Pulsed nucleation deposition of tungsten layers

[0001] The present application is a continuation-in-part of application Serial No. 10/023,125, filed Dec. 17, 2001, entitled Pulse Nucleation Enhanced Nucleation Technique For Improved Step Coverage and Better Gap Fill For WCVD Process, which in turn claims priority from U.S. provisional application No. 60/305,307, filed Jul. 13, 2001, entitled Pulse Nucleation Enhanced Nucleation Technique For Improved Step Coverage and Better Gap Fill For WCVD Process. Each of these applications is incorporated herein by reference in its entirety.

[0002] 1. Field of the Invention

[0003] The present invention generally relates to a method of forming tungsten layers and, more particularly forming tungsten layers using a pulsed nucleation deposition process.

[0004] 2. Description of the Related Art

[0005] Integrated circuits have evolved into complex devices that can include millions of components (e.g., transistors, capacitors and resistors) on a single chip. The evolution of chip designs continually requires faster circuitry and greater circuit density. The demands for greater circuit density necessitate a reduction in the dimensions of the integrated circuit components as well as improved layer uniformity.

[0006] In particular, metal layers, such as those used for interconnect, contact, and plug fill applications, are subject to increasingly stringent requirements related to step coverage and layer uniformity. Tungsten layers, for example, are particularly difficult to deposit with uniform step coverage. Typically, tungsten layers are formed by chemical vapor deposition (CVD), using a two step process. A bulk tungsten layer is typically deposited by reacting tungsten hexafluoride (WF₆) with hydrogen (H₂) on a substrate. In order to reduce the time required to initiate the reaction between the tungsten hexafluoride (WF₆) and the hydrogen (H₂) (i.e., reduce the incubation time), a nucleation layer is typically deposited prior to depositing the bulk tungsten layer. Unfortunately, the nucleation layer, typically deposited by reacting tungsten hexafluoride (WF₆) with silane (SiH₄), has sub-optimal step coverage. As a result, voids may form in the tungsten nucleation layer during deposition, adversely impacting the electrical performance of devices fabricated therefrom.

[0007] Therefore, a need exists in the art for a method of depositing a tungsten nucleation layer with improved step coverage.

[0008] Embodiments of the present invention generally relate to a method of forming a tungsten nucleation layer using a sequential deposition process. The tungsten nucleation layer is formed by reacting pulses of a tungsten-containing precursor and a reducing gas in a process chamber to deposit tungsten on the substrate. Thereafter, reaction by-products generated from the tungsten deposition are removed from the process chamber. After the reaction by-products are removed from the process chamber, a flow of the reducing gas is provided to the process chamber to react with residual tungsten-containing precursor remaining therein. Such a deposition process forms tungsten nucleation layers having good step coverage. The sequential deposition process of reacting pulses of the tungsten-containing precursor and the reducing gas, removing reaction by-products, and than providing a flow of the reducing gas to the process chamber may be repeated until a desired thickness for the tungsten nucleation layer is formed.

[0009] The formation of the tungsten nucleation layer is compatible with integrated circuit fabrication processes. In one integrated circuit fabrication process, the tungsten nucleation layer is formed prior to tungsten plug formation. For such an embodiment, a preferred process sequence includes depositing a tungsten nucleation layer in apertures defined in a dielectric material layer formed on a substrate. The tungsten nucleation layer is formed using a sequential deposition process in which pulses of a tungsten-containing precursor and a reducing gas are reacted, reaction by-products removed, and than a flow of reducing gas is provided to a process chamber. The sequential deposition process may be repeated until a desired thickness for the tungsten nucleation layer is deposited. Thereafter, the tungsten plug is completed when a bulk tungsten layer is deposited on the tungsten nucleation layer to fill the apertures.

[0010] So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

[0011] It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0012] FIG. 1 is a schematic cross-sectional view of a processing chamber that may be used to practice embodiments described herein;

[0013] FIG. 2 illustrates a process sequence for tungsten nucleation layer formation using deposition techniques according to embodiments described herein; and

[0014] FIGS. 3A-3B depict cross-sectional views of a substrate at different stages of an integrated circuit fabrication sequence.

[0015] Embodiments described herein relate to a method for depositing a tungsten nucleation layer on a substrate. FIG. 1 depicts a schematic cross-sectional view of a chemical vapor deposition (CVD) process chamber 100 that may be used to practice embodiments of the invention described herein. The process chamber 100 may be part of a processing system (not shown) that includes multiple processing chambers connected to a central transfer chamber (not shown) and serviced by a robot (not shown).

[0016] The process chamber 100 includes walls 106, a bottom 108, and a lid 110 that define a process volume 112. The walls 106 and bottom 108 are typically fabricated from a unitary block of aluminum. The walls 106 may have conduits (not shown) therein through which a fluid may be passed to control the temperature of the walls 106. The process chamber 100 may also include a pumping ring 114 that couples the process volume 112 to an exhaust port 116 as well as other pumping components (not shown).

[0017] A heated support assembly 138 is centrally disposed within the process chamber 100. The support assembly 138 supports a substrate 300 during a deposition process. The support assembly 138 generally is fabricated from aluminum, ceramic or a combination of aluminum and ceramic and typically includes a vacuum port (not shown) and at least one or more heating elements 132.

[0018] The vacuum port may be used to apply a vacuum between the substrate 300 and the substrate support 138, so as to secure the substrate 300 to the substrate support 138 during the deposition process. The one or more heating elements 132, may be, for example, electrodes disposed in the substrate support 138, and coupled to a power source 130, to heat the substrate support 138 and substrate 300 positioned thereon to a predetermined temperature.

[0019] Generally, the support assembly 138 is coupled to a stem 142. The stem 142 provides a conduit for electrical leads, vacuum and gas supply lines between the support assembly 138 and other components of the process chamber 100. Additionally, the stem 142 couples the support assembly 138 to a lift system 144 that moves the support assembly 138 between an elevated position (as shown in FIG. 1) and a lowered position (not shown). Bellows 146 provide a vacuum seal between the chamber volume 112 and the atmosphere outside the chamber 102 while facilitating the movement of the support assembly 138.

[0020] The support assembly 138 additionally supports a circumscribing shadow ring 148. The shadow ring 148 is annular in form and typically comprises a ceramic material such as, for example, aluminum nitride. Generally, the shadow ring 148 prevents deposition at the edge of the substrate 300 and support assembly 138.

[0021] The lid 110 is supported by the walls 106 and may be removed to service the process chamber 100. The lid 110 is generally comprised of aluminum and may additionally have heat transfer fluid channels 124 formed therein. The heat transfer fluid channels 124 are coupled to a fluid source (not shown) that flows a heat transfer fluid through the lid 110. Fluid flowing through the channels 124 regulates the temperature of the lid 110.

[0022] A mixing block 134 is disposed in the lid 110. The mixing block 134 is coupled to gas sources 104. Generally, individual gas streams from the gas sources 104 are combined in the mixing block 134. These gases are mixed into a single homogeneous gas flow in the mixing block 134 and introduced into the process volume 112 after passing through a showerhead 118 that diffuses the gas flow outwardly towards the chamber walls 106.

[0023] The showerhead 118 is generally coupled to an interior side 120 of the lid 110. A perforated blocker plate 136 may optionally be disposed in the plenum 122 between the showerhead 118 and lid 110. Gases (i.e., process and other gases) that enter the chamber 102 through the mixing block 134 are first diffused by the blocker plate 136 as the gases fill the plenum 122 behind the showerhead 118. The gases then pass through the showerhead 118 and into the chamber 102. The blocker plate 136 and the showerhead 118 are configured to provide a uniform flow of gases to the process chamber 100. Uniform gas flow is desirable to promote uniform layer formation on the substrate 300.

[0024] The CVD process chamber 100 is controlled by a microprocessor controller 154. The microprocessor controller may be one of any form of general purpose computer processor (CPU) that can be used in an industrial setting for controlling various chambers and sub-processors. The computer processor may use any suitable memory, such as random access memory, read only memory, floppy disc drive, hard disk, or any other form of digital storage local or remote. Various support circuits may be coupled to the CPU for supporting the processor in a conventional manner. Software routines as required may be stored in the memory or executed by a second CPU that is remotely located.

[0025] The software routines are executed after the substrate is positioned on the substrate support. The software routines, when executed, transform the general purpose computer into a specific process computer that controls the chamber operation so that a chamber process is performed. Alternatively, the software routines may be performed in hardware as an application specific integrated circuit or other type of hardware implementation, or a combination of software and hardware.

[0026] Tungsten Nucleation Layer Formation

[0027] FIG. 2 illustrates a process sequence 200 detailing the various steps used for the formation of a tungsten nucleation layer. These steps may be performed in a CVD process chamber similar to that described above with reference to FIG. 1. As shown in step 202, a substrate is provided to the CVD process chamber. The substrate may be, for example, a silicon substrate, which may or may not have one or more material layers disposed thereon. The one or more material layers, for example, may be an oxide layer having a sub-quarter micron aperture therein. Sub-quarter micron apertures typically have aspect ratios (e.g., the feature width divided by the feature length) greater than about 8:1.

[0028] The sub-quarter micron features may have a barrier layer formed therein. The barrier layer may be, for example, titanium nitride (TiN). The barrier layer generally serves to prevent diffusion of the tungsten into surrounding material layers and to enhance the adhesion of the tungsten layer.

[0029] In step 204, a tungsten nucleation layer is deposited on the substrate conformably in the sub-quarter micron features. The tungsten nucleation layer may be formed, for example, from a reaction of a tungsten-containing precursor such as, for example, tungsten hexafluoride (WF₆) with a reducing gas such as for example, silane (SiH₄), disilane (Si₂H₆), borane (BH₃) and diborane (B₂H₆), among others. In one embodiment, tungsten hexafluoride (WF₆) is reacted with silane (SiH₄). Helium (He) and argon (Ar), or other inert gases, may also be provided along with the reactant gases either singly or in combination.

[0030] Typically, the reaction may be performed at a tungsten hexafluoride (WF₆) flow rate of about 5 sccm (standard cubic centimeters per minute) to about 100 sccm and a silane (SiH₄) flow rate of about 1 sccm to about 100 sccm. The tungsten-containing precursor and the reducing gas may be provided to the process chamber in a tungsten-containing precursor to reducing gas ratio of about 1:1 to about 5:1. It is believed that such ratios for the tungsten-containing precursor and the reducing provide good step coverage for the tungsten nucleation layer. A total pressure of about 0.5 torr to about 100 torr and a pedestal temperature of about 200 C. to about 600 C. may be used.

[0031] The tungsten-containing precursor and the reducing gas may be flowed or pulsed into the process chamber for a time period of about 0.1 seconds to about 10 seconds, and preferably for about 1 second to about 3 seconds. The time period of reactant gas flow should be generally long enough to provide a sufficiently high deposition rate for the tungsten nucleation layer.

[0032] Referring to step 206, after the flow or pulse of the reactant gases has ceased, the process chamber is purged/pumped to remove reaction by-products generated during nucleation layer deposition. The production of these reaction by-products at or near the surface of the tungsten nucleation layer are believed to form a rough tungsten surface and contribute to non-conformality and poor step coverage during tungsten nucleation layer deposition. Removal of the reaction by-products using a purge/pump step may provide nucleation layers with improved step coverage.

[0033] The purge gas may be one or more gases selected from the group of argon (Ar), nitrogen (N₂), helium (He) neon (Ne) and xenon (Xe), among others. Typically, the process chamber is purged by providing thereto a purge gas at a flow rate of about 5 sccm to about 5000 sccm, for up to about 10 seconds.

[0034] Alternatively or in addition to introducing the purge gas, the process chamber may be depressurized in order to remove the residual reactant gases as well as any reaction by-products. The depressurization of the process chamber may result in the chamber pressure being reduced to a pressure in a range of about 0.01 torr to about 40 torr within a time period of about 0.5 seconds to about 20 seconds.

[0035] Depending upon the ratio of the tungsten-containing precursor to the reducing gas in the reactant gas mixture, as well as the removal time, excess tungsten-containing precursor may remain in the process chamber after the purge/pump step 206. Referring to step 208, after the purge/pump step is performed, a flow of the reducing gas may be provided to the process chamber. The flow of the reducing gas functions to react with excess tungsten-containing precursor that may be present near the surface of the nucleation layer, particularly in regions adjacent to the high aspect ration features after the purge/pump step 208.

[0036] The reducing gas may be provided to the process chamber as a pulse or flow for a time period within a range of about 0.1 seconds to about 10 seconds. In general, the time period should be long enough for the reducing gas to react with excess tungsten-containing precursor in the process chamber. Particularly for high aspect ratio features, it is believed that such flow of reducing gas may react with any excess tungsten-containing precursor in the vicinity of the feature improving the step coverage therefore and depositing tungsten in a void-free manner within such features.

[0037] Referring to step 210, after the flow or pulse of the reducing gas has ceased, the process chamber is purged/pumped to remove reaction by-product as well as residual gases remaining therein. The purge gas may be one or more gases selected from the group of argon (Ar), nitrogen (N₂), helium (He) neon (Ne) and xenon (Xe), among others. Typically, the process chamber is purged by providing thereto a purge gas at a flow rate of about 5 sccm to about 5000 sccm, for up to about 10 seconds.

[0038] Alternatively or in addition to introducing the purge gas, the process chamber may be depressurized in order to remove the residual reactant gases as well as any reaction by-products. The depressurization of the process chamber may result in the chamber pressure being reduced to a pressure in a range of about 0.01 torr to about 40 torr within a time period of about 0.5 seconds to about 20 seconds.

[0039] Referring to step 212, after purge/pump step 210, a total thickness of the tungsten nucleation layer will be formed on the substrate. Depending on specific device requirements, steps 204 through 210 may be repeated until a desired thickness for the tungsten nucleation layer is achieved. Thereafter, when the desired thickness for the tungsten nucleation layer is achieved the process is stopped as indicated by step 214.

[0040] The determination of the thickness of the tungsten nucleation layer may be performed using conventional processes such as, for example, spectroscopic measurements. Alternatively, the thickness of the nucleation layer may be estimated by performing a calculation based upon the deposition rate as determined using, for example, various process variables such as flow rates, temperature and pressure.

[0041] While steps 206 through 210 are depicted as three distinct steps in FIG. 2, steps 206 through 210 may take place simultaneously. For example, a purge gas may be provided to the process chamber simultaneously with the reducing gas to improve overall process throughput.

[0042] Additionally, process variables, such the temperature, pressure, gas flow rates, flow/pulse times, and the like, may be adjusted throughout the nucleation layer deposition process in order to optimize layer deposition rate. For example, early in the deposition process, a low ratio for the tungsten-containing precursor and reducing gas (e.g., ratio of about 1:1) may be used in step 204 in order to, for example, minimize the adverse effects of fluorine on underlying material layers. In subsequent deposition cylces, the ratio for the tungsten-containing precursor and reducing gas may be increased (e.g., ratio of 3:1).

[0043] In an exemplary tungsten nucleation layer deposition process, a silicon substrate 300 having a feature 310 formed in a dielectric layer 301 was provided, as shown in FIG. 3A. The substrate 300 had a titanium nitride (TiN) barrier layer 304 conformably deposited on the feature 310. A tungsten nucleation layer 306 was formed on the titanium nitride (TiN) barrier layer within the feature 310, as shown in FIG. 3B. The tungsten nucleation layer was formed as described above with reference to FIG. 2.

[0044] Specifically, a gas mixture comprising tungsten hexafluoride (WF₆) and silane (SiH₄) was provided to the process chamber for a time period of about 1.5 seconds. The respective flow rates of the tungsten hexafluoride (WF₆) and silane (SiH₄) were about 30 sccm and about 10 sccm, respectively. An argon (Ar) carrier gas was provided to the chamber at a flow rate of about 1500 sccm along with tungsten hexafluoride (WF₆). A nitrogen (N₂) carrier gas was provided to the chamber at a flow rate of about 300 sccm along with the silane (SiH₄).

[0045] The gas mixture was removed from the process chamber by providing a pulse of argon (Ar) for a duration of about 2 seconds. Thereafter, a flow of silane (SiH₄) gas was provided to the chamber at a flow rate of about 20 sccm for about 1 second. A pulse of argon (Ar) was then provided for about 2 seconds in order purge the process chamber. This process sequence was repeated by providing the gas mixture, the argon purge, the flow of silane (SiH₄), and the argon purge until a thickness of about 250 for the tungsten nucleation layer was achieved. The resulting tungsten nucleation layer had a step coverage, as determined by transmission electron microscopy (TEM) of about 100%.

[0046] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.