RF plasma method

Details RF plasma method RF plasma method

2000-04-27

2001-08-07

Mills, Gregory (Department: 1763)

Etching a substrate: processes

Gas phase etching of substrate

Application of energy to the gaseous etchant or to the...

C438S710000, C438S758000, C427S569000, C134S001100

Reexamination Certificate

active

06270687

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates to an RF plasma etch reactor, and more particularly to such a reactor employing an internal inductive coil antenna and electrically conductive chamber walls.
2. Background Art
A typical inductively coupled plasma etch reactor of the type currently available is depicted in FIG.
1
. This reactor has a vacuum chamber
10
surrounded by an inductive coil
12
. A workpiece
14
, usually a semiconductor wafer, is supported inside the chamber
10
on a pedestal
16
. An inductive coil antenna
12
is wound around the outside of the chamber
10
and connected to a radio frequency (RF) power generator
18
through an impedance matching network
20
to provide RF power into the chamber. In addition, a bias RF power generator
22
and associated impedance matching circuit
24
is connected to the pedestal
16
and used to impose a bias on the workpiece
14
. The chamber walls
30
are composed of an electrically insulating material, typically quartz or ceramic, so as to minimize attenuation of the RF power coupled into the chamber
10
. Underlying the insulative chamber walls
30
and surrounding the pedestal
16
is a portion
34
of the chamber constructed of a conductive material. This conductive portion
34
is electrically grounded and serves as the ground for the RF power supplied to the pedestal
16
. There are also cooling channels
32
formed within the conductive portion
34
. Coolant fluid is pumped through the channels
32
to transfer heat away from the interior of the chamber
10
so that the chamber temperature can be maintained at a particular level desired for the etch process being performed. The exterior of the chamber walls
30
are also cooled for the same reason. However, as insulative materials such as quartz and ceramic cannot be easily formed with internal cooling channels, the exterior surface of the walls
30
are cooled, typically by forced air convection methods. Etchant gas is introduced into the chamber
10
through gas injection ports
26
. A vacuum pump
28
evacuates the chamber
10
to a desired chamber pressure.
In operation, an etchant gas is introduced into the interior of the chamber
10
and RF power inductively coupled from the coil
12
generates a plasma within the chamber. The plasma produces etchant species (e.g. ions and radicals) from the etchant gas which are used to etch the workpiece
14
. A key component of anisotropic etching processes is the bombardment of the workpiece
14
with ions produced in the plasma. The energy and directionality exhibited by the ions and their density within the plasma are important factors which, to a large part, determine the quality of the resulting etched workpiece
14
. These factors substantially determine etch uniformity, etch rate, photoresist selectivity, the straightness of the etch profile, and the smoothness of etch feature sidewalls. For example, a high plasma ion energy at the surface of the workpiece
14
is desirable so as to prevent isotropic etching and to maximize the etching rate. However, ion energy which is too high will produce poor etching results, such as high photoresist loss, and can cause damage to the devices being formed on the workpiece
14
. Therefore, the plasma ion energy is ideally kept relatively near but below a threshold at which the etch quality begins to deteriorate significantly and/or where device damage becomes unacceptable. Similarly, a high plasma ion density is desirable so as to maintain a high etch rate. Essentially, the more ions there are, regardless of their energy, the faster the workpiece
14
is etched.
In the inductively coupled reactor of
FIG. 1
, the plasma ion density is substantially controlled by the amount of RF power coupled into the chamber via the coil
12
. For the most part, the more power coupled, the higher the plasma ion density. Thus, in most cases, the plasma ion density can be held to a desired level by selecting the appropriate amount of RF power to be supplied by the RF power generator
18
to the coil
12
. The RF power coupled into the chamber by the coil
12
, however, does not significantly affect the plasma ion energy at the surface of the workpiece
14
. Control of the ion energy at the surface of the workpiece is conventionally accomplished by capacitively coupling RF power into the chamber via the to the pedestal
16
using the bias RF power generator
22
. Ideally, the bias power supplied to the pedestal
16
will not significantly affect the ion density produced in the chamber
10
, thereby decoupling the control of ion density and ion energy.
The plasma ion energy controlled by the bias RF power applied to the pedestal
16
is, however, affected by the ratio of the surface area of the pedestal to the surface area of the grounded portion
34
of the chamber. The pedestal
16
acts as the cathode and the grounded portion
34
serves as the anode to form a capacitively coupled circuit. Since the majority of the interior surface of the chamber
10
is formed by the insulative chamber walls
30
to maximize the inductive coupling of power into the chamber from the coil
12
, the surface area associated with the grounded portion
34
is necessarily limited, and typically not too much larger than that of the pedestal
16
. An ion energy control problem results because the surface areas of the grounded portion
34
and the pedestal
16
are too close in size in a conventional inductively coupled etch reactor. When the surface area of the pedestal
16
is less than that of the grounded portion
34
, the average voltage (often referred to as the DC bias voltage) at the surface of the workpiece
14
is negative. This average negative voltage is employed to draw the positively charged ions from the plasma to the workpiece
14
. However, if the surface area of the pedestal
16
is only slightly smaller than the surface area of the grounded portion (as is typically the case in a conventional inductively coupled plasma etch reactor), the average negative voltage at the surface of the workpiece
14
is relatively small. This small bias voltage results in a weak attracting force and so a relatively low average ion energy. A higher negative bias voltage value than can typically be obtained using a conventional inductively coupled plasma etch reactor is necessary to optimize the plasma ion energy so as to ensure the maximum etch rate and no significant damage to the devices being formed on the workpiece
14
. Ideally, the surface area of the grounded portion
34
would be sufficiently large in comparison with that of the pedestal
16
so as to produce the maximum possible negative average voltage at the surface of the workpiece
14
, i.e. one half the peak to peak voltage.
The previously-described inductively coupled etch reactor has in the past been used to etch aluminum from the surface of the workpiece
14
. This etching process produced byproducts comprising mostly aluminum chlorides (AlCl
x
) and fragments of photoresist which tend to deposit on the walls of the reactor chamber
10
. The byproducts of an aluminum etch have no significant effect on the plasma characteristics (e.g. plasma ion density and energy) because they are almost totally non-conductive. However, it is also desirable to etch other metals from the surface of a workpiece
14
, such as copper (Cu), platinum (Pt), tantalum (Ta), rhodium (Rh), and titanium (Ti), among others. Etching these metals presents a problem when using the conventional etch reactor of
FIG. 1
because the etching by-products of these metals tend to be conductive. Thus, a conductive coating forms on the chamber walls. This conductive coating has the effect of attenuating the RF power coupled into the chamber by the coil
12
. The coil
12
produces a magnetic field which results in power being coupled into the chamber. When the interior surface of the chamber under the coil
12
is coated with a conductive material, eddy currents are produced in this material, thereby attenuating the magnetic field to some extent and reducing the am

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