Method for controlling etch rate when using consumable electrodes during plasma etching

A method and apparatus to improve process control during plasma etching of semiconductor substrates. Improvements are directed towards controlling the rate of etching when using consumable electrodes. Consumable electrode materials are used to increase selectivity in certain plasma etching processes as in via. contact. or in SOG etch. A consumable electrode material has a significant effect on processing time due to changing gap dimension between electrodes. This invention teaches how to adjust for process variables by using feedback from two strategically placed pressure manometers.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention applies generally to plasma etching of semiconductor 
materials and more particularly to a novel process and apparatus for 
maintaining the gap between a consumable upper electrode and a lower 
electode, thereby, maintaining a more effective etching rate. 
2. Description of the Prior Art 
The following three documents relate to various methods dealing with 
moveable electrodes in plasma etch systems. 
U.S. Pat. No. 5,354,413 issued Oct. 11, 1994 to Smesny et al, discloses an 
electrode position controller for a semiconductor etching device that is 
calibrated each time the etching device is turned on. 
U.S. Pat. No. 5,344,542 issued Sep. 6, 1994 to Mahar et al, teaches a 
plasma etch system that has electrodes that are moveable so as to provide 
a selectable gap for either a single or multiple step processing modes. 
U.S. Pat. No. 5,336,355 issued Aug. 9, 1994 to Zarowin et al, shows a 
method and apparatus for confinement of a plasma etch region for precision 
shaping or contouring surfaces of substances and films. 
It is well known that during the manufacturing of silicon-based 
semiconductor devices the goal of the contact or via etch processes is to 
obtain a minimum dimension, high aspect ratio hole in silicon dioxide with 
straight walls, and selectivity to etch silicon or polysilicon. 
The gas ratio of F:C (Fluorine:Carbon) limits the selectivity of silicon 
dioxide to etch either silicon or polysilicon during the etching process. 
Varying this ratio directly affects selectivity. Fundamentally, 
selectivity can be increased by reducing the F radical concentration or by 
increasing the C radical concentration. The fluorine atoms contribute to a 
faster etch rate on silicon than on silicon dioxide thereby reducing 
selectivity. Carbon atoms, on the other hand, operate as a polymer source 
that slows the etch rate thus increases selectivity. It has been found 
that selectivity can be increased significantly by adding H.sub.2 which 
reacts with F to form HF so that the F concentration in process is 
reduced. It is also found that selectivity is reduced by adding oxygen, 
which combines with carbon from reaction gas of CF.sub.3 and liberates F 
so that the F radical concentration is increased. 
In addition to adding a different gas to achieve a desired selectivity, the 
same result is achieved by using a commercially available electrode made 
from either silicon or graphite materials for varying the F:C ratio. 
Silicon works to dissipate the F radical concentration while graphite 
provides the carbon source to vary the F:C ratio. The electodes erode with 
time thus affecting the etch rate of silicon dioxide. The rate may 
increase or decrease as the electrode is consumed. 
The gap dimension and parallism between the two electrodes are critical 
calibration steps that are performed manually during machine maintenance. 
This procedure is sufficient for non-consumable electrodes due to its 
planar and smooth surface, however, when using consumable electrodes, the 
profile erodes forming a concave shape with enlarged gas distribution 
holes thereby precluding accurate calibration. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus to improve process 
control and selectivity during plasma etching of semiconductor substrates 
when using consumable electrodes. 
It is a primary object of the present invention to improve etch rate by 
providing an in-process feedback that is used for updating gap counts for 
automatically driving a gap driver for adjusting the gap height to a home 
position for consumable electrodes. 
It is another object of the present invention to further improve the etch 
rate by providing a method and apparatus for monitoring and holding the 
gas pressure gradient between electrodes to a desired and constant value. 
In accordance with the objects of this invention, a new method is provided 
to improve process control during plasma etching of semiconductor 
materials. Gap height can be automatically adjusted on-line or off-line. 
On-line denotes that the gap controller adjusts the gap height assuring 
the upper electrode gap pressure (GP) equals set point for each part 
before the etch process begins. Off-line denotes that adjustment is done 
only after a preset number of wafers have been processed. When the preset 
number of wafers are completed and the GP still equals the set point 
(meaning the electrode gap is still within an acceptable gap dimension), 
the wafer count is reset to zero and the etching process continues without 
need to reset the gap height. 
Two capacitance manometers are used, to obtain and control the pressure 
gradient between the upper and lower electrodes. The first manometer 
senses the chamber pressure (CP) and provides pressure feedback to a 
pressure controller that adjusts a throttle valve to proportionately 
maintain a constant chamber pressure. A second manometer is used to 
provide gap pressure feedback to a gap controller thus converting the 
upper electrode gap pressure into dimensional gap counts used for driving 
the gap driver to a new position. The etching process does not start until 
the CP and GP meet set point. The desired control options, on-line or 
off-line, gap height, tolerances, CP, GP, and wafer count can be written 
in process recipes or installed in equipment configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An aspect of the present invention comprises a method and an apparatus for 
controlling the etch rate of a semiconductor wafer utilizing consumable 
electrodes in a plasma etching apparatus. Some of the upper electrodes are 
designed to react with ionized gas to increase selectivity, for example, 
silicon doped with boron is used in the silicon dioxide (SiO2) etching 
process. Because the upper electrode's surface wears, its parallism and 
planarity relation to the bottom electrode is diminished with time and 
requires periodic replacement. Since most of the present processes use a 
fixed distance between the upper and lower electrodes, the pressure 
gradient between them becomes an important component in preserving etch 
rate in a pressure dominant process. While the upper electrode is 
consumed, the pressure gradient between electrodes shifts, thereby, 
affecting the etch rate. To counteract the effects of upper electrode 
consumption, gap height adjustment is necessary. 
Referring now to the drawings, and FIG. 1 in particular, there is 
illustrated a plasma etching apparatus or chamber 3 is generally indicated 
comprising a chamber wall 6, evacuation ports 18 and 19 which are 
connected to throttle valve 20 to an external pumping source 22 to 
maintain the desired pressure in chamber 3 and at least one interlock 15 
through which a wafer 10 can be introduced into chamber 3. Within chamber 
3 is an rf source (not shown) and an external power supply (not shown) 
that are used to generate the plasma field 9, and a wafer support pedestal 
resting on the bottom electrode 5 upon which a wafer 10 is positioned for 
etching by the plasma generated by the rf source. An inlet port 21 is also 
provided in chamber wall 6 for admission of process gas into etching 
chamber 3 from an external process gas source (not shown). 
In accordance with the invention, means are provided for adjustment of gap 
height 50 by either on line or off-line process adjustment contained in 
set point control box 41 modes. The pressure gradient within the gap 
height 50 is used as feedback given by two manometers. The first manometer 
8 senses the pressure within the chamber 3 by way of part 17 and is 
controlled by pressure controller 34 via throttle valve 20. This pressure 
is widely used as the process pressure representing the pressure around 
the lower electrode 5 and wafer 10. The second manometer 7 senses the 
pressure between the upper 4 and lower 5 electrodes within the plasma 
field gap 9. Referring now to FIG. 2, illustrating the underside surface 
of the upper electrode 4, the average pressure between the upper and lower 
electrode is transmitted from three ports 11, 12, 13 into pipe 1 and 
thereon to the gap controller 33. The three ports are equally spaced on 
the face of the upper electrode clamp 16 dividing the circle into 
120.degree. between port centers to produce a uniform pressure feedback 
between the electrodes. The pressure feedback is used to drive the gap 
driver 32 via gap controller 33. Based on this feedback, the gap 
controller repeatedly calculates new gap counts for the gap driver 32 to 
drive the gap assembly to a destined position. The etching process will 
not begin unless both the chamber and gap pressures match preset process 
parameters. The desired control options on-line or off-line, gap height 
tolerances, chamber and gap pressures, and wafer count can be set in 
process recipes or equipment configuration. 
The on-line and off-line process flow charts are rendered in FIGS. 3A and 
3B. The on-line process 61 shown in FIG. 3B provides gap height 
compensation for each wafer before the etching process begins by comparing 
the gap pressure to the gap set point pressure. The off-line process 60 
differs since the process continues until the preset wafer count reaches a 
preset count and then, and only then, is the gap pressure compared to the 
gap set point pressure. If the gap pressure is not in agreement with the 
set point pressure, then the gap driver readjusts the gap in response to 
the gap controller. If the gap pressure is in agreement, the wafer count 
is reset to zero and the process continues. 
Process selectivity is a major consideration of the present invention with 
methods to neutralize the effects of electrode consumption on etch rate 
during certain etching processes as in contact, via, or in SOG etching. 
Comparing FIG. 4 with FIG. 5 typifies the erosive effects when using 
consumable electrodes materials. FIG. 4 illustrates an unused consumable 
upper electrode 4 having a uniform and parallel gap 71 relative to lower 
electrode 5. FIG. 5 embodies the reduced surfaces of upper electrode 4 in 
the active etching areas after prolonged use. The concave surface of the 
upper electrode 4 and enlarged gas distribution holes 24, though 
exaggerated, are indications of the wearing results of the plasma etching 
process. Dimensions of gap 72 and gas distribution hole diameters 24 in 
FIG. 5 are no longer equal to gap 71 and gas distribution hole diameters 
23 in FIG. 5. 
FIG. 4, in a self explanatory manner, also illustrates the contrapositive 
effect of non-consumable electrodes, that is, no change in gap 71 or in 
the gas distribution hole diameters 23 after prolonged use. 
One important factor of plasma etching is the chamber pressure that is 
monitored by a manometer giving feedback to a pressure controller for 
opening or closing a throttle valve to stabilize the desired chamber 
pressure. The second important factor is the gap height, the distance 
between the upper and lower electrodes preset to a given value and driven 
by a gap driver. Because of the erosion of the upper electrode, the preset 
gap height is no longer the desired value. Although the chamber pressure 
can be controlled at set point, the pressure gradient between the upper 
and lower electrodes has changed. The pressure gradient represents the 
dynamics of chamber gas flow. As the pressure gradient changes, etching 
time increases or reduces. 
Referring to FIG. 6, tests were conducted using a Lam Research Rainbow 
Model 4520 Reactive Ion Etcher 81. The chamber 82 included a 400 kHz RF 
source with a split power configuration (not shown), a silicon upper 
electrode 83, an anodized lower electrode 84 a gas inlet line 88, a gap 
driver 89, a dry pump 87 for chamber vacuum, a top-side clamp system and 
two (10 torr) capacitance manomometers 85 and 86. Manometer 86 is used as 
chamber pressure feedback to control chamber pressure via pressure 
controller and throttle valve control (not shown). Manometer 85 is 
designed to monitor the stability of plasma confinement by monitoring the 
pressure around the upper electrode 83. Since the sensing port 90 is 
immediate to the upper electrode it also precisely measures the pressure 
changes associated with upper electrode wear. 
As previously stated, manometer 85 designed for sensing plasma confinement 
stability, is used in this test to sense the pressure around the upper 
electrode for interpolating upper electrode wear thus validating a primary 
object of this invention. 
The test includes 3,500 wafers. During the first wet clean cycle, the gap 
pressure, (upper electrode pressure) is observed and the relation between 
gap pressure and etch rate is recorded. A gap pressure value is chosen for 
an etch rate of 4500 angstroms/min. The gap pressure value was chosen as a 
process parameter for continuous wet clean cycles. The electrode is 
consumed with increasing RF process time affecting the gap pressure and 
etch rate. To maintain the gap pressure within the process parameter, the 
gap height was adjusted within a limited available range. Etch rate and 
etch uniformity were checked after every 500 wafers processed based on a 
controlled gap pressure. 
The etch rate 92 and gap pressure 91 trends are plotted in FIG. 7. During 
the first wet clean cycle, the gap pressure was 324 mtorr, the etch rate 
at 4700 angstroms/minute. At the start of the second cycle, the gap height 
was reduced by 10 counts or 0.1 mm, the gap pressure had reduced to 312 
mtorr and the etch rate between 4500 and 4600 A/min. The ratio between gap 
height, gap pressure(GP) and etch rate is 1:1.2:10 respectively. Based on 
this ratio, the gap height is reduced by 4 counts (0.04 mm) during the 
third cycle. The gap pressure(GP) was 312 mtorr, and etch rate reduced by 
50A/min. or 4500.+-.50A/min. The total gap height reduction between the 
first and third cycle was used as a basis for establishing the set point 
for the GP of 315.+-.5 mtorr. After four cycles or 14,000 processed 
wafers, the upper electrode is replaced at its expected serviceable life. 
The new electrode with its smooth surface affects the GP. The gap height 
is increased to 0.11 mm (11 counts) to maintain the GP at the established 
set point of 315 mtorr. FIG. 7 depicts a significant correlation between 
etch rate and the GP, suggesting that the etch rate can be controlled by 
the GP at an established range. 
FIG. 8 plots the relation between gap count 97 and GP 96 during the wet 
clean cycle, with the gap count held at a constant value. However, GP 
varies with increasing number of wafers processed indicating gap height 
adjustment is necessary. Adjustment is done after every 500 wafers are 
processed. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made without departing from the spirit and scope of the invention.