The Kinetics of LPCVD Tungsten Deposition in a Single Wafer Reactor

Abstract

The rate equation for the low pressure chemical vapor deposition (LPCVD) of tungsten from tungsten hexafluoride on (100) silicon was experimentally determined in a laboratory scale single wafer vacuum reactor. The reactor was designed and built as a high vacuum stainless steel system with a minimum of heat and mass transfer limitations. The tungsten film deposition is initiated by the silicon reduction of tungsten hexafluoride. In the absence of hydrogen, the silicon reduction results in a 10-40 nm self-limiting tungsten deposit, with the thickness dependent upon the native oxide layer prior to deposition. The hydrogen reduction of tungsten hexafluoride is one-half order in hydrogen, zero order in tungsten hexa-1 fluoride, and has an activation energy of 73,000 J mol-at temperatures from 561 to 683 K, and pressures from 0.067-1.3 kPa 4 1 05 8 1 0 (0.5 to 10 torr). The pre-exponential factor was found to be 6.8 • 10 nms Pa " (4.7 • 10 A rain-torr-~). A mechanism is proposed and two possible rate limiting steps yield rate expressions consistent with the observed kinetics. The rate limiting step could be either the addition of adsorbed monatomic hydrogen to adsorbed partially fluorinated tungsten or hydrogen fluoride desorption. In the absence of hydrogen the deposition was perfectly selective over all ranges of temperature and pressure. During the hydrogen reduction selectivity was lost in less than 600s at temperatures above 653 K (380~ Due to the observation that loss of selectivity is an activated process, it is concluded that deposition of tungsten on oxide is initiated by a chemical reaction rather than by random surface defects. Due to the high vacuum procedures, films were made with tungsten resistivities of 6 ~/cm, approaching the ideal 5.5 ~/cm for bulk tungsten.For very large scale integration (VLSI), a low resistance metal film is needed due to performance limitations of aluminum and polysilicon (1). As a result of these limitations, tungsten has been st,adied and successfully used for multilevel metalization by several facilities (2,3,4)

Surface Morphology Evolution of Chemical Vapor-Deposited Tungsten Films on Si(100)

“…This value, which is lower than those reported for tungsten films (67-73 kJ/ mol), [3] may be the result of using a cold wall reactor system, where an accurate measurement of the deposition temperature is difficult, especially at low pressures. For T > 720 K, u increases slightly with T and the growth process is controlled by the rate at which the reactants are supplied to the growing film (regime II).…”

“…[2,3] In CVD an external source maintains a fixed concentration of reactant molecules at a distance above the film surface. [4] Then, gas diffusion drives the molecules through the diffusion layer [2] towards the film surface.…”

Modelling of the Deposition of Molybdenum on Silicon from Molybdenum Hexafluoride and Hydrogen

“…studied the deposition of molybdenum by reduction of MoF, by Si [4] and H2 [5] at 120-1300 Pa and 523-673 K . They observed a self-limiting reduction reaction by Si, similar to the reduction of WF, by Si [7,8]. Self-limiting thicknesses of 30-130 nm at 130 Pa (MoF,/He= 113) and 523-673 K were found.…”

Step Coverage Predictions Using Combined Reactor Scale and Feature Scale Models for Blanket Tungsten LPCVD

“…(3) (3) is based on an analysis of deposition rate data of McConica and Krishnanami [23], van der Putte [24] and the mechanistic study of Arora and Pollard [25]. This expression reduces to the form of Eq.…”