The invention relates generally to plasma reactors and their operation. In particular, the invention relates to the composition of parts of the chamber facing the plasma in a plasma etch reactor.
Dry plasma etching is the preferred process for etching features on a silicon wafer being fabricated into semiconductor integrated circuits. Typically, one or more planar layers are deposited over the previously defined substrate, and a layer of photoresist mask or a hard mask is deposited over the planar layers and patterned to leave apertures exposing portions of the planar layers. An etching gas admitted into the etching reactor is then excited into a plasma state, and it acts on the portions of the planar layers exposed by the mask to remove those exposed portions. The dry plasma etching process has proved to be very effective at defining extremely small features with low production of deleterious particles.
The field of plasma etching is typically divided among silicon etching, oxide etching (typically of SiO2), and metal etching. Each uses its preferred chemistry and presents its own problems. However, many problems are common among them, and the etching chambers dedicated to different ones of the uses tend to resemble each other.
The most prevalent use of metal etching is to define interconnects (and accompanying contacts or vias) in a layer of aluminum or aluminum alloy deposited over an interlayer dielectric. Recently, copper interconnects have been developed. Once the generally planar aluminum layer has been deposited over the interlayer dielectric and into the contact or via holes, a photomask is deposited and defined over the aluminum layer. Then, an etching gas is admitted into the plasma etch chamber and excited into the plasma state. A typical etching gas is a halide-containing gas, usually F, Cl, or Br. The halogen reacts with the material being etched to typically form a volatile byproduct. It has long been known that a chlorine-based chemistry is effective at etching aluminum. See, for example, U.S. Pat. No. 5,387,556 to Xiaobing et al. Gaseous hydrochloric acid (HCl) is the prototypical chlorine-based etchant. However, HCl is no longer considered the optimum aluminum etchant.
Aluminum quickly forms an overlying layer of a native oxide of alumina (Al2O3) and related materials forming a residue over the metallic aluminum being etched. Alumina is a relatively stable material and resistant to reductive breakdown, even by HCl. For these reasons, a plasma etch of boron trichloride (BCl3), often in conjunction with HCl or Cl2, is typically used for etching aluminum and its alloys. Wang et al. in U.S. Pat. No. 5,219,485 use a similar chemistry to etch silicides in order to avoid residues from the silicide etch.
However, the use of a powerful chlorine-based etchant like BCl3 introduces a problem originating from the fact that the chamber body is most economically made of aluminum, for example the alloy A16061-T6, and the chamber dome is usually made of alumina. The seminal problem is that a chamber having an aluminum body and which is used for etching aluminum must balance the etching of the aluminum portion of the substrate against the etching of the chamber body. Typically, the aluminum chamber body is anodized, that is, electrochemically processed to be covered with a moderately thin coating of alumina, to provide some protection for the aluminum. Nonetheless, though usually to a lesser extent, a chlorine-based etchant can also attack the alumina, whether in the dome or the thin anodized layer on the chamber body. The physical integrity of the aluminum chamber and alumina dome are important, but a more important problem arises from the fact that the etching of these parts is likely to produce aluminum-based particles that deleteriously fall on the wafer and reduces the yield of functioning integrated circuits. As a result, the chamber wall in a plasma reactor intended for aluminum etching advantageously should not be composed of aluminum, even with a coating of alumina. Alumina is relatively resistant to a chlorine-based etch, though not impregnable. However, as will be explained later, fluorine is often also used, which more readily etches alumina.
In U.S. patent application Ser. No. 08/770,092 filed Dec. 19, 1996, Shih et al. (including the two present inventors plus others) describe a protective coating of boron carbide (nominally B4C) applied to the aluminum chamber walls. A similar disclosure appears in European Patent Application EP-849,767-A2. This patent application is incorporated herein by reference in its entirety. The boron carbide coating has been applied to a high-density plasma reactor, known as the Decoupled Plasma Source (DPS) Metal Etch Chamber available from Applied Materials, Inc. of Santa Clara, Calif.
A schematic representation of the commercial DPS chamber is illustrated in the cross-sectional view of FIG. 1. An upper, main processing compartment 10 is bounded by a curved ceramic dome 12 typically of alumina, an upper housing 14 typically of aluminum to which the ceramic dome 12 is sealed, and a movable pedestal wall 16 that is vertically movable to engage and seal within an inwardly extending annular shelf 18 of the upper housing 14. The upper housing 14 rests on and is sealed to a lower housing 20, and a bellows 22 is sealed to the bottom of the lower housing 20 and to a stem 24 extending downwardly from the pedestal wall 16. An electrode 19 may be included at the center of the dome 12. A lower compartment 26 is defined generally by the walls of the lower housing 20 and the lower edge of the annular shelf 18. During plasma processing, the movable pedestal wall 16 seals the upper compartment 10 from the lower compartment 22 by engaging and sealing itself to the annular shelf 18 of the upper housing 14.
A vertical actuator 28 connected to the bottom of the stem 24 can move the pedestal wall 16 into and out of engagement with the annular shelf 18. An unillustrated robot blade can transfer a wafer 30 into the lower compartment through a loadlock slit 32 in the lower housing 20 and its unillustrated slit valve when the vertical actuator 28 has lowered the pedestal wall 16 to a position to receive the wafer 30 on its upper surface. The pedestal wall 16 typically includes an electrostatic chuck to selectively hold the wafer 30 by electrostatic attraction exerted by an electrical signal applied to the chuck. After the wafer has been placed on the pedestal wall 16, the vertical actuator 28 raises the pedestal wall 16 so that it seals the upper compartment 10 and places the wafer within the upper compartment 10.
The upper housing 14 also includes a turbo port 38 connecting to an integral pumping stack 40. A vacuum pumping system 42 mated with the bottom of a pumping stack 40 pumps the upper compartment 10 as well as the lower compartment 26 when it is opened to the upper compartment 10. A poppet valve 44 fixed to the upper housing 14 over the pumping stack 40 can selectively isolate the upper compartment 10 from the vacuum pumping system 42.
Processing gas, which for etching aluminum typically includes BCl3 and Cl2 as well as possibly CF4, CHF3, N2, Ar, etc., is injected into the sealed upper compartment 10 through a plurality, typically four, of unillustrated gas nozzles fixed to the radially inner ends of respective gas orifices 46 penetrating the upper housing 14 near its top. RF power is applied to an inductive coil 48 wrapped around the curved dome 12 so as to create a high-density plasma of the processing gas within the upper compartment 10. RF power is also applied to the wafer pedestal 16 and possibly to an unillustrated counter electrode fixed in middle of the curved dome 12 so as to bias the plasma to effect the desired etching of the wafer.
According to Shih et al., a coating of boron carbide is plasma sprayed onto the inside of the aluminum chamber housing 14 to protect it from the chlorine plasma. Although the original disclosure suggests coating only an upper portion of the chamber wall 14 in a band around the gas ports 46 and leaving the lower portion as anodized aluminum, more recent results show the advantage of extending the boron carbide coating to all portions of the aluminum chamber wall 14 exposed to the plasma. Plasma-sprayed boron carbide has proven to be extremely durable against a chlorine plasma, particularly with the extended wall coverage. While in the past metal etch chamber walls needed to be replaced after approximately 20,000 wafers were etched, the B4C-coated wall has been used for more than 100,000 wafers. Such a lifetime typically defines the demarcation between a consumable part subject to contractually enforced minimum lifetimes and a non-consumable part not subject to further contract limitations.
The boron carbide coating, however, has not solved the entire durability problem associated with chlorine-based plasma etching because boron carbide cannot be beneficially coated onto all chamber parts. A schematic illustration of DPS chamber of FIG. 1 is presented in FIG. 2 emphasizing its electrical characteristics. The wafer pedestal 16 is usually electrically conductive and is electrically biased by a bias RF power supply 50 though an RF coupling capacitor 52 so as to control the DC self-bias seen by the wafer 30. The DC self-bias controls the energy of plasma ions drawn across the plasma sheath and incident on the wafer 30. The B4C-coated chamber wall 14 is typically electrically grounded or at a minimum held at a predetermined DC potential to define a grounding plane (or anode) relative to the RF biased pedestal electrode 16. The chamber wall 14 is thus usually formed of a metal, preferably aluminum for reasons of economy. Boron carbide in its pure crystalline forms is a semiconductor with a bandgap of about 1.6 eV. Typical electrical resistivities are 0.1 to 1 ohm-cm at room temperature and 0.038 ohm-cm at 600xc2x0 C. Boron carbide thus has sufficient electrical conductivity to extend the grounding into the interior of the chamber, particularly at the higher temperatures experienced in HDP etching.
The DPS chamber is a high-density plasma (HDP) reactor, a high-density plasma being defined as having an ionization density of at least 1011cmxe2x88x923 throughout the region it extends with the except of plasma sheaths. Most of the plasma power is inductively coupled into the chamber by the inductive coil 48 wrapped around the dome 12 and powered by a source RF power supply 58. The actual electrical connections to the inductive coil 48 are more complex than the single direct connection illustrated. The power applied to the coil 48 creates an RF magnetic field inside the chamber, which induces a circumferential electric field that powers the plasma. However, at these high frequencies an electrically conductive dome 12 would short the RF magnetic field. As a result, the electrical resistivity of the dome 12 must be high, and it is usually composed of an insulative ceramic.
The most typical ceramic material for the dome is xcex1-alumina having a resistivity of over 1012 ohm-cm in its purer forms. It has a chemical composition of approximately Al2O3, to about 99.5% or higher, but some silica is usually included in commercial products. Alumina is widely used in semiconductor fabrication equipment and is economically available in large, complex shapes. However, we have found that an alumina dome causes substantial problems in an HDP metal etch reactor.
Visually, three zones seem to develop within the dome that are differently affected by either a chlorine-based etchant or a chlorine-based etchant with additional fluorocarbon. As illustrated in FIG. 2, an inner zone 60 at the middle of the alumina dome 12 not having the inductive coil 48 as its back suffers relatively little damage although sometimes it is covered by a polymeric coating. Scanning electron micrographs (SEMs) show an unetched or slightly etched, compacted granular structure. A coil zone 62 is formed as an annular region generally located beneath the innermost windings of the inductive coil 48 show substantial damage. The alumina is etched into pinnacles having diameters of a few hundred nanometers. It is believed that the pinnacles had been separated by grain boundaries prior to etching. It is further believed that the pinnacles eventually break off, probably when the bottom grain boundaries are reached, and they cause a severe particle problem. An edge zone 64 is formed as an annular region generally located beneath the outermost windings of the coil 48 and also beneath the outer coil-free region of the dome. Under some conditions, the edge zone 64 is etched, but into more of a cavernous structure than in the coil zone 62. Under other conditions, the edge region 64 is covered with a dark polymeric coating and it left unetched. In all cases, etching of the grain boundaries is observed. It is believed that the grain boundaries include a silicon-rich glassy phase that is easily attacked by either chlorine or fluorine. However, at the coil zone 62 or edge zone 64, all portions are observed to be etched. The dome etching is likely to produce tiny alumina particles which fall onto the wafer causing defects. The addition of a fluorocarbon tends to increase the observed polymerization
A further problem with using alumina as the dome material arises from the need to etch relatively deep (≈1 xcexcm) and narrow (less than 0.25 xcexcm) holes into the metal. Some sidewall passivation is needed with these high aspect ratios. A fluorocarbon gas such as carbon tetrachloride (CF4) is sometimes added to the plasma etching gas and to deposit as a protective polymer on the vertical sidewalls. However, such a fluorocarbon plasma, because it is fluorine-based, attacks alumina, thereby forming volatile CO or CO2 and harmful AlF3 particulates.
Clearly, uncoated alumina is unsatisfactory for the dome material in the reactor of FIGS. 1 and 2. A more robust material is desired.
A first approach to solving the dome problem would be to coat it with boron carbide. While such an approach may work with proper constraints, boron carbide introduces inherent design problems. Boron carbide has a low but finite electrical resistivity in the range of 0.04 to 1 ohm-cm. In view of the need for 100,000-wafer lifetimes, significant film thicknesses are required even for small erosion rates. For B4C-coated aluminum, Shih et al. suggest a minimum coating thickness of 125 xcexcm, and a thickness of 1 mm would be desirable for a margin of error. Such thicknesses of boron carbide present too high a resistive loss when used inside an RF inductive coil.
Silicon or more specifically polysilicon has been used as a dome material in oxide etching since its resistivity can be increased to almost 200 ohm-cm. However, silicon is expensive, prone to cracking, and is subject to some etching. For metal etching in the DPS chamber, even the stated resistivity for polysilicon is believed to be insufficiently high for an RF window.
Steger has suggested in U.S. Pat. No. 5,268,200 the use of a conductive carbon coating on metal walls in a plasma etching chamber. Steger does not specify the crystallographic state of the metal coating, but in view of its low electrical resistivity (less than 2xc3x9710xe2x88x924 ohm-cm) and low formation temperature (below 500xc2x0 C.), it is likely graphitic. The very high electrical conductivity makes Steger""s coating unsuitable for coating a dome.
Thus, no satisfactory dome material or dome coating has to date been found.
Accordingly, an object of the invention is to provide a protective coating suitably formed over a ceramic coating that has a high electrical resistivity but is not significantly etched by a plasma, particularly a chlorine-based plasma.
According to one aspect of the invention, a diamond layer is coated onto an insulating substrate, particularly a ceramic one, used in a plasma reactor.
In a particular aspect of the invention, the diamond coating is applied to a chamber wall behind which is wound an RF inductive coil.
Diamond coatings can be advantageously applied to alumina, silicon nitride, silicon carbide, or polysilicon bulk substrates.