Inductively coupled RF plasma reactor having an antenna...

Details Inductively coupled RF plasma reactor having an antenna... Inductively coupled RF plasma reactor having an antenna...

2000-09-29

2002-09-03

McDonald, Rodney G. (Department: 1753)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With microwave gas energizing means

C204S298060, C204S298080, C204S298310, C204S298340, C118S7230IR, C118S7230IR, C118S7230ER, C118S7230ER

Reexamination Certificate

active

06444085

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The invention is related to inductively coupled RF plasma reactors of the type having a reactor chamber ceiling overlying a workpiece being processed and an inductive coil antenna adjacent the ceiling.
2. Background Art
Inductively coupled RF plasma reactors are employed to perform a variety of processes on workpieces such as semiconductor wafers. Referring to
FIG. 1
, one type of inductively coupled RF plasma reactor has a reactor chamber
10
including a ceiling
12
and a cylindrical side wall
14
. A pedestal
16
supports the workpiece
18
, such as a semiconductor wafer, so that the workpiece generally lies in a workpiece support plane, and a bias RF power generator is coupled to the pedestal
16
. A generally planar coil antenna
20
overlies the ceiling
12
and is coupled to a plasma source RF power generator
22
. A chief advantage of inductively coupled RF plasma reactors over other types such as capacitively coupled ones, is that a higher ion density can be achieved with the inductively coupled type.
Adequate etch selectivity is achieved by operating at higher chamber pressure. (The term etch selectivity refers to the ratio of etch rates of two different materials exposed to etching in the reactor.) This is because the polymerization processes typically employed in a high density plasma etch reactor to protect underlying non-oxygen-containing (e.g., silicon, polysilicon or photoresist) layers during etching of an overlying oxygen-containing (e.g., silicon dioxide) layer are more efficient at higher chamber pressures (e.g., above about 20-500 mT) than at lower pressures. Polymer precursor gases (e.g., fluorocarbon or fluorohydrocarbon gases) in the chamber tend to polymerize strongly on non-oxygen-containing surfaces (such as silicon or photoresist), particularly at higher chamber pressures, and only weakly on oxygen-containing surfaces (such as silicon dioxide), so that the non-oxygen-containing surfaces are relatively well-protected from etching while oxygen-containing surfaces (such as silicon dioxide) are relatively unprotected and are etched. Such a polymerization process enhances the oxide-to-silicon etch selectivity better at higher chamber pressures because the polymerization rate is higher at higher pressures such as 100 mT. Therefore, it is desireable to operate at a relatively high chamber pressure when plasma-etching oxygen-containing layers over non-oxygen-containing layers. For example, under certain operating conditions such as a chamber pressure of 5 mT, an oxide-to-photoresist etch selectivity of less than 3:1 was obtained, and raising the pressure to the 50-mT range increased the selectivity to over 6:1. The oxide-to-polysilicon etch selectivity exhibited a similar behavior.
The problem with increasing the chamber pressure (in order to increase etch selectivity) is that plasma ion spatial density distribution across the wafer surface becomes less uniform. There are two reasons this occurs: (1) the electron mean free path in the plasma decreases with pressure; and (2) the inductive field skin depth in the plasma increases with pressure. How these two factors affect plasma ion spatial density distribution will now be explained.
With regard to item 1 above, the electron-to-neutral species elastic collision mean free path length, which is inversely proportional to chamber pressure, determines the extent to which electrons can avoid recombination with other gas particles and diffuse through the plasma to produce a more uniform electron and ion distribution in the chamber. Typically, electrons are not generated uniformly throughout the chamber (due, for example, to a non-uniform inductive antenna pattern) and electron diffusion through the plasma compensates for this and provides greater electron and plasma ion spatial density distribution uniformity. (Electron spatial density distribution across the wafer surface directly affects plasma ion spatial density distribution because plasma ions are produced by collisions of process gas particles with energetic electrons.) Increasing chamber pressure suppresses electron diffusion in the plasma, thereby reducing (degrading) plasma ion spatial density distribution uniformity.
This problem may be understood by reference to
FIG. 1
, in which the inductive antenna
20
, due to its circular symmetry, has an antenna pattern (i.e., a spatial distribution of the magnitude of the induced electric field) with a null or local minimum along the antenna axis of symmetry so that very few if any electrons are produced over the wafer center. At low chamber pressures, electron diffusion into the space (“gap”) between the antenna
20
and the workpiece
18
is sufficient to transport electrons into the region near the wafer center despite the lack of electron production in that region, thereby providing a more uniform plasma distribution at the wafer surface. With increasing pressure, electron diffusion decreases and so plasma ion distribution becomes less uniform.
A related problem is that the overall plasma density is greater near the ceiling
12
(where the density of hot electrons is greatest) than at the workpiece
18
, and falls off more rapidly away from the ceiling
12
as chamber pressure is increased. For example, the electron mean free path in an argon plasma with a mean electron temperature of 5 eV at a chamber pressure of 1 mT is on the order of 10 cm, at 10 mT it is 1.0 cm and at 100 mT it is 0.1 cm. Thus in a typical application, for a 5 cm ceiling-to-workpiece gap, most of the electrons generated near the ceiling
12
reach the workpiece at a chamber pressure of 1 mT (for a maximum ion density at the workpiece), and a significant number at 10 mT, while at 100 mT few do (for a minimal ion density at the workpiece). Accordingly, it may be said that a high pressure regime is one in which the mean free path length is about {fraction (1/10)} or more of the ceiling-to-workpiece gap. One way of increasing the overall plasma ion density at the workpiece
18
(in order to increase etch rate and reactor throughput) without decreasing the chamber pressure is to narrow the gap so that the mean free path length becomes a greater fraction of the gap. However, this exacerbates other problems created by increasing chamber pressure, as will be described further below.
With regard to item (2) above, the inductive field skin depth corresponds to the depth through the plasma—measured downward from the ceiling
12
—within which the inductive field of the antenna
20
is nearly completely absorbed.
FIG. 2
illustrates how skin depth in an argon plasma increases with chamber pressure above a threshold pressure of about 0.003 mT (below which the skin depth is virtually constant over pressure).
FIG. 2
also illustrates in the dashed-line curve how electron-to-neutral elastic collision mean free path length decreases linearly with increasing pressure. The skin depth function graphed in
FIG. 2
assumes a source frequency of 2 MHz and an argon plasma density of 5·10
17
electrons/m
3
. (It should be noted that the corresponding plasma density for an electro-negative gas is less, so that the curve of
FIG. 2
would be shifted upward with the introduction of an electro-negative gas.) The graph of
FIG. 2
was derived using a collision cross-section for an electron temperature of 5 eV in argon. (It should be noted that with a molecular gas such as C
2
F
6
instead of argon, the collision cross-section is greater so that the skin depth is greater at a given pressure and the entire curve of
FIG. 2
is shifted upward.) If the chamber pressure is such that the inductive field is absorbed within a small fraction—e.g., {fraction (1/10)}th—of the ceiling-to-workpiece gap adjacent the ceiling
12
(corresponding to a pressure of 1 mT for a 5 cm gap in the example of FIG.
2
), then electron diffusion—throughout the remaining {fraction (9/10)}ths of the gap—produces a more uniform plasma ion distribution at the workpiece surface. However, as pressure increases and skin depth increases—e.g., beyond a

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