Modulated power for ionized metal plasma deposition

Details Modulated power for ionized metal plasma deposition Modulated power for ionized metal plasma deposition

1997-08-07

2001-05-22

Mills, Gregory (Department: 1763)

Chemistry: electrical and wave energy

Apparatus

Coating, forming or etching by sputtering

C204S298060, C204S298340, C156S345420, C118S7230IR, C118S7230AN, C118S7230IR

Reexamination Certificate

active

06235169

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 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 being deposited, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched openings including trenches and holes of semiconductor devices having openings with a high depth to width aspect ratio, may not adequately coat the walls of the openings, particularly the bottom walls. If a large amount of material is being deposited, the deposited material can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively biasing (or self biasing) the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by 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, in the range of 10
11
-10
13
ions/cm
3
.
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.
Although ionizing the deposition material facilitates deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges. This “center thick” deposition profile is undesirable in many applications where a uniformity of deposition thickness is needed.
As described in copending application Ser. No. 08/680,335, filed Jul. 10, 1996, (now abandoned), entitled “Coils for Generating a Plasma and for Sputtering” by Jaim Nulman et al., which is assigned to the assignee of the present application and is incorporated herein by reference in its entirety, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to the coil can cause the coil to develop a negative bias which will attract positive ions which can impact the coil causing material to be sputtered from the coil. Because the material sputtered from the coil tends to deposit more thickly at the periphery of the wafer, the center thick tendency for material sputtered from the primary target can be compensated by the edge thick tendency for material sputtered from the coil. As a result, uniformity can be improved. However, a need exists for improved control over the quantity of material sputtered from the coil to increase further the degree of uniformity of deposition that may be achieved.
SUMMARY OF THE PREFERRED EMBODIMENTS
It is an object of the present invention to provide an improved method and apparatus for sputter depositing a layer which enhances uniformity of deposition of sputtered material.
These and other objects and advantages are achieved by a plasma generating apparatus in which, in accordance with one aspect of the invention, the RF energy applied to a coil for the purpose of inductively coupling RF energy into a plasma, is modulated so as to enhance the ability to control the bias level of the coil. It is believed that the power level of the modulation can control the sputtering rate of the coil to provide enhanced control over the uniformity of deposition. As a result, improved uniformity may be achieved. In addition, the power level of modulation may be controlled separately from power level of the RF energy being applied to the coil to ionize the plasma. Consequently, the ionization rate and the coil sputtering rate may be more controlled more independently of each other to provide greater latitude in optimizing these factors.
In the illustrated embodiment, an RF generator is coupled to one end of a coil in a sputtering chamber through an impedance matching network which is tuned to minimize the reflection of RF energy back to the generator. A second generator is coupled to the other end of the coil through a filter. The second generator, referred to herein as a modulation generator, modulates the RF energy supplied by the RF generator at a lower frequency such as a few hundred kilohertz. It is believed that this low frequency modulation of the RF signal applied to the coil directly affects the biasing induced on the coil and therefore directly affects the sputtering rate of the coil. Moreover, the impedance matching network tends to block the lower frequency signal such that much of the power contributed by the modulation generator is expended primarily through the plasma. Hence, it is believed that the sputtering rate may be controlled by the power level of the modulation generator with a degree of independence from the power level provided by the RF generator. Hence these two power levels may be controlled to optimize plasma ionization rate and coil sputtering more independently. As a consequence, the amount of material deposited at the edges may be controlled more independently and the uniformity of the thickness of the deposition may be improved.


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