Reactive Ion Etch (RIE) is used to form shapes on work pieces such as semiconductor wafers. In a typical RIE process, radio frequency (RF) or microwave power is used to excite a gas to form a plasma. The plasma facilitates etching the shapes into the work piece. A typical use of the RIE process is in etching very fine holes, typically minimum dimension sized holes, through a layer, e.g., an insulating layer, to an underlying layer, commonly referred to as "opening contacts" or "opening vias" through the layer. RIE is also used to open minimum width trenches through one or more work piece layers. Typically, these minimum dimension shapes are deeper than at least one of their surface dimensions and, therefore, have a high aspect ratio.
FIG. 1A is a schematic diagram of a conventional diode RIE system 100, such as an MXP chamber of the AME 5X00 series, manufactured by Applied Materials Corp. The RIE system includes an etching chamber 2. A window 4 is formed in one portion of etching chamber 2 for viewing the RIE process from outside etching chamber 2. A slot 6 is formed in another portion of etching chamber 2 through which a work piece such as a semiconductor wafer may be inserted for etching. A pump 8 is connected to etching chamber 2 for creating a vacuum in etching chamber 2. Magnetic coils 10a, 10b, 12a, and 12b are disposed about the perimeter of etching chamber 2. Each magnetic coil is offset by about 90 degrees from the magnetic coils on either side. For example, magnetic coil 10a is offset by about 90 degrees from magnetic coils 12a and 12b, respectively. Magnetic coil 10a faces magnetic coil 10b across etching chamber 2 and, similarly, magnetic coil 12a faces magnetic coil 12b across etching chamber 2.
Magnetic coils 10a, 10b, 12a, and 12b are typically driven by an AC power source (not shown) providing sinusoidal functions. In particular, magnetic coils 10a and 10b are driven by a first sinusoidal function provided by the AC power source, and magnetic coils 12a and 12b are driven by a second sinusoidal function offset by approximately 90 degrees with respect to the first sinusoidal function. As such, at a first phase corresponding to a maximum of the first sinusoidal function, a magnetic field passes between magnetic coils 10a and 10b across etching chamber 2. Similarly, at a second phase corresponding to a maximum of the second sinusoidal function, a magnetic field passes between magnetic coils 12a and 12b. In this way, magnetic coils 10a, 10b, 12a, and 12b produce a magnetic field which rotates over time with respect to etching chamber 2. The magnetic field produced by magnetic coils 10a, 10b, 12a, and 12b typically rotates at a rate of about 0.2 Hertz (Hz).
FIG. 1B illustrates a cross-sectional view of the conventional diode RIE system 100 of FIG. 1A. Gas is injected into etching chamber 2 through shower head 24 and is pumped out of etching chamber 2 by pump 8. A semiconductor wafer 14 has been inserted through slot 6 (shown in FIG. 1A), dropped onto a work piece chuck 16, and clamped to a work piece chuck electrode 18 formed as a portion of work piece chuck 16. Work piece chuck electrode 18 is driven by a power supply 20. Power supply 20 creates a radio frequency (RF) field through work piece chuck electrode 18 which, in turn, produces a plasma 22. Plasma 22 may diffuse slightly along the sides of work piece chuck 18. A rotating electromagnetic field 26 is produced by magnetic coils 10a, 10b, 12a, and 12b, as described with reference to FIG. 1A. Specifically, FIG. 1B illustrates electromagnetic field 26 at a time corresponding to the first phase described above, such that rotating electromagnetic field 26 passes between magnetic coils 10a and 10b across etching chamber 2.
Because electron plasma 22 is generated by AC power source 20 through work piece chuck electrode 18, electron plasma 22 is capacitively driven. As such, the plasma efficiency of conventional diode RIE system 100 decreases as the density of plasma 22 increases. Therefore, for the conventional diode RIE system to operate efficiently, the density of plasma 22 is low. Such an RIE system 100 often results, however, in less-than-desirable etch rates, plasma non-uniformities, and ionic charging.
Effects from ionic charging, known to those skilled in the art as "aspect ratio charging effects," typically occur at the bottom of high aspect ratio structures, especially if some part of the structure is an insulator. These aspect ratio charging effects include oxide damage, device damage, threshold voltage shifts, polysilicon notching, and reduction of ion current at the bottom of trenches and at the bottom of vias that impede or even stop etching (known as RIE lag).
As line widths have decreased and aspect ratios of etch structures have increased, aspect ratio charging effects have become more problematic. These charging effects are due, at least in part, to the fact that, in the conventional RIE system, electrons diffuse out of the plasma due to high thermal energy and densities while positive ions are extracted. Thus, positive ions are accelerated to various locations on the structure and electrons spread and are collected near mask surfaces until a potential develops which equalizes these two particle fluxes. There remains a need, therefore, to eliminate aspect ratio charging effects.