Plasma density modulator for improved plasma density...

Details Plasma density modulator for improved plasma density... Plasma density modulator for improved plasma density...

1998-10-13

2001-05-29

McDonald, Rodney (Department: 1753)

Chemistry: electrical and wave energy

Processes and products

Coating, forming or etching by sputtering

C204S298060, C204S298120, C204S298160, C204S298170, C204S298180, C204S298190, C204S298200

Reexamination Certificate

active

06238528

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to plasma generators, and more particularly, to a method and apparatus for generating a plasma to sputter deposit a layer of material in the fabrication of semiconductor devices.
BACKGROUND OF THE INVENTION
Plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent to the target impact the surface of the target to dislodge, i.e., “sputter” material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate on which they are being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices with a high depth to width aspect ratio can bridge over the opening causing undesirable cavities in the deposition layer. To prevent such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate to position vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered in a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term “dense plasma” is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance-matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
Ionized physical vapor deposition (PVD) typically requires the chamber to be operated at a relatively high pressure. As a result, the frequency of collisions between the plasma ions and the deposition material atoms is increased and the scattering of the deposition atoms towards the shields is likewise increased. This scattering of the deposition atoms typically causes the thickness of the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions. Such non-uniformity of deposition is often undesirable in the fabrication of semiconductor devices.
SUMMARY OF THE PREFERRED EMBODIMENTS
It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter-depositing a layer which obviate, for practical purposes, the above-mentioned limitations.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention, a plasma-generating apparatus that improves the uniformity of a high density plasma including, for example, molecular nitrogen (N
2
) and argon (Ar), by optimizing a ratio of RF power from a first coil, surrounding and inductively coupled into the high density plasma, to RF power from a second coil, positioned closer to the high density plasma. An increase in RF power supplied to the second coil positioned above the central region relative to RF power suppled to the first coil surrounding the high density plasma can increase the relative density of the plasma toward the center of the high density plasma. It is believed that RF power supplied to the second coil positioned above the central region of the substrate tends to add more electrons into the central region of the high density plasma to compensate for electrons recombining with plasma ions. A balance can thus be struck between RF power supplied to the first and second coils to increase plasma uniformity in the high density plasma, which may cause an increase in the uniformity of ionization of the sputtered target material atoms by the high density plasma.
In a preferred embodiment, an apparatus for energizing a plasma within a semiconductor fabrication system to sputter material from a target onto a substrate may include a semiconductor fabrication chamber having a plasma generation region adjacent the target. A first coil carried by the chamber is positioned surrounding the plasma generation region to couple energy into the plasma generation region. It is believed that the plasma generated by the first coil not only ionizes the sputtered material but also contributes to the total number of ions impacting the target. Moreover, it is believed that the ions produced by the inductively coupled plasma tend to etch preferentially the periphery of a target as compared to the center of a target. Hence, by shaping the target to be like the frustrum of a hollow cone without a center, and by adjusting the ratio of power supplied to the first coil to power supplied to the target, the amount of material deposited on the periphery of the substrate relative to the center of the substrate can similarly be adjusted so as to achieve an improved degree of uniformity of deposition of the target material sputtered onto the substrate.
A second coil carried by the chamber is positioned above a central region of the plasma generation region to couple energy into the plasma generation region. It is believed that the plasma generated by the second coil replenishes the supply of electrons in the high density plasma to compensate for the loss of electrons to various recombination processes. Moreover, it is believed that the inductively coupled plasma produced by the first coil tends to be most dense preferentially at the periphery of the high density plasma, whereas it is believed that the inductively coupled plasma produced by the second coil tends to be most dense preferentially at the central region of the high density plasma. Hence, by adjusting the ratio of power supplied to the first and second coils, the density profile of the high density plasma can be adjusted so as to achieve an improved degree of uniformity of density of the high density plasma.


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