern the white turbidity visually under natural light the white turbidity can be seen under a condenser type lamp. It is not possible to completely prevent the occurrence of white turbidity by this method.
As opposed to this, the apparatus shown in
FIG. 7
uses SF
6
gas, has a plasma discharge position far away from the silicon wafer W, and sprays only activated species gas G to the silicon wafer W. Therefore, so long as the silicon wafer W is etched by this apparatus, no disturbance occurs in the crystal structure of the silicon wafer W. The present inventors used this apparatus and added hydrogen (H
2
) gas to the SF
6
gas to locally etch the silicon wafer W, then inspected the surface of the silicon wafer W visually, whereupon they discerned no white turbidity. This is believed to be because the occurrence of white turbidity was suppressed by the presence of the hydrogen fluoride (HF) produced by the reaction between the fluorine (F) radicals and H
2
. Further, with this method, since the ions in the activated species gas G do not strike the silicon wafer W. the crystal structure of the surface portion of the silicon wafer W is not disturbed Further, it is believed that by heating the silicon wafer W to a predetermined temperature, it is possible to completely prevent the occurrence of white turbidity on the surface of the silicon wafer W.
Note that as shown in
FIG. 10
, there is a technique for etching the surface of a silicon wafer W by adding H
2
gas to SF
6
gas.
This technique arranges the silicon wafer W inside a high atmospheric pressure environment of 1500 Torr. A drum shaped electrode
300
longer than the diameter of the silicon wafer W is brought close to the silicon wafer W and the SF
6
gas with the added H
2
gas is interposed in the slight clearance between the electrode
300
and the silicon wafer W. In this state, plasma discharge of the gas is caused by a high frequency of 150 MHZ, whereby the surface of the silicon wafer W is etched. Further, in this technique as well, the plasma discharge position is made close to the silicon wafer W, so similar problems arise as with the technique shown in FIG.
9
. Further, since the atmospheric pressure is high, the temperature of the gas rises. As a result, the temperature of the wafer rises and problems such as warping of the wafer occurs. Further, it is not possible to suppress the occurrence of white turbidity.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a wafer etching method which adds hydrogen gas, ammonia gas or mixed gas containing one of these gases to sulfur hexafluoride gas and thereby suppresses the occurrence of white turbidity at the surface of the wafer at the time of etching and enables a high quality mirror polish to the wafer.
To achieve this object, according to an aspect of he present invention, there is provided a wafer etching method comprising: a plasma generation step for converting sulfur hexafluoride gas to plasma at a discharge position in a discharge tube to generate; an activated species gas; and a spraying step for spraying the activated species gas onto a relatively thick portion of the wafer, in a state where a nozzle portion of the discharge tube leading the activated species gas generated at the discharge position to the wafer side is made to face the relatively thick portion of the wafer, so as to locally etch the relatively thick portion, wherein hydrogen gas, ammonia gas or mixed gas containing one of these gases is added to the activated species gas in a predetermined ratio.
Due to this configuration, in the plasma generation step, the sulfur hexafluoride gas is converted to plasma at the discharge position in the discharge tube to generate an activated species gas. Further, in the spraying step, the activated species gas generated at the discharge position is led by the nozzle portion to the wafer side and the activated species gas from the nozzle portion facing a relatively thick portion of the wafer is sprayed to the relatively thick portion whereby the relatively thick portion is locally etched. At this time, since the wafer is etched by the activated species gas comprised primarily of the sulfur hexafluoride gas, the etching rate is extremely high. Further, since the activated species gas is sprayed for etching from a nozzle portion away from the discharge position where t

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