Multifrequency plasma reactor

A multifrequency plasma reactor includes first, second and third power generators operably coupled to at least one of an upper and lower electrode for generating power signals. The plasma reactor further includes a controller for selectively activating the power generators according to an activation profile that results in the formation of a desirable narrow gap via in a semiconductor wafer. A method of generating a plasma in the reactor for etching the semiconductor wafer is also described by way of configuring the power generators according to various activation configurations during various phases of the etching process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma reactor and, in particular, to a multiple frequency plasma reactor in which the frequencies and the powers associated therewith are individually controllable.

2. State of the Art

Semiconductor fabrication techniques are used to form integrated circuits on wafers and frequently include plasma-assisted processes for etching materials from the semiconductor wafer. Such plasma etching processes, also known as “dry etching,” are conventionally performed in a plasma reactor which utilizes radio frequency (RF) power generators to provide power to one or more electrodes within a vacuum chamber containing a gas at a predetermined pressure as defined by a specific process. The plasma reactor also includes a matching network for efficiently coupling power from the RF power generator to the electrode within the vacuum chamber.

Dry etching of a semiconductor wafer occurs within a vacuum chamber when electric fields between the electrodes within the vacuum chamber cause electrons present in the gas within the vacuum chamber to initially collide with gas molecules. With time, the electrons gain more energy and collide with the gas molecules to form an excited or ionized species. Eventually, a plasma is formed in which excitation and recombination of the atoms with electrons within the plasma are balanced. Highly reactive ions and radical species result in the plasma and are used to etch materials from the semiconductor wafer. Electric and magnetic fields within the vacuum chamber are used to control the etching processes on the semiconductor wafer.

One conventional RF-powered plasma reactor is a single-frequency diode reactor. In a single-frequency diode reactor, RF energy is conventionally applied to the wafer table on which the semiconductor wafer is located with an electrode located above the wafer serving as a grounded electrode. In such an arrangement, the plasma forms above the wafer and the ions are accelerated downward, as a result of an electric field formed between the plasma and the negatively charged wafer, into the wafer to physically etch materials from the wafer. Different frequencies presented at the electrode cause different physical phenomena in the plasma, which may or may not be desirable for a particular semiconductor process.

Another conventional RF-powered reactor includes a dual-frequency reactor which generally permits one RF frequency to be applied to a first powered electrode located away from the wafer and which predominantly controls and powers the plasma. A second RF frequency electrode provides a bias to the wafer to control the potential (e.g., sheath potential) between the second powered electrode and the plasma. Such a configuration generally assumes a capacitively coupled arrangement, which results in the formation of a self-induced DC bias to the wafer. Dual-frequency systems generally permit higher ion densities in the plasma, which results in a higher ion flux into the wafer. Such an approach significantly affects etch rates as a higher density of ions generally induces a higher etch rate.

Yet another conventional RF-powered reactor includes a dual-frequency reactor which applies two RF frequencies to a biasing electrode to control the potential between the biasing electrode and the plasma. Another electrode is located away from the wafer and is coupled to a reference potential, such as ground. The two frequencies typically perform separate functions, with one frequency dominating the ion energy while the other frequency dominates the plasma energy.

Though various arrangements for providing power to the plasma of a plasma reactor have been described, each heretofore-described configuration includes corresponding shortcomings. Therefore, there exists a need for an improved configuration which provides for a flexible solution to the foregoing problems and deficiencies.

BRIEF SUMMARY OF THE INVENTION

A multifrequency plasma reactor and method of etching a semiconductor wafer is provided. In one embodiment, a plasma reactor includes first, second and third power generators which are coupled to corresponding upper and lower electrodes for generating power signals. The plasma reactor further includes a controller for selectively activating the power generators according to an activation profile that results in the formation of a desirable narrow gap via on a semiconductor wafer.

In another embodiment of the present invention, a plasma reactor includes a vacuum chamber which includes upper and lower electrodes therein. First, second and third power generators couple to the upper and lower electrodes, the power generators selectively activated by a controller according to a specific activation profile.

In yet another embodiment of the present invention, a method of generating a plasma in a plasma reactor for etching a semiconductor wafer during an etch process is provided. First, second and third power generators are configured and operated according to a first activation configuration during a first phase of the etch process. The power generators are reconfigured and operated according to a second activation configuration during a second phase of the etch process. In a yet further embodiment of the present invention, an etching method is provided wherein first, second and third power signals are generated at upper and lower electrodes with the power generators being individually activated to control the etching of the semiconductor wafer.

In yet another embodiment of the present invention, a method for etching a semiconductor wafer is provided. A plasma reactor is provided which includes three power generators coupled to upper and lower electrodes. A controller selectively activates the power generators and, by controlling the power generators, the etching process is further controlled.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “semiconductor” includes all bulk semiconductor substrates including silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), silicon-on-glass (SOG), gallium arsenide (GaAs), and indium phosphide (InP), etc. A triple-frequency plasma reactor10for processing semiconductor devices is shown atFIG. 1. A first, or upper, power generator12is utilized to generate plasma14within vacuum chamber16. Plasma reactor10further includes a second, or lower, high-frequency power generator18and a third, or lower, low-frequency power generator20used to bias the substrate of wafer22as located upon wafer table24. In the present embodiment, plasma reactor10is a parallel plate reactor having an upper electrode26and a lower electrode28. Additionally, power generators12,18and20are capacitively coupled via respective capacitors30-34.

Upper power generator12may be configured to generate a variably powered RF signal of, by way of example and not limitation, between 1 and 2 kilowatts of power at a frequency of approximately 40 to 100 megahertz. Additionally, lower high-frequency power generator18may be configured, by way of example and not limitation, to generate a variably powered RF signal of approximately 1 to 2 kilowatts of power and operate at a frequency range of approximately 13.5 to 60 megahertz. Yet further, lower low-frequency power generator20, by way of example and not limitation, may be configured to generate a variably powered RF signal of approximately 1 to 2 kilowatts of power at an operational frequency of approximately 1 to 13.5 megahertz. While specific frequencies and powers have been identified as examples, other rules may be applied for identifying frequencies and powers according to a specific process. Such rules may include guidance for selecting a frequency for the lower high-frequency power generator18, namely that the lower high-frequency power generator18operates at a frequency greater than three times the frequency of the lower low-frequency power generator20. Another rule may include that the upper power generator12be configured to operate at a frequency of at least that of the lower high-frequency power generator18.

Additionally, proper operation of a plasma reactor requires sound grounding techniques. Grounding plates36are illustrated and grounding may further take place through the use of a matchbox (not shown) or a counter electrode (not shown), the configuration and implementation of which is appreciated by those of ordinary skill in the art. Generally, a matchbox matches the impedance with the chamber and the generator. In short, the matchbox matches the impedance on both sides of the generator in order to minimize reflected power, which otherwise would result in an ineffective coupling of power into the plasma.

The exact frequencies of operation for the power generators may be selected to correspond to internationally recognized industrial/scientific/medical (ISM) apparatus frequencies or the output frequencies of commercially available RF power supplies. Utilization of a frequency in the VHF signal band for upper power generator12is desirable as frequencies in this range are more effective than lower frequencies at breaking down etch gases into reactive radicals and initiating a plasma. Furthermore, the required pressure within vacuum chamber16may be reduced through the use of such frequencies. Additionally, higher frequencies beyond the VHF signal band also become more expensive to generate and to couple into the plasma. The triple-frequency plasma reactor10may further include a controller38operably and controllably coupled with power generators12,18and20. Controller38may be programmable and may control the power generators in both wattage and frequency and may be further responsive to a configured duty cycle which enables a reconfiguration of the operation of the power generators during a semiconductor wafer treatment process.

While embodiments of the present invention contemplate various operational parameters on the corresponding power generators, as defined herein, the term “inactive” or similar terminology as applied to a power generator includes the deactivation of the entire power generator and further includes the reduction in dominating power of a specific power generator. Therefore, in lieu of disabling or turning off a power generator, a reduction in power, for example, from one or more kilowatts to one or more hundreds of watts results in the same overall effect while allowing some beneficial effects from the continued operation, albeit at a reduced level, of various power generators.

FIGS. 2A and 2Bare illustrative cross-sectional profiles of narrow gap vias which may be formed by the triple-frequency plasma reactor of the present invention. The cross-sectional illustrations are not to scale and are presented herein for illustrative purposes only of the various narrow gap profiles attainable through various combinations of the excitation of power generators12,18and20ofFIG. 1. InFIG. 2A, the cross-sectional view as illustrated results from the configuration of triple-frequency plasma reactor10(FIG. 1) according to the configuration or power profile ofFIG. 3A. InFIG. 3A, the generator signal40of upper power generator12(FIG. 1) is inactive while generator signal42of lower high-frequency power generator18(FIG. 1) and generator signal44of lower low-frequency power generator20(FIG. 1) are set to active or defined levels. Such a configuration results in a profile of a narrow gap via46which is directionally etched as defined by a mask48through, for example, a glass or other insulative layer50to a contact or target layer52. It should be noted that narrow gap via46assumes a bowed profile as a result of, for example, polymer buildup around the throat of the via.

The configuration or power profile ofFIG. 3Aprovides processing benefits including good mask or photoresist selectivity (i.e., the mask endurance through the plasma bombardment is relatively robust). Another benefit of the present configuration is that resultant narrow gap vias exhibit a desirable relatively large opening at the bottoms thereof. The present configuration further exhibits some less desirable characteristics, namely the bowing nature that occurs in the upper section of the narrow gap via as a result of the constriction at the throat portion or upper portion of the via.

InFIG. 2B, the cross-sectional view as illustrated results from the configuration of triple-frequency plasma reactor10(FIG. 1) according to the power profile ofFIG. 3B. InFIG. 3B, the generator signal54of upper power generator12(FIG. 1) is set to active for a defined level while generator signal56of lower high-frequency power generator18(FIG. 1) is inactive. Furthermore, generator signal58of lower low-frequency power generator20(FIG. 1) is set to an active or defined level. Such a configuration results in a profile of a narrow gap via60which is directionally etched as defined by a mask62through, for example, a glass or other insulative layer64to a contact or target layer66. It should be noted that narrow gap via60assumes a tapered narrowing profile as the depth through insulative layer64increases.

The configuration or power profile ofFIG. 3Bprovides processing benefits including a good initial profile at the throat or top of the narrow gap via. The present configuration further exhibits some less desirable characteristics, namely the appreciable narrowing of the via as the depth into the via increases. Therefore, the contact area at the bottom of the via must be accounted for with the depth and initial opening size at the top of the via.

FIG. 3CandFIG. 3Drepresent other configurations of excitation of power generators12,18and20of the triple-frequency plasma reactor10. Specifically, inFIG. 3C, the generator signal68of upper power generator12(FIG. 1) is set to an active or defined level as is the generator signal70of lower high-frequency power generator18(FIG. 1). Generator signal72of lower low-frequency power generator20(FIG. 1) is inactive. Referring toFIG. 3D, all frequencies74,76and78are set to active or defined levels for creation of plasma as used in a specific dry etching semiconductor process.

FIG. 4is a power profile of the excitation of the respective power generators of the triple-frequency plasma reactor10, in accordance with another embodiment of the present invention. The previous embodiments have illustrated a static configuration of the various power generators of the triple-frequency plasma reactor and the corresponding narrow gap vias resulting therefrom. In the present embodiment, a dynamic excitation of power generators12,18and20is illustrated by way of the formation of duty cycles associated with each of the power generators. Those of ordinary skill in the art appreciate that narrow gap vias are typically formed for the further formation of an electrical connection through the via to the corresponding target layer, such as a conductive trace or a pad. Ideally, the formation of a narrow gap via having sidewalls perpendicular with the target layer and with an adequate aspect ratio for accommodating a reliable filling of the narrow gap via is desirable. However, as previously illustrated inFIGS. 2A and 2B, various profiles of narrow gap vias exhibit desirable and undesirable profile characteristics.

Formation of a narrow gap via occurs as the plasma etching process proceeds over a continuum of time as defined by an etch rate and a resulting profile. The present embodiment varies the excitation of the power generators to advantageously formulate the plasma and the resulting electrical fields to select desirable etching characteristics over an entire etching process. InFIG. 4, various duty cycles are defined for the respective frequencies. In a first phase80, generator signal82of upper power generator12(FIG. 1) is set to an active or defined level. Generator signal84of lower high-frequency power generator18(FIG. 1) is inactive. Additionally, generator signal86of lower low-frequency power generator20(FIG. 1) is also set to an active or defined level during first phase80. The first phase configuration of power generators12,18and20of the triple-frequency plasma reactor10(all ofFIG. 1) similarly corresponds to the configuration as illustrated above with regard toFIG. 3Band correspondingly with the formation of an acceptable initial opening of narrow gap via60ofFIG. 2B. Correspondingly, the narrow gap via88ofFIG. 5illustrates the formation of an initial opening during first phase80.

Returning toFIG. 4, a second phase90alters the excitation of power generators12,18and20in an arrangement wherein generator signal82of upper power generator12(FIG. 1) is inactive while generator signal84of lower high-frequency power generator18(FIG. 1) and generator signal86of lower low-frequency power generator20(FIG. 1) are set to active or defined levels. Such a configuration results, during second phase90, of a more widened profile than would otherwise be attainable through the previous configuration as illustrated with reference to first phase80. Such a resulting narrow gap via profile is illustrated with reference toFIG. 5. As a large aperture is desirable when mating with a target layer, such as target layer92ofFIG. 5, a reconfiguration of the excitation of power generators12,18and20is desirable. With reference toFIG. 4, a third phase94reconfigures the excitation in a manner consistent with the excitation of first phase80, namely generator signals82and86of upper power generator12and lower low-frequency power generator20, respectively, are set to active or defined levels while generator signal84of lower high-frequency power generator18(FIG. 1) is inactive. Such a configuration of the excitation of the corresponding power generators enables the formation of a more desirably larger aperture when coupling with target layer92.

FIG. 6is a flowchart of a variable duty cycle multiple frequency plasma reactor, in accordance with an embodiment of the present invention. InFIG. 6, the power generators are configured100for a first phase. The status or completion of the first phase is queried102until the completion of the first phase. The power generators are reconfigured104for a subsequent phase with the duration of that phase queried106until its completion. Upon its completion, the determination of the last phase is queried108with any remaining phases being reconfigured104until each phase is completed.