Method and apparatus for gas feed control in a dry etching process

A method and apparatus for obtaining a uniform gaseous molecular field under high vacuum conditions encountered in a dry etching process. In the method a source of a gas is provided and introduced into a manifold. The manifold feeds at least one nozzle and the gas is passed through the manifold and through the nozzle into a chamber maintained under vacuum conditions. The pressure of the gas and the configuration of the manifold and nozzle are such that the gas is caused to exit from the nozzle into the chamber under vacuum at sonic velocity.

BACKGROUND OF THE INVENTION 
The present invention relates to a dry etching process and to an etching 
apparatus and more particularly relates to techniques for controlling 
etchant gas feed and control of the reactor flow during a dry etching 
process. 
Plasma-type etching processes using, for example, tetrafluoromethane 
(CF.sub.4) gas, have come to be widely used in place of solution-type 
etching in the production of semiconductor devices. Plasma-type etching 
utilizes a reactive gas in a radio frequency glow discharge to etch 
silicon and its compounds, such as silicon nitride and silicon oxide, and 
also molybdenum layers and tungsten layers. Plasma-type etching and 
reactive ion etching have sometimes been referred to as dry etching or dry 
process technology. The basic aim in dry process technology is to provide 
most favorable conditions of etching rate, uniformity of etching, 
selectivity and etch definition. Prior art dry etching processes were 
sometimes defective in that an uneven distribution of positive and 
negative ions in a high electric and magnetic field lead to irregular 
etching. 
The general flow characteristics of the etchant gas in dry process 
technology, due to the high vacuum used in this technology, are in the 
domain of rarefied gas dynamics. Specifically, the gas flow occurs in the 
ranges of free-molecular flow and the transition flow regime which occurs 
between free-molecular flow and continuum flow. Thus, the flow behavior in 
the reactivity zone is characterized by predominately molecular collision 
phenomena. Continuum fluid mechanics are not useful to predict mass flow 
effects under the reactor conditions encountered in dry processing 
techniques. 
In general, the present state of dry processing technology is such that 
etching performance variation occur under various operating conditions. 
The principal variation which affects etching performance is the character 
and uniformity of the gas field in the reactivity zone surrounding the 
etching object. The present invention is directed to new techniques and 
apparatus for generating and controlling uniform, highly effective gas 
field characteristics in a reactivity zone. 
SUMMARY OF THE INVENTION 
Heretofore, it has been believed in dry processing technology that the 
etchant gas attains a uniform flow distribution through natural transport 
phenomena caused by the flow characteristics set up by the outflow of gas 
occasioned by the high vacuum field used in the process. In accordance 
with the present invention it has been discovered that the flow pattern 
caused by the vacuum takeoff is highly non-uniform and that a significant 
improvement in etching uniformity can be achieved by introducing the 
etchant gas into the reactivity zone at sonic velocity. 
In the method of the present invention for obtaining a uniform gaseous 
molecular field under high vacuum conditions a source of an etchant gas is 
provided at a relatively high pressure compared to the vacuum pressure in 
the reactivity zone. The high pressure gas source in introduced into a 
manifold which feeds at least one nozzle. The nozzle has a configuration 
such that passage of the gas from the relatively high pressure of the 
manifold to the vacuum pressure in the reactivity zone causes the gas to 
exit from the nozzle at sonic velocity. The sonic velocity attitude of the 
gas at the exit end of the nozzle causes the gas molecules to form a 
highly swirled flow pattern which achieves a highly uniform gas field in 
the reactivity zone adjacent the etching object.

DETAILED DESCRIPTION OF THE INVENTION 
Turning first to FIG. 1, there is shown apparatus for dry etching or for 
carrying out vapor or gaseous deposition on the exposed surfaces of a 
substrate 29, which may be wafers or slices of semiconductor material. The 
apparatus described herein can be used for other treatment of the 
substrates in addition to etching and deposition. For example, the 
apparatus can be used for sputtering processes whereby material is added 
to or removed from the exposed surfaces of the substrate. 
The apparatus in FIG. 1 consists of a base plate 11, a sidewall 13 and a 
top plate 15. The base plate, sidewall and top plate are connected in 
sealing relationship to define an evacuable chamber. A gas manifold 
assembly 17 is located within the evacuable chamber. A rotatable carrier 
19 is positioned beneath the gas manifold assembly 17. The space between 
the rotatable carrier 19 and the gas manifold assembly 17 defines a 
reaction zone 21. A first electrode 23 and a second electrode 25, which 
are both parallel cylindrical plate-like members, are contained within the 
reactor. The first electrode is electrically coupled to an RF power source 
(not shown) and the second electrode is electrically coupled to a 
reference potential which is typically ground potential. A drive mechanism 
27 is provided to turn the rotatable carrier to position a substrate 29 in 
a desired location. A vacuum source (not shown) is coupled to port 31 to 
evacuate the reaction zone 21. A view port 33 is provided to observe the 
progress of the etching procedure during a run. The upper surface of the 
rotatable carrier and the lower surface of the gas manifold assembly are 
provided with a quartz liner 35 for protection against the etching 
atmosphere. When the RF source is activated and appropriate gases are 
introduced into the reaction zone, a glow discharge reaction occurs in the 
space between the two electrodes. The quartz liners 35 protect the 
surfaces of the first electrode and the rotatable carrier during the glow 
discharge reaction. 
The gas manifold assembly 17 comprises a gas inlet port 37, a gas manifold 
39 and nozzle assembly 41. A plurality of coolant passages 43 is provided 
to cool the gas manifold assembly 17 during the etching process. 
The operating principles of the gas nozzle assembly 41 are best understood 
by reference to FIGS. 2-4. As shown in FIGS. 2-4 the gas nozzle assembly 
comprises a passage member 45 and a backing member 47. The passage member 
45 is machined to provide a channel 49 and grooves 51. The grooves 51 
intersect the channel 45 and and extend from the channel 45 to the 
peripheral edge of the passage member 45. The passage member 45 is then 
affixed to the backing member 47 by suitable means, such as bolts or 
screws (not shown). When assembled in fixed relationship, the channel 49 
and intersecting grooves 51 define a manifold and an intersecting 
plurality of passages which act as nozzles. A gas feed port 53 is affixed 
to the second member 47 and interconnects with the channel 49 of the first 
member via a port 55 drilled through the second member 47. 
As shown in FIG. 3, a preferred configuration for the grooves 51 is a 
triangular cross section. The triangular cross section is preferred for 
ease of manufacture. Other cross sectional shapes, such as semicircular, 
rectangular, square or polygonal are also suitable. It is also feasible to 
machine the surface of both the passage member and the backing member to 
provide circular, ovoid or rhomboid shapes. When both the passage member 
and backing member are machined, however, registration of the two members 
is required. For this reason, it is preferred to machine only the passage 
member. 
In operation, etchant gas at a relatively low absolute pressure and a 
relatively high pressure compared to the vacuum chamber pressure is 
introduced into the manifold created by channel 49 and passes through the 
gas passages created by grooves 51. The pressure of the etchant gas is 
preferably from about 1 to about 10 psig. The gas passages are preferably 
in the range of from about 0.5 to about 1.0 inch long and preferably have 
a cross sectional area in the range of from about 2 to about 
5.times.10.sup.-6 in.sup.2. The preferred ratio of the gas passage length 
to the cross sectional area is in the range of from about 
0.1.times.10.sup.6 to about 0.5.times.10.sup.6 in.sup.-1. The preferred 
dimension parameters of the gas passage provide a nozzle configuration 
such that sonic velocity of the etchant gas can be obtained at the 
relatively low absolute pressure of the etchant gas in the manifold. 
Gas passage routes having the length, cross sectional area and length to 
cross sectional area ratio parameters as described herein provides an 
aggregate passage resistance to the flow of the gas such that the manifold 
pressure is essentially constant throughout the length of the manifold. 
The gas passage parameters described herein also provide an energy 
dissipation factor such that the passage of the gas through the passage 
results in attainment of sonic velocity upon exit of the gas from the 
nozzle. The attainment of sonic velocity promotes an explosive discharge 
from the vacuum terminus of the nozzle which engenders a highly swirled 
and uniform dissipation of gas molecules in the reaction zone. 
As shown in FIG. 5 the gas nozzle construction of the present invention is 
adapted to provide for the influx of two or more gases and to promote 
mixing of the gases as they emerge into the evacuable chamber. In the 
nozzle construction of FIG. 5 two passage members 45 are machined with 
channels and grooves to provide the manifold and gas passages described 
hereinabove. The two passage members 45 are joined in metal to metal 
contact with a second member 47 to create the manifold and gas passages 
described heretofore. 
As shown in FIG. 5 the backing member 47 terminates prior to the passage 
members 45. As shown in FIG. 6 this provides for a mixing section 57 
wherein the two gas species are highly comingled and form a uniform gas 
mixture prior to emerging into the reaction zone 21. The spacing of the 
terminus of the backing member 47 from the terminus of the passage members 
45 is not critical and, in fact, can be less than, equal to or greater 
than the terminus of the members 45. The emergence of the two gas species 
at sonic velocity creates a highly swirled relationship of the two gases 
regardless of the location of the terminus of the backing member 47. 
A preferred embodiment of the gas nozzle assembly 41 is shown in FIG. 7. 
The nozzle assembly 41 of FIG. 7 can be considered to be a form of the 
nozzle assembly 41 of FIG. 2 wherein the flat plate of FIG. 2 is formed 
into a conical member. As shown in FIG. 7, the nozzle assembly comprises 
an attachment shaft 59, a nozzle body 61 and an insert 63. As best seen in 
FIG. 8, the insert 63 has triangular grooves 51 machined along the conical 
surface thereof. The grooves are used to form gas passages as described 
hereinabove in connection with the construction of FIGS. 2-4. A retaining 
ring 65 is used to hold the shaft 59 and the insert 63 in fixed axial 
relationship while permitting the insert 63 to be rotated about the shaft 
59. Spanner holes 69 are provided to hold the insert as the shaft 59 is 
attached to a threaded socket 71 in top plate 73 by means of the threaded 
end 75 of the shaft. A hexagonal socket 77 is provided to accept an allen 
wrench for turning shaft 59 into engagement with top plate 73. 
To assemble the gas nozzle assembly of FIG. 7, the shaft 59 is first 
inserted into the mating aperture 67 of the insert 63 and retaining ring 
65 is attached. The insert is free to rotate on the shaft. The assembled 
insert 63 and shaft 59 are then mated with the nozzle body 61. The insert 
and shaft assembly are held in position by means of a spanner and the 
shaft is screwed into the top plate by means of an allen wrench. An O-ring 
67 is provided to block passage of any gas between the mating surfaces of 
the shaft 59 and the insert 63. 
When positioned in place the grooves of the insert 63 are adjacent the 
surface of a mating block 79 to provide a gas passage. O-ring 81 is 
provided to prevent any gas leakage between the surface shown. 
The present invention can be used to provide variable passage lengths for 
specific gas flow characteristics. An insert with variable passage lengths 
is shown in FIG. 9. As shown in FIG. 9, the insert consists of an 
obliquely truncated cylinder having grooves machined in the surface of the 
cylinder. Because of the oblique truncation, the grooves have a variable 
length. When the obliquely truncated cylindrical insert 63 is inserted 
into mating contact with the nozzle body 61, the position of the variable 
passages 51 can be preset by holding the insert 63 in a desired position 
as the shaft 59 is screwed into position. 
The various structural features defined herein with relationship to the 
different embodiments may be combined in a manner different than described 
herein and some modifications and variations may be made to these 
different embodiments without departing from the spirit and scope of the 
invention as set forth in the appended claims.