Plasma etching system

A relatively small amount of high frequency RF power is mixed with a predominantly low frequency RF power to provide an improved etch rate uniformity of a semiconductor wafer in a low frequency plasma etching system.

BACKGROUND OF THE INVENTION 
Low frequency plasma etching systems are well known. In such systems, a 
planar reactor is provided into which a gas mixture, such as Argon, carbon 
tetrafluoride, and fluoroform is injected at a low pressure. A pair of 
electrodes within the reactor receives power from a source of low 
frequency outside the reactor and the gas is ionized to form a plasma. A 
semiconductor wafer, such as silicon with a silicon dioxide film, is 
placed on one of the electrodes. The applied power to the electrodes and 
plasma formed within the reactor cause etching of the film in areas where 
no photoresist material is present. 
One of the problems in many low frequency plasma etching systems is that 
there are a number of non-uniformity problems which are evidenced by some 
areas of the wafer etching faster than other areas. These non-uniformities 
are of various sorts. Some of them involve very gradual non-uniformities 
over the large areas of the wafers while others are very localized. Such 
localized patterns include small spots which tend to etch very fast. In 
other cases, small, narrow rings are formed all around the wafer at a 
certain radii where etching occurs as much as 10 or 20% faster than the 
rest of the wafer. Non-uniformities are visible to the eye when the wafer 
is removed from the reactor after being partially etched. 
Non-uniformities, being visible, appear as colored interference patterns on 
a wafer with a silicon dioxide film. Some parts of the surface of the 
wafer become defective and may not yield working devices when the wafer is 
ultimately diced. In semiconductor device fabrication, sufficient over 
etch time is required to insure that the film is completely etched in the 
slowest area of the wafer. This means that in areas of faster etching, the 
substrate under the film being etched is subject to attack and possible 
damage by the plasma. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide an improved low frequency 
etching process. 
It is a further object of this invention to provide an improved low 
frequency etching process which reduces non-uniformities of high spatial 
frequency, such as fast etching rings, spots and the like. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, a plasma etching process includes 
a planar reactor having gas therein and a pair of electrodes with one of 
the electrodes disposed to receive thereon a wafer to be etched. Sources 
of power connected across the electrodes include a source of relatively 
low radio frequency signals having a predetermined output power for 
providing the main source of energy in the etching process and a source of 
relatively high radio frequency signals having a predetermined output 
power substantially lower than the source of low radio frequency signals. 
The low and high radio frequency signals are applied to a combining 
circuit with the combined signals being applied across the electrodes. The 
combining circuit also includes means for electrically isolating the 
sources of high and low frequency signals from each other. 
Other objects and advantages of the present invention will be apparent and 
suggest themselves to those skilled in the art, from a reading of the 
following specification and claims, taken in conjunction with the 
accompanying drawing.

DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a low frequency plasma etching system 10 comprises a 
source of low frequency RF signals 12. The output signals from the source 
12 are applied through a low frequency matching network 14. In a normal 
low frequency plasma etching circuit used heretofore, the output signals 
from the low frequency source would be applied across a pair of electrodes 
16 and 18 to ionize gas to form a plasma in a planar reactor 20. A wafer 
22, having suitable photoresist material thereon, is disposed on the 
electrode 18 for etching. 
The present invention involves modification of the conventional low 
frequency RF etching system. The modification involves the use of a source 
of high frequency RF signals and combining such high frequency signals 
with the low frequency signals from source 12. In the embodiment 
illustrated, low frequency signals from the matching network 14 and high 
frequency signals from the source 24 are applied to a high frequency 
matching and combining circuit 26. 
In practicing the present invention, the power of the high frequency 
signals from source 24 is increased until the desired smoothness of etch 
uniformity is obtained, as measured by the partly etched films. It has 
been found that the power of the high frequency signals from source 24 may 
be between 5% and 10% of the low frequency power from the source 12. High 
selectivity etching of SiO.sub.2 and various glass films on 100 mm and 125 
mm silicon wafers were successful when the powers applied from the low and 
high frequency sources were in these ranges. It was also found that the 
mixing of the relatively low levels of high frequency signals with the 
high levels of low frequency signals did not materially affect the process 
results in the system, but only improved the etch uniformity. 
In the embodiment illustrated in FIG. 1, typical requirements for SiO.sub.2 
etching of 125 mm wafers would be 450 Watts, 400 kHZ from the source of 
low frequency signals 12; 30 Watts at 27.13 MHz as from the source of high 
frequency signals 24, 2.0 torr process pressure in chamber 23 of the 
planar reactor 20. The electrodes 16 and 18 would be made of graphite and 
aluminum, respectively. The gap between wafer 22 and electrode 16 would be 
about 0.25 inches with a gas flow of about 200 sccm for Argon, 40 sccm for 
CF.sub.4 and 40 sccm for CHF.sub.3 gas. 
The high frequency matching and combining circuit 26 may take a variety of 
different forms. One such form is illustrated in FIG. 2. The purpose of 
the circuit 26 is to isolate the low and high frequency sources 12 and 24 
from each other. 
Referring to FIG. 2, a high frequency trap 28 is tuned to the frequency of 
the high frequency source 24, for example 27.13 MHz, and comprises a 
capacitor 30 and an inductor 32 connected in parallel. The trap 28 
prevents signals from the source 24 being fed back to the source 12. At 
the same time, low frequency signals from the source 12 are permitted to 
pass through the trap 28 without being attenuated. 
A capacitor 34 effectively isolates the high frequency source 24 from the 
low frequency signals which pass through the trap 28. 
A match network comprising a capacitor 35, an inductor 36 and the capacitor 
34 matches the impedance of the high frequency source 24 at line 38 to the 
load. In one embodiment, the impedance of the high frequency source 24 may 
be 50 ohms, while the load impedance approximates 300-400 ohms resistance 
in parallel with 100-200 pF capacitance. 
The capacitors 35 and 34 are adjustable to provide minimum high frequency 
power being reflected back to the source 24 while plasma is being excited 
by the low frequency signals from the source 12. When the low frequency 
discharge is not present, the load impedance changes greatly, and the high 
frequency signal from the source 24 is poorly matched with the load. 
The theory as to why the addition of a relatively small level of high 
frequency signals improves a low frequency etching system is not precisely 
understood. The operating hypothesis, however, is that adding the high 
frequency discharge maintains a plasma at all times. It is known that in 
low frequency etching processes involving 400 kilohertz alone that the 
plasma essentially goes out on alternate half cycles. This is evident by 
viewing the light coming out with a photomultiplier from the plasma which 
goes off at every zero crossing as nearly as can be detected. 
When the 27 megahertz signals are added, there is sufficient power to 
maintain a steady discharge within the plasma chamber even without the low 
frequency signals. It is a weak discharge, but it is there. The ions in 
the plasma chamber maintain optical emission all the time. It is possible 
that the instabilities and non-uniformities in the etched wafers arise as 
a result of the constant turning off and turning on of the plasma. By 
maintaining a sort of a uniform ionization over the whole wafer all the 
time, the etching becomes more uniform.