Method for improving uniformity and reducing etch rate variation of etching polysilicon

[0002] 1. Field of the Invention

[0003] The invention relates to a method of plasma etching polysilicon with HBr and apparatus for improving plasma etching uniformity.

[0004] 2. Description of the Related Art

[0005] Plasma processing systems wherein an antenna coupled to a radiofrequency (RF) source energizes gas into a plasma state within a process chamber are disclosed in U.S. Pat. Nos. 4,948,458; 5,198,718; 5,241,245; 5,304,279; 5,401,350; and 5,571,366. In such systems, the antenna is located outside the process chamber and the RF energy is supplied into the chamber through a dielectric wall or window. Such processing systems can be used for a variety of semiconductor processing applications such as etching, deposition, resist stripping, etc. When such systems are used for plasma etching a batch of consecutively processed semiconductor substrates, the etching rate and/or uniformity may change during the course of etching the batch of substrates. Such variation in etch rate/uniformity is undesirable since the features etched into the substrate may fall outside acceptable product parameters.

[0006] Polysilicon etching techniques are disclosed in U.S. Pat. Nos. 5,242,536; 5,314,573; 5,336,365; 5,368,684; and 5,763,327. Of these, the '536 patent discloses anisotropic polysilicon etching in a parallel plate plasma etcher using HBr gas along with Cl₂and He. The '573 patent discloses anisotropic polysilicon etching in a parallel plate plasma etcher using HBr gas along with HCI or Cl₂. The '365 patent discloses anisotropic polysilicon etching in an electron cyclotron resonance (ECR) plasma etcher using HBr gas along with Cl₂in a first step and HBr and He in a second step. The '684 patent discloses anisotropic polysilicon etching in a magnetron plasma etcher using HBr gas, a mixture of HBr and Cl₂or a mixture of HBr and HCl. The '327 patent discloses polysilicon etching in a plasma etcher using Cl₂, Br₂or HBr, He and O_2.

[0007] Techniques for etching polysilicon using HBr alone or in combination with other gasses are described in U.S. Pat. Nos. 5,160,407; 5,180,464; 5,560,804; 5,591,664; 5,665,203; 5,670,018; 5,792,692; 5,801,077; 5,804,489; 5,861,343; 5,932,115; and 5,994,234.

[0008] U.S. Pat. No. 6,022,809 discloses a composite shadow ring used in a plasma etch chamber, the ring including an insert which does not generate contaminating oxygen gas when bombarded by a gas plasma such as a fluorine-containing plasma. The shadow ring is movably mounted in the bottom of a vacuum chamber such that a wafer can be loaded onto or off of an electrostatic chuck and plasma is generated in the chamber by a coil antenna surrounding a sidewall of the chamber. The shadow ring includes an outer body portion of silicon dioxide and an inner ring-shaped insert portion of silicon. During via etching of a spin-on-glass (SOG) layer the insert is intended to reduce generation of oxygen gas which can inhibit or attack polymeric sidewall passivation layers in the via openings thereby reducing tungsten plug loss near the peripheral edge of the wafer.

[0009] While there have been proposals in the prior art to improve polysilicon etch processes and while attempts have been made to prevent generation of oxygen during etching by use of pure silicon materials, there is a need in the art for improvements in etching uniformity of polysilicon.

[0010] The invention provides a method of consecutively processing a series of semiconductor substrates with minimal plasma etch rate variation following cleaning with fluorine-containing gas and/or seasoning of the plasma etch chamber. The method includes steps of (a) placing a semiconductor substrate on a substrate support in a plasma etching chamber, (b) maintaining a vacuum in the chamber, (c) processing the substrate by supplying an HBr-containing etching gas to the chamber and energizing the etching gas to form a plasma in the chamber, (d) removing the substrate from the chamber; and (e) consecutively processing additional substrates in the chamber by repeating steps (a-d), the etching step being carried out by maintaining a recombination rate of H and Br on a member surrounding the substrate at a rate sufficient to offset a rate at which Br is consumed across the substrate.

[0011] According to a preferred feature of the invention, the semiconductor substrate can include a layer of polysilicon which is etched with HBr during step (c) and/or the chamber is maintained at a vacuum pressure of less than 100 mTorr during step (c). During step (c) an RF bias of no more than 500 watts can be applied to the substrate by the substrate support.

[0012] In the preferred embodiment, the etching gas is energized into a plasma state by a planar or non-planar antenna separated from the interior of the chamber by a dielectric member. Preferably, the dielectric member is at least coextensive with the substrate support and the etching gas is supplied to the chamber by at least one gas inlet in the dielectric member.

[0013] The invention will be described in greater detail with reference to the accompanying drawing wherein:

[0014] FIG. 1 shows a schematic cross sectional view of a plasma etching chamber which can be used to carry out the method of the invention.

[0015] The invention provides an improved plasma etching method capable of providing more repeatable results when etching a batch of consecutively processed semiconductor substrates. In particular, it has been found that during the course of etching a batch of semiconductor wafers, the etch rate can vary significantly depending on whether the etching is preceded by an in-situ chamber clean or seasoning of the chamber by running one or more conditioning wafers through the chamber prior to processing production wafers. According to the invention, it has been surprisingly discovered that the etch rate following cleaning or seasoning can be made more uniform by using silicon carbide as the material of the edge ring surrounding the wafer.

[0016] The method of the invention can be carried out in any suitable plasma etch reactor wherein individual substrates are consecutively processed. A preferred plasma etching reactor is an inductively coupled plasma reactor such as shown in FIG. 1. As shown, the apparatus includes a chamber 2, a substrate support 4, an edge ring 6 surrounding a substrate 8, a dielectric window 10, an inductively coupled plasma source 12, and an outlet 14 leading to a vacuum pump. In addition, etching gas is supplied to the chamber through a gas inlet 18 in the dielectric window 10. The plasma source is preferably a planar antenna such as a spiral coil supplied with RF energy from a suitable RF generator 20 and the substrate support preferably includes an electrode supplied with RF energy to apply a suitable RF bias to the substrate. The edge ring 6 can be entirely of CVD SiC or a composite such as sintered SiC or other material coated with SiC. However, it is preferable for the entire surface of the edge ring exposed to the plasma in the chamber to be of silicon carbide. Further, to minimize electrical field effects, the edge ring 6 can have its upper surface located above the exposed surface of the substrate 8, e.g. the top of the edge ring can be 1-2 mm above the surface of the substrate.

[0017] According to the invention, the edge ring of silicon carbide provides etch rate stabilization during batch processing of individual semiconductor substrates. Moreover, the edge ring can be maintained free from deposition build-up due to ion bombardment produced by the RF bias. Such self cleaning is advantageous in maintaining the etch rate stabilization since deposition on the edge ring can alter H and Br recombination which causes etch rate drift. The reactor can include any suitable source for generating plasma in the chamber (e.g., parallel plate, inductive coupled, ECR, magnetron, helicon, etc.) and any suitable gas supply for delivering etching gas into the chamber (e.g., a gas ring, showerhead, gas injector mounted in an opening extending through the dielectric window, etc.).

[0018] In the embodiment shown in FIG. 1, the vacuum processing chamber preferably includes a substrate holder providing an electrostatic clamping force to a substrate and the edge ring confines the plasma in an area above the substrate while it is He backcooled. A source of energy for maintaining a high density (e.g. at least 10⁹ions/cm³) plasma in the chamber such as a planar antenna powered by a suitable RF source and associated RF impedance matching circuitry can be used to inductively couple RF energy into the chamber so as to provide a high density plasma. A vacuum pump preferably maintains the interior of the chamber at a desired pressure (e.g., below 50 mTorr, typically 1-20 mTorr).

[0019] The reactor shown in FIG. 1 includes a substantially planar dielectric window 10 of uniform thickness between the antenna 12 and the interior of the processing chamber. However, other antenna and/or widow configurations can be used such as a non-planar antenna and/or a non-planar dielectric window. Further, while any suitable gas supply arrangement can be used, in the embodiment shown a centrally located gas inlet 18 is provided in the dielectric window. The inlet 18 can be supplied gas from one or more gas supplies 22. The substrate holder can include conventional features such as a lift pin arrangement (not shown) for lifting the substrate.

[0020] According to the invention it has been discovered that use of a silicon carbide edge ring can improve HBr etch uniformity. In order to compare the effectiveness of a silicon carbide edge ring to other materials, tests were carried out using edge rings of quartz, silicon nitride and alumina. Compared to these other materials, the silicon carbide edge ring provides a much more repeatable etch rate after chamber cleaning using fluorine-based gas chemistries or after chamber seasoning wherein conditioning wafers are processed prior to etching production wafers. A diode laser was used to measure HBr dissociation percentages during polysilicon etching. The tests were carried out after chamber seasoning (wherein 6 bare silicon wafers were processed prior to processing production wafers) and after waferless cleaning (WAC) of the chamber (wherein the chamber was cleaned using a fluorine-based cleaning gas while the substrate support was uncovered). The plasma etch chamber used in the tests was a LAM 2300™ etch chamber and quartz, alumina and silicon carbide were used as edge ring materials. The dissociation percentages are set forth in the following table.

[0021] During plasma etching of polysilicon with HBr, silicon combines with Br and is removed as a gas. Accordingly, it is desirable to maintain an HBr dissociation rate at the inner portion of an etched wafer at the same rate as at the outer periphery of the wafer. However, because different edge ring materials cause H and Br to recombine at different rates, edge ring materials which provide a high H and Br recombination rate can be used to provide an etch rate at the outer periphery of the wafer at the same rate as at the center of the wafer. In contrast, if the recombination rate at the edge ring is low, nonuniform etching can result since more Br is available for etching the periphery of the wafer compared to the inner portion of the wafer.

[0022] According to the invention, uniformity of polysilicon etching can be improved by increasing the recombination rate at the edge of the wafer. In particular, the edge ring material is selected to increase the recombination rate and the width of the edge ring can be increased to enhance this effect. For example, in a chamber designed to etch 200 mm wafers, the edge ring width can be selected to be at least 10% of the wafer diameter, i.e., at least 20 mm, preferably 25 mm or wider in the case of a 200 mm wafer.

[0023] A silicon carbide edge ring in accordance with the invention provides additional advantages. For instance, compared to an alumina edge ring which can be a source of aluminum contamination, an edge ring made of high purity silicon carbide such as chemically deposited SiC (CVD SiC) does not present such contamination problems. Compared to a quartz edge ring which is chemically attacked by fluorine-based cleaning gas chemistries, a silicon carbide edge ring is more chemically stable. Further, when it is desired to carry out polysilicon etching at a target etch rate after WAC or chamber seasoning, a silicon carbide edge ring provides the least affected polysilicon etch rate compared to a quartz edge ring, a silicon nitride edge ring or an alumina edge ring.

[0024] The process according to the invention can be carried out using pure HBr or HBr in combination with other gases. For instance, the polysilicon etch can be carried out using HBr alone or in combination with one or more other gases during main etching and the gas chemistry can be changed during overetching. An example of suitable gas chemistries follows.

[0025] According to a first embodiment of etching polysilicon such as masked silicide/polysilicon layers, the main etch can be carried out using HBr and Cl₂with or without non-etching gases such as argon, helium or nitrogen. If desired, the Cl₂could be replaced with another Cl source such as HCl. The process parameters such as power to the plasma source, wafer temperature, RF bias, chamber vacuum pressure, etc. can be set in accordance with the type of plasma reactor being used. For an inductively coupled plasma source, the antenna power can be in the range of 200 to 1000 watts, the temperature of the wafer can be 10 to 100° C., the chamber pressure can be 5 to 50 mTorr, the RF bias can be less than 500 watts, etc. In the overetching step, the gas can be modified to include oxygen and helium. For instance, the overetch can be carried out using HBr, Cl₂, He and O₂. Alternatively, the Cl₂can be terminated during the overetching step.

[0026] According to a second embodiment, a polycide film comprising a lower layer of doped polysilicon and an upper layer of tungsten silicide can be etched using HBr and SF₆and overetching can be carried out by terminating the supply of SF₆and adjusting the flow of HBr to a desired level. Other fluorine gases besides SF₆can also be used, such gases including NF₃, ClF₃, F₂, HF, C₂F₆, CHF₃, CF₄, CH₂F₆, C₃F₈, C₄F₈, or the like. Further, N₂and/or O₂can be added for sidewall protection and gases such as argon and/or helium can be added for sputtering, dilution or cooling effects, if desired.

[0027] According to a third embodiment, the polysilicon etching can be preceded by a step of removing any surface oxide on the polysilicon layer. For instance, a surface oxide which may have formed on a polysilicon layer during a preceding step such as stripping of a lithographic mask can be removed using a suitable gas such as CF₄. Then, the polysilicon on a 300 mm wafer can be etched using a mixture of 50-300 sccm HBr, 20-150 sccm Cl₂, 1-20 sccm of a He/O₂mixture (e.g., 70% He and 30% O₂). In a following step the Cl₂can be terminated and in a third step to achieve overetching the power and chamber pressure can be reduced while using the Cl₂-free HBr and He/O₂gas mixture.

[0028] In a fourth embodiment, a tungsten silicide layer can be etched with a Cl₂/O₂mixture and a doped polysilicon layer beneath the tungsten silicide film can be etched with HBr alone or a HBr/O₂mixture. During the etching steps, the plasma source power can be adjusted to provide a desired plasma density. For instance, the plasma density can be adjusted on the order of 1×10¹⁰cm⁻³. However, lower plasma densities on the order of 1×10⁹cm^×3or higher plasma densities on the order of at least 1×10¹¹cm^{33 3}can also be used.

[0029] The foregoing has described the principles, preferred embodiments and modes of operation of the present application. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.