Dry etching apparatus

A dry etching apparatus using microwaves according to the present invention is equipped with means for impressing such an AC voltage upon a sample as has a frequency ranging from 100 KHx to 10 MHx. Consequently, the sample has its surface prevented from being charged up no matter which it might be made of an insulator or might have its surface covered with an insulator. As a result, the etching rate can be maintained at a high level even for such sample.

BACKGROUND OF THE INVNETION 
1. Field of the Invention 
The present invention relates to an equipment to manufacture solid surfaces 
with ion-using technology, and more particularly to improvements in an 
apparatus for etching the solid surfaces with ions in plasma. 
2. Description of the Prior Art 
In a semiconductor field or in a field requiring for the etching technique 
as fine as several microns or less, the recent increase in share is 
directed to the dry etching process which resorts to the use of either an 
ion beam or the ions or active atoms (or compounds) in plasma. As compared 
with the former method using the ion beam, the latter method using the 
ions or the like in the plasma, e.g., the RF (Radio Frequency) sputter 
etching method or the microwave plasma etching method has a higher ion 
density so that it is featured by a higher etching rate. Therefore, there 
have been made a variety of investigations, among which a method for 
generating the plasma with the use of microwave discharge is found 
especially excellent because: (1) since the discharge can be broken down 
even under a low pressure (which is lower than 1.times.10.sup.-3 Torr), 
the direction of the ions is uniform; (2) it is possible to generate the 
plasma of especially high density; and (3) since the discharge is broken 
down without any electrode, the sample can be prevented from having its 
surface contaminated, and active gases can be used (e.g., U.S. Pat. No. 
4,101,411). 
However, the microwave plasma etching process has a fault that its etching 
rate is lower than that of the RF sputter etching process which has been 
frequently used according to the prior art. This is because the sample in 
the microwave plasma etching apparatus at the initial stage is held under 
a floating potential (about -20 volts) with respect to the plasma. 
In order to eliminate this fault, therefore, there has been made a 
proposal, in which a higher negative potential than that floating 
potential is impressed upon the sample. More specifically, if the sample 
is supplied with a negative voltage from the outside, the ions are 
accelerated to impinge upon the sample surface so that the etching rate is 
increased. 
As a result, in case the sample is made of a conductor, the etching rate 
can be increased by the method described in the above. Here, the sample to 
be etched in a semiconductor process has its surface made of an insulator 
such as a film of SiO.sub.2 or a photo-resist. Consequently, the 
insulating surface is charged up by the ions. As a result, the negative 
voltage impressed upon the sample from the output is offset to apply no 
voltage to the sample so that the etching rate cannot be finally 
increased. 
SUMMARY OF THE INVENTION 
It is therefore, an object of the present invention to provide a dry 
etching apparatus which can etch a sample, even if its surface is made of 
an insulator, with the use of microwave plasma without any reduction in 
the etching rate. 
In order to attain the above object, the microwave plasma etching apparatus 
according to the present invention is characterized in that there is 
provided means for impressing an AC voltage upon the sample and in that 
the frequency of that AC voltage is set to range from 100 KHz to 10 MHz. 
According to the constructional characteristics of the present invention, 
the etching process can be carried out without any reduction in its rate 
even if the sample is made of an insulator or has its surface covered with 
an insulator. 
Incidentally, the reasons for limiting the frequency range are as follows: 
If the frequency of the AC voltage impressed becomes lower than the lower 
limit of 100 KHz, the charge-up preventing effect of the insulating sample 
almost disappears; and If the frequency exceeds the upper limit of 10 MHz, 
an impedance matching circuit is required, when the AC voltage is 
introduced into the etching apparatus, so that the construction of the 
apparatus becomes complex. 
In other words, the charge-up preventing mechanism according to the present 
invention can be analyzed in the following. During one half cycle in which 
the negative voltage of the high frequency AC voltage is impressed upon 
the insulating sample, the ions from the plasma impinge upon the sample 
surface thereby to effect the etching treatment. Since, however, the 
sample is made of an insulator, the coming ions other than those which are 
coupled and neutralized with the electrons having already impinged will 
effect the charge-up in the sample surface. During the next half cycle of 
the high frequency AC voltage, i.e., the half cylce in which the positive 
voltage is impressed upon the insulating sample, the ions from the plasma 
impinge upon the sample surface so that they are coupled and neutralized 
with the ions which have been charged up in the sample surface. During 
this neutralization, the etching treatment is not accomplished. During the 
subsequent half cycle, since the insulating sample is supplied with the 
negative voltage, the ions from the plasma enter the sample surface 
thereby to effect the etching treatment. Similar phenomena are repeated on 
and on. Specifically, the etching and neutralizing actions are alternately 
effected so that the charge-up during the etching treatment can be 
eliminated by the subsequent neutralizing treatment. On the other hand, 
for a low frequency, the charge-up preventing effect is not attained for 
the following reason. That is to say, the insulating sample can be thought 
to constitute a kind of condenser having a preset capacity. As a result, 
for the low frequency, the sample has such a high impedance as to make it 
hard for the AC current to flow. As a result, it becomes almost impossible 
to attain the charge-up preventing effect. For a high frequency higher a 
preset level, on the contrary, the impedance is so reduced as to make it 
feasible for the AC current to flow so that the charge-up preventing 
effect can be attained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the diagrammatical construction of a microwave plasma etching 
apparatus according to the present invention. As shown, the microwaves (of 
2.45 GHz), which are generated by a microwave generator (e.g., a 
magnetron) 1, are guided through a waveguide 2 until they are absorbed by 
the etching gases (which are introduced through a leak valve 6), which are 
controlled to a pressure lower than 1 atm. and which are reserved in such 
a discharge room 3 as is made of an insulator and as is placed in mirror 
magnetic fields established by mirror magnetic field making coils 4 and a 
permanent magnet 5. An etching room 7 has its inside composed of a gas 
inlet 17, an evacuating outlet 18 and a substrate stage 9 made of a 
conductor. Here, the area of the discharge room 3 and the etching room 7 
forms a vacuum volume. The active ions in plasma 19 thus generated move 
along the mirror magnetic fields to impinge a sample 8 so that they etch 
the surface of the sample 8. At this time, the sample 8 and the substrate 
stage 9 are supplied with a high frequency AC voltage, as shown in FIG. 1. 
The voltage impressing means will be described in the following. A high 
frequency electric power supply 10 is connected with the primary coil of a 
transformer 20 which has no core and which has a ratio of about unit 
between the input and output voltages. The transformer 20 has its 
secondary coil connected at its one terminal with the substrate stage 9 
and at its other with a condenser 12. This condenser 12 has its other 
terminal grounded to the earth. Here, the position of the condenser 12 may 
be located at the side of the substrate stage 9. On the other hand, an 
insulator 11 is mounted in the wall of the etching room 7 thereby to 
electrically float the substrate stage 9. Since the substrate 9 is made of 
an electric conductor, the high frequency voltage impressed thereupon is 
simultaneously applied to the sample 8. In short, the closed circuit at 
the secondary side of the transformer 20 is composed of the earth--the 
wall of the etching room 7--the plasma 19--the sample 8--the substrate 
stage 9--the secondary coil--the condenser 12--the earth. The condenser 12 
is made to have a capacity of about 0.1 .mu.F and to act to isolate the 
sample 8 from the earth in a direct current manner while allowing only a 
high frequency current to flow. By the use of the condenser 12, it is 
possible to restrict the electron current which flows as a direct current 
from the plasma 19 into the surface of the sample 8. As a result, it is 
possible to restrict the unnecessary temperature rise of the sample 8. The 
transformer 20 employs a coreless transformer, which may be replaced by a 
transformer using a core material if this material has good high frequency 
characteristics and large permeability. 
The effects of the present invention will be described in the following. 
FIG. 2 illustrates the changes coming from the kinds of the sample 8 by 
taking the frequency of the AC voltage to be impressed upon the sample 8 
in an abscissa and by taking the etching rate at that time in an ordinate. 
According to the measuring conditions in this instance: C.sub.2 F.sub.6 
+He (10%) is used as the etching gases; (1) a substrate of Si and (2) a 
substrate of Si having a surface of SiO.sub.2 having a thickness of 1.6 
.mu.m are prepared as the sample 8; and a voltage of 120 V is used as the 
amplitude of the AC voltage to be impressed. A curve 21 corresponds to the 
case, in which the sample 8 is made of a substrate (1), i.e., Si. Si is a 
semiconductor so that no charge-up phenomenon takes place. Consequently, 
the etching rate can be substantially constant irrespective of the 
frequency of the voltage impressed. On the contrary, a curve 22 
corresponds to the case, in which the sample 8 is made of a substrate (2), 
i.e., Si having a surface of SiO.sub.2. As is apparent from this curve, 
when the frequency of the voltage impressed is gradually increased from 0 
(i.e., the condition under which a negative voltage is impressed upon the 
sample (2)) until the frequency becomes higher than 100 KHz, the etching 
rate is also gradually increased until it is saturated in the vicinity of 
about 1 MHz. This can be explained by that the charge-up phenomana are 
gradually decreased, when the frequency becomes higher than 100 KHz, until 
they completely disappear at the frequency of about 1 MHz. As is apparent 
from the curve 22, more specifically, the etching rate can be 
substantially doubled in case the AC voltage of 1 MHz is impressed, as is 
different from the conventional case in which no AC voltage is impressed 
upon the sample (2) (i.e., the condition under which a direct negative 
voltage is impressed upon the sample (2)). 
The difference in the etching rate between the Si indicated by the curve 21 
and the SiO.sub.2 indicated by the curve 22 results from that in the 
material. These different etching rates are varied with the kind and 
pressure of the etching gases, but the tendency of the voltage to be 
impressed for preventing the charge-up against the frequency is absolutely 
the same. 
Turning to FIG. 3, the etching rate is plotted against the amplitude of the 
AC voltage to be impressed so that the changes coming from the kinds of 
the sample 8 may be illustrated. According to the measuring conditions in 
this instance, however, C.sub.2 F.sub.6 gases are used as the etching 
gases, and the frequency of the impression voltage is set at 800 KHz, but 
the sample is the same as that which has been used in the example 
explained with reference to FIG. 2. 
The influences from the amplitude of the impression voltage are such that 
the etching rate is increased with the increase in the amplitude, as shown 
in FIG. 3. This tendency is common between the case (for a curve 23), in 
which the sample is made of Si, and the case (for a curve 24), in which 
the sample is made of SiO.sub.2, and is expressed by a substantially 
linear increase while leaving a difference in the gradient between the two 
curves. As is apparent from FIG. 3, in the case of the sampe of SiO.sub.2 
(or the curve 24), the etching rate for the case, in which the amplitude 
of the impression voltage is 150 V, becomes four times of that for the 
case of the zero impression voltage (i.e., for the conventional case, in 
which no AC voltage is impressed upon the sample). 
Another embodiment of the present invention will now be described in the 
following. FIG. 4 shows the diagrammatical construction of the dry etching 
apparatus according to the second embodiment. The differences from the 
apparatus shown in FIG. 1 reside in the two points: that a rotating stage 
13 is mounted in the etching room 7; and that the means for impressing the 
AC voltage upon the sample 8 placed upon the rotating stage 13 is made 
different. Therefore, only these differences will be described in detail 
while others being omitted. In order to prevent the temperature rise in 
the sample 8 and to etch a number of the samples 8 at a time, it is known 
effective to turn the sample 8. As concrete means for those purposes, the 
sample 8 is placed through a substrate stage 14 made of an insulator upon 
the rotating stage 13 which is fixed to a shaft of rotation 25. The means 
for impressing the AC voltage upon the sample 8 on such rotating stage 13 
will be described in the following. First of all, a rotary coil 16, which 
is fixed to the rotating stage 13, is connected in series with the 
condenser 12. The rotary coil 16 has its other terminal contacting with 
the sample 8, whereas the condenser 12 has its other terminal connected 
with the rotating stage 13. And, this stage 13 itself is grounded to the 
earth. There is no fear of the sample 8 and the rotating stage 13 being 
short-circuited because they are isolated by the substrate stage 14 which 
is made of an insulator. Here, the connection between the sample 8 and the 
other terminal of the rotary coil 16 may be made in the following manner. 
Namely, the substrate stage 14 made of the insulator is formed at the side 
of the sample 8 with a conducting layer, with which is connected the other 
terminal of the rotary coil 16. Thus, the desired electric connection can 
be attained more reliably and simply. 
A stationary coil 15, which is not rotated, is connected through another 
transformer 26 with the high frequency electric power supply 10. It is 
quite natural that the stationary coil 15 may be connected with the high 
frequency electric power supply directly not through the transformer 26. 
There is connected between the rotating stage 13 and the sample 8 placed 
thereon a series circuit which is composed of the rotary coil 16 and the 
condenser 12. Incidentally, the condenser 12 need not be used for each of 
the rotary coils 16 but can be commonly connected therewith. With these 
construction arrangements, at the instant when the rotating stage 13 is 
turned so that the rotary coil 16 placed on the rotating stage 13 reaches 
a position above the stationary coil 15, an inductive coupling takes place 
between the stationary coil 15 and the rotary coil 16 so that the high 
frequency voltage induced in the rotary coil 16 is impressed upon the 
sample 8. Since the rotating srage 13 is rotating with a constant speed, 
the coupling between the stationary coil 15 and the rotary coil 16 is 
promptly weakened. Since, at this time, the sample 8 has already been 
dislocated from the etching region by the plasma 19, no charge-up 
phenomenon takes place in the surface. Thus, the respective samples 8 
never fail to be supplied with the AC voltage unless they are subjected to 
the etching treatments by the plasma 19 so that they are brought into the 
same condition as that of the apparatus shown in FIG. 1. 
Here, in case the permanent magnet 5 is made of a conductor, an eddy 
current is established in the permanent magnet 5 by the high frequency 
waves if the stationary coil 15 is placed just above the permanent magnet 
5. In this instance, similar effects can be obtained by placing the 
stationary coil 15 at a position, which is spaced by a preset angle of 
rotation from the permanent magnet 5, so that the rotary coil 16 may come 
to a position above the aforementioned stationary coil 15 when the sample 
8 connected therewith just enters the plasma 19. 
On the other hand, since the amplitude of the aforementioned high frequency 
voltage, i.e., 100 to 150 V is remarkably low in comparison with that of 
the voltage, which is to be impressed upon the sample by the conventional 
RF sputter etching apparatus, i.e., several KV, the damage which is to be 
exerted upon the sample during the etching treatment can be wholly 
neglected. Moreover, by suitably selecting the amplitude of the high 
frequency voltage and the pressure of the etching gases, not only the 
etching rate but also the sectional shape of the sample during the etching 
process can be controlled. 
Still moreover, the waveform of the AC voltage to be impressed may be 
either positive or negative pulses of sine or square shape. In short, it 
is sufficient that positive and negative voltages can be alternately 
applied to the sample. Incidentally, the experiments conducted by applying 
the pulses have revealed that the higher etching rate can be attained by 
making the duration time of the positive voltage longer than that of the 
negative voltage. This is thought to come from the fact that the ratio of 
the etching period in one cycle can be enlarged by making the during time 
of the negative voltage than that of the positive voltage so that the 
etching rate can be improved. 
According to the dry etching apparatus of the present invention thus far 
described, the sample can be etched without any reduction in the etching 
rate no matter which it is made of an insulator or a conductor covered 
with an insulator. As a result, the etching time is shortened while 
reducing the production cost.