1. Field of the Invention
This invention relates to the processing of semiconductor substrates. More particularly, this invention relates to improvements in the process of depositing refractory metal layers on semiconductor substrates.
2. Description of the Related Art
The semiconductor processing industry continues to strive for larger production yields while increasing the uniformity of layers deposited on substrates having increasing larger surface areas. These same factors in combination with new materials also provide higher integration of circuits per unit area of the substrate. As circuit integration increases, the need for greater uniformity and process control regarding layer thickness rises. As a result, various technologies have been developed to deposit layers on substrates in a cost-effective manner, while maintaining control over the characteristics of the layer. Chemical Vapor Deposition (CVD) is one of the most common deposition processes employed for depositing layers on a substrate. CVD is a flux-dependent deposition technique that requires precise control of the substrate temperature and precursors introduced into the processing chamber in order to produce a desired layer of uniform thickness. These requirements become more critical as substrate size increases, creating a need for more complexity in chamber design and gas flow technique to maintain adequate uniformity.
A variant of CVD that demonstrates superior step coverage, compared to CVD, and is Atomic Layer Deposition (ALD). ALD is based upon Atomic Layer Epitaxy (ALE) that was originally employed to fabricate electroluminescent displays. ALD employs chemisorption to deposit a saturated monolayer of reactive precursor molecules on a substrate surface. This is achieved by alternatingly pulsing an appropriate reactive precursor into a deposition chamber. Each injection of a reactive precursor is separated by an inert gas purge to provide a new atomic layer additive to previous deposited layers to form a uniform layer on the substrate. The cycle is repeated to form the layer to a desired thickness. A drawback with ALD techniques is that the deposition rate is much lower than typical CVD techniques by at least on order of magnitude.
Employing the aforementioned deposition techniques it is seen that formation of a layer at a high deposition rate while providing adequate step coverage are conflicting characteristics often necessitating sacrificing one to obtain the other. This has been prevalent when depositing refractory metal layers cover gaps or vias during formation of contacts that interconnect adjacent metallic layers separated by a dielectric layer has proved problematic. Historically, CVD techniques have been employed to deposit conductive material in order to inexpensively and quickly form contacts. Due to the increasing integration of semiconductor circuitry, tungsten has been used based upon the superior step coverage of tungsten. As a result, deposition of tungsten employing CVD techniques enjoys wide application in semiconductor processing due to the high throughput of the process.
Depositing tungsten in this manner, however, is attendant with several disadvantages. For example, blanket deposition of a tungsten layer on a semiconductor wafer is time-consuming at temperatures below 400xc2x0 C. The deposition rate of tungsten may be improved by increasing the deposition temperature to, e.g., about 500xc2x0 C. to about 550xc2x0 C. Temperatures in this range may compromise the structural and operational integrity of the underlying portions of the integrated circuit being formed. Tungsten has also frustrated photolithography steps during the manufacturing process by providing a relatively rough surface having a reflectivity of 20% or less than that of a silicon substrate. Finally, tungsten has proven difficult to deposit uniformly. This has been shown by variance in tungsten layers"" thickness of greater than 1%, which frustrates control of the resistivity of the layer. Several prior attempts to overcome the aforementioned drawbacks have been attempted.
For example, in U.S. Pat. No. 5,028,565 to Chang et al., which is assigned to the assignee of the present invention, a method is disclosed to improve, inter alia, uniformity of tungsten layers by varying the deposition chemistry. The method includes, in pertinent part, formation of a nucleation layer over an intermediate, barrier, layer before depositing the tungsten layer via bulk deposition. The nucleation layer is formed from a gaseous mixture of tungsten hexafluoride, hydrogen, silane and argon. The nucleation layer is described as providing a layer of growth sites to promote uniform deposition of a tungsten layer thereon. The benefits provided by the nucleation layer are described as being dependent upon the barrier layer present. For example, were the barrier layer formed from titanium nitride the tungsten layer""s thickness uniformity is improved as much as 15%. Were the barrier layer formed from sputtered tungsten or sputtered titanium tungsten the benefits provided by the nucleation layer are not substantial.
U.S. Pat. No. 5,879,459 to Gadgil et al. discloses an apparatus that takes advantage of ALD. To that end, the apparatus, a low profile, compact atomic layer deposition reactor (LP-CAR) has a body with a substrate processing region adapted to serve a single substrate or a planar array of substrates, as well as a valve, and a port for substrate loading and unloading. In some embodiments multiple reactors are stacked vertically and share a common robotic handler interface with a CVD system. In this manner, the robotic handler may manipulate substrates associated with both the CVD system and the LP-CAR The compact reactor is distinguished by having individual injectors, each of which comprises a charge tube formed between a charge valve and an injection valve. The charge valve connects the charge tube to a pressure regulated supply, and the injection valve opens the charge tube into the compact reactor. Rapidly cycling the valves injects fixed mass-charges of gas or vapor into the compact reactor.
What is needed, however, is a technique to deposit conductive layers having a deposition rate comparable to CVD techniques while providing the step coverage associated with ALD techniques.
A method and system to form a refractory metal layer on a substrate features a bifurcated deposition process that includes nucleating a substrate, when disposed in a first processing chamber, using ALD techniques to serially expose the substrate to first and second reactive gases followed by forming a layer, employing chemical vapor deposition to subject the nucleation layer, when disposed in a second processing chamber, to a bulk deposition of a compound contained in one of the first and second reactive gases. To that end, the system includes first and second processing chambers, each of which includes a holder, disposed therein to support the substrate. A gas delivery system and a pressure control system is in fluid communication with the processing chamber. A temperature control system is in thermal communication therewith. A robotic handler is disposed between the first and second processing chambers. A controller is in electrical communication with gas delivery systems, temperature control systems, pressure control systems, and the robotic handler. A memory is in data communication with the controller. The memory comprises a computer-readable medium having a computer-readable program embodied therein. The computer-readable program includes instructions for controlling the operation of the two processing chambers.