Method for producing deep vertical structures in silicon substrates

A method is provided for producing deep substantially vertical structures in silicon substrates, wherein in a first step, the silicon substrate is anisotropically plasma etched to a first predetermined depth, thereby creating a first structure. Subsequently, the surface of the substrate is covered conformally with an etch-resistant coating, and the horizontal parts of said coating are selectively removed. Following this removal, the substrate is anisotropically plasma etched at low temperatures to a second predetermined depth with a mixture of SF.sub.6 /O.sub.2, whereby a second structure is created. Finally, the vertical parts of the coating are removed.

FIELD OF THE INVENTION 
The present invention relates to anisotropic etching in silicon substrates 
and especially to a method for producing deep substantially vertical 
structures in silicon substrates at low temperatures. 
BACKGROUND OF THE INVENTION 
Anisotropic etching in silicon requires gas mixtures which are able to 
cause an "in-situ" passivation of the side walls of the structure to be 
etched. The process parameters are adjusted so that, e.g., carbon 
containing polymers will form on the sidewall of the structure (of. P. H. 
Singer, "Today's Plasma Etch Chemistries", Semiconductor International, 
vol. 11, no. 4, p. 68 (1988)). 
For the manufacture of the so called "deep capacitor trenches", Y. T. Lii 
et al., Electrochem. Soc. Proc., Vol. 92-1, p. 158 ff., as well as J. A. 
Bondur et al., Electrochem. Soc. Proc., Vol. 90-1, p. 176 ff., use gas 
mixtures which comprise HBr, He/O.sub.2 or NF.sub.3. Typical etch rates 
(in RF plasmas) are 0.1 .mu.m/min to a maximum of 0.7 .mu.m/min and 
typical etch depths amount to a maximum of 20 .mu.m. However, for the 
manufacture of micromechanical structures, substantially higher etch rates 
have to be achieved. Using SF.sub.6 in combination with compact microwave- 
or helicon plasmas, etch rates up to 10 times greater can be realized. 
Lower temperature or cryogenic etching has been shown to improve etch 
anisotropy (cf. W. Varthue et al., "Electron Cyclotron Resonance Plasma 
Etching of Photoresist at Cryogenic Temperatures", J. Appl. Phys., vol. 
72, no. 7, p. 3050 (1992)). This result is thought to occur because 
spontaneous sidewall reactions are reduced at lower temperatures. The 
reduced spontaneous reaction rate reduces the etching of the sidewalls. 
However, for very great etch depths, sharp anisotropic etch profiles and, 
simultaneously, high etch rates can not be achieved by simply etching at 
cryogenic temperatures. When etching to great depths, typically greater 
than 50 .mu.m, the side wall passivation in the area of the surface of the 
etched structure will, in turn, be destroyed and an isotropic profile 
showing a partially destroyed side wall results (1 in FIG. 1, FIG. 2). 
It is therefore an object of the present invention to provide a 
cost-effective and reliable method for producing deep substantially 
vertical structures in silicon substrates. 
It is a further object of the invention that these structures can be 
produced at high etch rates thereby showing substantially anisotropic 
profiles. 
BRIEF SUMMARY OF THE INVENTION 
These objects are achieved by a process comprising the steps of: 
a) anisotropically plasma etching a portion of the silicon substrate to a 
first predetermined depth thereby creating a first structure, 
b) conformally coating the surface of said silicon substrate and said 
anisotropically etched portion with an etch-resistant coating, 
c) selectively removing the horizontal parts of said coating, 
d) anisotropically plasma etching said anisotropically etched portion of 
said silicon substrate at low temperatures to a second predetermined depth 
with a mixture of SF.sub.6 /O.sub.2, and 
e) removing the vertical parts of said coating, thereby forming a deep 
substantially vertical structure. 
By means of this process, the area of the sidewalls lying near the surface 
of the substrate is, in addition to the insufficient "in-situ"-coating, 
protected by a specific protective coating. 
The sidewall passivation thus is sufficient to protect the pre-etehed 
structure so that the sidewalls are not destroyed when the substrate is 
further etched. 
It was found that adding O.sub.2 to the SF.sub.6 gas at low temperatures 
results in creating substantially anisotropic profiles in the substrates. 
These and other objects, features and advantages will become apparent from 
the following detailed description of the various aspects of the invention 
taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The process sequence for the method according to the invention is 
schematically shown in FIG. 3. According to FIG. 3a), the substrate 2 is 
first of all etched anisotropically, e.g. by RIE (reactive ion etching) or 
by using a gas mixture of SF.sub.6 /O.sub.2 through a first etch mask (not 
shown) to a first predetermined depth d.sub.1 by means of an anisotropic 
etch step. SiO.sub.2 as an etch mask material was found to be 
advantageous. Thus a first structure is created. The value of this first 
predetermined etch depth depends on the desired overall depth of the fully 
etched structure. It is in the range of about 10 to about 50 micrometer, 
preferably about 40 micrometer. For this first etching step ail suitable 
etchants could be used and the etching can take place either at room 
temperature or at lower temperatures. It has, however, been found that a 
mixture of SF.sub.6 and O.sub.2, especially at low temperature is 
advantageous. 
When the first depth d.sub.1 has been reached, the etching is stopped and 
the substrate 2 is conformally coated with an etch-resistant coating 4 
(FIG. 3b)). This coating can, e.g., be made of thermal oxide. In this case 
the substrate has to be removed from the etching reactor and is coated 
with SiO.sub.2 or other suitable thermal oxide in a separate deposition 
chamber. After deposition, the substrate is again entered into the etching 
reactor and the horizontal parts 6 of the oxide coating are removed by an 
anisotropic etch step, e.g., using argon. This anisotropic etch step is 
preferably performed with an argon flow of 200 sccm at 0.4 Pa and -200 V 
DC-bias. 
After this etch step has been performed, only the vertical parts 8 of the 
coating will remain (FIG. 3c)). 
In the next step, the silicon will be etched with a gas mixture of SF.sub.6 
and O.sub.2 at low temperatures (using the first etch mask) until a second 
predetermined etch depth d.sub.2 is reached (FIG. 3d)). Thus a second 
structure is created. The value of this second predetermined etch depth is 
in the range of about 30 to about 90 micrometers, preferably about 50 
micrometers. In this context, low temperatures shall mean such 
temperatures where acceptable high etch rates, i.e. in the range of 2-6 
.mu.m/min can be expected. It was found that the temperature should be in 
the range of about -80.degree. C. to -120.degree. C. and preferably is 
-100.degree. C. 
After removal of the remaining protective coating 8 (and the remaining 
mask), e.g., by etching with a suitable etchant, the desired profile is 
achieved (FIG. 3e)). 
The steps as shown in FIGS. 3a-3e may be repeated as necessary to achieve 
the desired structure. 
As already mentioned above, when using thermal oxide as a protective 
coating, the substrate has to be removed from the etch reactor to be able 
to deposit the oxide in a separate deposition chamber. 
In an especially advantageous embodiment of the present invention, the 
protective coating is therefore formed of an ice film. In this case, the 
substrate can be left in the etch reactor when applying the protective 
coating. This can be realized by the following process steps: 
In the first step, the substrate is anisotropically etched to the first 
predetermined depth d.sub.1. This can be achieved by RIE etching or by 
using a gas mixture of about 5 to 30% O.sub.2 and about 95 to 70% 
SF.sub.6. In an especially advantageous embodiment a gas mixture of 20% 
O.sub.2 and 80% SF.sub.6 is used. The addition of a small amount of 
O.sub.2 to SF.sub.6 at low temperatures has been shown to further improve 
etch anisotropy. The substrate is preferably etched for a time of about 10 
minutes applying a microwave power of about 1500 W, and a substrate bias 
of about -25 V at a pressure of about 1 Pa. In this case the etching was 
performed at -100.degree. C., a suitable temperature range is about 
-80.degree. C. to about -120.degree. C. After the etching has been 
completed, the plasma is switched off and the reactor is pumped down to a 
basis vacuum of about 10.sup.-3 Pa. 
In a second step, water vapor is condensed on the so structured surface of 
the substrate. FIG. 5 schematically shows a respective arrangement. In 
case the first etching step was performed at room temperature, the 
substrate has first to be cooled down to the condensation temperature. By 
means of a metering valve 10 water vapor is introduced into the reaction 
chamber 12 from a little vacuum vessel 14 filled with water. The water 
vapor condenses on the cooled substrate supported by a succeptor 16 and 
forms an ice film on the surface of the first structure produced during 
the first etching step. For this step a water partial pressure of about 
0.1-0.5 Pa has turned out to be suitable. Within a few minutes a 
substantially isotropic ice film having a thickness of a few micrometers 
is formed on the substrate. The formation of this film can be controlled 
"in-situ" by means of an interferometer 18. A film thickness on the 
horizontal parts of the coating of between 1 and 1.5 .mu.m has proved to 
be appropriate. Since only half of the half-space at the sidewall is 
available for adsorption if a step is present in the substrate, the 
thickness of the ice film at the sidewall should be about half the 
thickness of the ice film on the horizontal parts of the coating. The 
sublimation rate at a temperature of about -100.degree. C. is so small 
that the so formed ice film is stable over a longer period of time. 
The third step of the method according to the invention comprises sputter 
etching the ice film with a suitable etchant, e.g., argon, on the 
horizontal parts of the substrate whereas the film will retain on the 
vertical parts due to the strong anisotropy of the etching step (FIG. 
3c)). Suitable process parameters for this step are e.g., an argon gas 
flow of 20 seem, a chamber pressure of 0.4 Pa, a microwave power of 1500 W 
and a DC-bias of -200 V. The end of the etching step can also be monitored 
by means of an interferometer 18. 
Subsequently, the etching of the silicon is continued until the desired 
etch depth is achieved. After the etching at low temperature the substrate 
is brought to room temperature. The ice film acting as a passivation layer 
on the sidewall thereby evaporates without leaving any residue. The result 
of this process is shown in FIG. 4. 
By means of the present invention, deep substantially vertical structures 
can be produced in silicon substrates using high etch rates. 
The method according to the present invention is especially advantageous 
for the formation of guide plates, e.g., for use in test probes. 
It will be apparent to those skilled in the art having regard to this 
disclosure that other modifications of this invention beyond those 
embodiments specifically described here may be made without departing from 
the spirit of the invention. Accordingly, such modifications are 
considered within the scope of the invention as limited solely by the 
appended claims.