Dry etching method

A dry etching method for silicon trench etching in which high anisotropy, high etchrate and low pollution may be achieved simultaneously. A single crystal silicon substrate is etched using a gas mixture of S.sub.2 Cl.sub.2 and S.sub.2 F.sub.2 while a wafer is cooled to about -70.degree. C. Etching proceeds by a mechanism in which a radical reaction by Cl* derived from S.sub.2 Cl.sub.2 and F* derived from S.sub.2 F.sub.2 is assisted by the incident energy of S.sup.+, SF.sup.+, SCl.sup.+ or Cl.sup.+ ions. The highly reactive F* radicals of a small atomic radius contribute to increasing the etchrate. Deposition of sulfur yielded from S.sub.2 Cl.sub.2 and S.sub.2 F.sub.2 provides for efficient sidewall protection to achieve high anisotropy. The conventional practice to add fluorine based gases with a view to increasing the etchrate is in need of an excess quantity of a deposition material to give rise to increased pollution by particles. There is no risk of pollution with the sulfur deposit according to the present invention because the sulfur deposit may easily be sublimed off by heating the wafer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a dry etching method employed for the preparation 
of semiconductor devices. More particularly, it relates to a dry etching 
method for performing high aspect ratio processing, such as silicon trench 
etching, with satisfactory anisotropy. 
2. Description of Related Art 
In keeping up with the high integration and high performance of 
semiconductor devices, such as VLSIs or ULSIs, the aspect ratio of various 
patterns in the semiconductor devices tends to be increased acutely. Thus, 
in the field of dry etching, there is a keen demand for a technology of 
performing high aspect ratio processing with satisfactory anisotropy. 
A typical example of high aspect ratio processing of the layer of the 
silicon based material is silicon trench etching aimed at forming 
capacitative elements or device isolation. Trench depth varies depending 
on the device type and usage and is of the order of 4 to 5 .mu.m, 1 .mu.m 
and 4 .mu.m for capacitative elements, device isolation for MOS 
transistors, and device isolation for bipolar transistors, respectively. 
The aperture diameter is of the order of 0.35 to 1.0 .mu.m in each case. 
With silicon trench etching, the cross-sectional shape of the trench is 
changed in a complex manner depending on mask patterns or etching 
parameters to present difficulties in trench filling or capacitance 
control to be performed in subsequent steps. Therefore, if fluorine based 
gases, with which etching proceeds mainly on the basis of radical 
reactions, are used for etching, difficulties are raised in achieving high 
anisotropy. For this reason, chlorine based gases, as typified by a gas 
mixture of SiCl.sub.4 and N.sub.2, are employed. With this gas mixture, 
Cl* are generated as a main etchant from SiCl.sub.4 while N.sub.2 is used 
for deposition of Si.sub.x N.sub.y Cl.sub.z which is used for sidewall 
protection for achieving high anisotopy. The present Assignee has 
previously proposed adding fluorine based gases capable of generating F*, 
such as ClF.sub.3, to the above gas system, in an amount which is not 
obstructive to anisotopy, with a view to elevating the etchrate which is 
determined by the reaction between Cl* and single crystal silicon. 
Meanwhile, low pollution is also demanded of dry etching, besides high 
anisotropy and high etchrate, as mentioned previously. Above all, in 
silicon trench etching, for which prolonged processing time is required, 
how to suppress particle generation during etching is crucial. Thus, for 
reducing pollution, it is desirable to diminish the generation of reaction 
products, such as Si.sub.x N.sub.y Cl.sub.z. However, in this case, 
anisotropy would be deteriorated because the effects of the fluorine based 
gases, added for increasing the etchrate, would be demonstrated 
pronouncedly. 
The present Assignee has also proposed a technique of etching a layer of a 
silicon based material using an etching gas consisting mainly of sulfur 
chloride, such as S.sub.2 Cl.sub.2, or sulfur bromide, such as S.sub.2 
Br.sub.2, as a gas system effective to suppress pollution by the 
particles. With this technique, anisotropy may be attained through 
sidewall protection and suppression of radical reaction which may be 
achieved by low temperature etching. If, for example, S.sub.2 Br.sub.2, is 
used, sidewall protection may be attained by a deposit mainly composed of 
sulfur dissociated from S.sub.2 Br.sub.2 and also containing SiBr.sub.x 
which is an etching reaction product. This deposit may easily be removed 
by sublimation or vaporization by heating the etched substrate to 
approximately 90.degree. to 150.degree. C. after completion of etching, so 
that there is no risk of pollution by the particles. Despite such an 
advantage, the etchrate is generally low with the prior art technique 
because the rate of the etching reaction itself is determined by the ion 
assist reaction by Cl.sup.+ or Br.sup.+. 
Thus a demand has been raised for a more excellent process in view of the 
difficulties encountered in simultaneously satisfying the requirements for 
high anisotropy, high etchrate and low pollution. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a dry etching method of 
etching a layer of a silicon based material which method will satisfy the 
above requirements simultaneously. 
As an approach to achieving high anisotropy, high etchrate and low 
pollution simultaneously, the present inventors have arrived at a concept 
of elevating the etchrate by the presence in the reaction system of a 
small amount of F* while intensive sidewall protection is assured by 
sublimable substances. Thus the present inventors have reached the concept 
of using a sulfur fluoride-sulfur chloride mixture as the etching gas 
composition. 
Among the compounds thought to be useful as sulfur fluorides are S.sub.2 
F.sub.2, SF.sub.2, SF.sub.4 and S.sub.2 F.sub.10. Besides, SF.sub.6 is 
also known as a stable sulfur fluoride compound and put to practical 
application as a dry etching gas. However, it has been demonstrated that 
the compound has a low S/F ratio (the ratio of the number of sulfur atoms 
to that of fluorine atoms in one molecule) and hence generates a large 
quantity of F* radicals, while it can hardly generate sulfur on 
dissociation by electrical discharge, so that this compound may not be 
used for the purpose of the present invention. 
Among the compounds thought to be useful as sulfur chlorides, on the other 
hand, there are S.sub.3 Cl.sub.2, S.sub.2 Cl.sub.2 and SCl.sub.2. 
Both the sulfur fluorides and the sulfur chlorides are capable of 
generating free sulfur in a plasma on dissociation by electrical 
discharge. Sulfur generated in this manner is easily precipitated on the 
substrate surface, depending on the operating conditions, if the substrate 
has been cooled to approximately below ambient or room temperature. Sulfur 
deposited on the ion incident surface may be readily sputtered off, 
whereas it continues to be deposited on the pattern sidewall where a 
lesser amount of ions are incident and where the sulfur functions as a 
sidewall protection film. Besides, high anisotropy is also assured because 
the substrate is cooled to a lower temperature and hence the radical 
reaction is suppressed to some extent. In addition, the deposited sulfur 
may easily be sublimed or burnt off by heating or processing the substrate 
with an oxygen plasma treatment after the completion of etching, so that, 
as a most crucial merit of the present invention, there is the least risk 
of pollution by the particles. 
On the other hand, Cl* derived from sulfur chlorides of F* derived from 
sulfur fluorides contribute to the etching of the layer of the silicon 
based material. The reaction by these radicals is assisted by the incident 
energy of the S.sup.+, SF.sup.+, SCl.sup.+ or Cl.sub.x.sup.+ ions. Among 
these, F* radicals are lesser in radius and may be intruded easily into 
the crystal lattices of single crystal silicon to expedite the chemical 
reaction, so that they may be said to be a highly reactive chemical 
species. For this reason, if sulfur fluorides are added to the gas mixture 
to supply F* to the etching system, the etchrate may be higher than when 
only chlorine based ions are used. 
Although attempts have been made in adding fluorine based gases for 
increasing the etchrate, it becomes necessary in this case to provide for 
intensive sidewall protection for assuring anisotropy, as a result of 
which pollution by particles is produced inevitably. The present invention 
is exempt from this problem because sulfur may be supplied from both 
sulfur chlorides and sulfur fluorides to provide for intensive sidewall 
protection and, besides, the deposited sulfur may be easily sublimed off. 
In addition, since Br is not contained in the etching gas employed in the 
present invention, there is no risk of pollution or the microloading 
effects brought about by an excess deposition of SiBr.sub.x. 
In this manner, in accordance with the present invention, high anisotropy, 
high etchrate and low pollution may be achieved simultaneously. 
Thus the present invention is highly useful in the preparation of 
semiconductor devices exhibiting high integration and high performance in 
conformity to the refined design rule.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1a and 1b, preferred embodiments of the present 
invention will be explained in detail. 
In the present embodiment, silicon trench etching was performed using a gas 
mixture of S.sub.2 F.sub.2 as a sulfur fluoride and of S.sub.2 Cl.sub.2 as 
a sulfur chloride. 
First, as shown in FIG. 1a, a substrate to be etched, or a wafer, including 
a silicon substrate 1 and a silicon oxide etching mask 2 formed thereon, 
was prepared. An aperture 3 about 0.5 .mu.m in diameter, was previously 
formed by patterning on the etching mask 2. 
The wafer was set in a magnetic microwave plasma etching device and cooled 
to about -70.degree. C. by circulating a cooling medium, such as ethanol, 
through a cooling conduit enclosed within a wafer setting electrode. The 
silicon substrate 1 was etched under the conditions of a S.sub.2 F.sub.2 
flow rate of 5 SCCM, a S.sub.2 Cl.sub.2 flow rare of 20 SCCM, a gas 
pressure of 1.3 Pa (10 mTorr), a microwave power of 850 W and an RF bias 
power of 100 W (2 MHz). 
These etching conditions have been selected for generating S.sup.+, 
SCl.sup.+, Cl.sub.x.sup.+ and SF.sup.+ ions in the plasma by discharge 
dissociation of S.sub.2 Cl.sub.2 and S.sub.2 F.sub.2, and for achieving 
high anisotropy by applying a high bias voltage under a low gas pressure. 
Thus the etching proceeded on the basis mainly of the ion assist reaction, 
at the same time that F* yielded by discharge dissociation of S.sub.2 
F.sub.2 produced a radical reaction to contribute to etching. Thus the 
high etchrate of approximately 0.5 .mu.m/min was achieved. 
In addition, with the present etching reaction system, sulfur yielded by 
discharge dissociation from both S.sub.2 F.sub.2 and S.sub.2 Cl.sub.2 was 
contacted with the wafer cooled to an extremely low temperature and was 
deposited on the pattern sidewall to form a sidewall protection film 4 as 
shown in FIG. 1b. Meanwhile, with a process in which processing is 
performed for an extended time under conditions of high incidention 
energy, such as with trench etching, the edge of the etching mask 2 tends 
to be retracted and rounded with the progress of the etching. If an 
aperture with an extremely small diameter in excess of the limit of the 
resolution of the photolithography is necessitated, a sidewall is 
occasionally formed in the etching mask 2 by etchback with reactive ion 
etching (RIE). If the mask edge is rounded in this manner, ions incident 
thereon tend to be scattered and converted into oblique incident 
components which then attack pattern sidewall sections to produce unusual 
shapes such as undercuts or bowing. In accordance with the present 
invention, since effective sidewall protection may be achieved by the 
sulfur as described above, a trench 3a with highly satisfactory 
anisotropic shape may be formed even with etching to a depth of 4 .mu.m. 
The sidewall protection film 4 may be sublimed off by heating the wafer to 
90.degree. C. or higher after the end of the etching so that no pollution 
by the particles was produced within the etching system. This heating may 
be achieved simultaneously by heating mainly intended for preventing 
dewing after low temperature etching. 
It is to be noted that the present invention is not limited to the above 
described embodiment. For example, a variety of addition gases may be 
added to the etching gas. For example, N.sub.2 may be added in expectation 
of intensive sidewall protection by reaction products. Alternatively, 
H.sub.2, H.sub.2 S or silane-based gases, which are capable of supplying 
H* and/or silicon based active species to the etching system, may be added 
for capturing excess halogen radicals for promoting sulfur deposition. 
Rare gases, such as He or Ar, may also be added for improving sputtering, 
cooling or diluting effects. 
With the above described process for silicon trench etching, it is 
intended, as premises, to form so-called deep trenches for producing 
capacitative elements. However, it may also be applied to formation of 
so-called shallow trenches used for device isolation in the preparation of 
MOS transistors. In this case, a wafer having a substrate structure 
similar to that in the gate process is etched using an organic resist 
material layer as a mask. Thus the sulfur deposited on the pattern 
sidewall may be removed for the first time by heating the wafer and 
ultimately by ashing which was originally intended for eliminating the 
organic resist material.