Nucleation of Tungsten on Titanium Nitride with Hydrogen Reduction of Tungsten Hexafluoride
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Nucleation of Tungsten on Titanium Nitride with Hydrogen Reduction of Tungsten Hexafluoride D. SRINIVAS,
R. FOSTER, S. MARCUS, R. ARORA, H. REBENNE
Materials Research Corporation, Phoenix, Arizona 85040
ABSTRACT In this work, a hydrogen (1H2 ) reduction process has been developed which gives tungsten (W) nucleation on titanium nitride (TiN) adhesion layers with a very short incubation time, eliminating the need for a silane (SiH-I4 ) reduced seed layer. The nucleation was found to be strongly dependent on the following factors: temperature of the substrate, total pressure in chamber, and gas introduction sequence into the reactor. The enhanced nucleation rate has been explained based on two competing reactions: dissociation of H 2 , and formation of titanium subfluorides on the TiN surface.
INTRODUCTION Tungsten films deposited by chemical vapor deposition (CVD) are an attractive alternative to sputtered aluminum as interconnect, contact, and via plug material in silicon integrated circuits with submicron features (1). W adhesion to dielectric materials is poor, so adhesion promoting layers like sputtered TiN and CVD TiN are used (2). A major drawback of W film is that it does not readily nucleate on TiN without using SiH 4 . In particular, SiH 4 reduction of tungsten hexafluoride (WF 6 ) on TiN occurs almost instantaneously, while H2 reduction requires incubation times on the order of minutes (3,4,5). Sil4 reduction process has the following disadvantages: a) silicon is incorporated in the W film resulting in high resistivity films, b) SiH4 reacts with WF 6 at room temperature causing problems such as W deposition in undesirable location and particulation c) SiH 4 reduction process yield poor step coverage in submicron features, and d) Sill4 is highly inflammable and causes handling problems. The H 2 reduction process does not have these disadvantages, but due to the long incubation time for W to nucleate on TiN with the H 2 reduction process, the common practice in the industry is to use a two step deposition process, a Sill4 reduction step to form the initial layer of W, followed by a H 2 reduction step to deposit the bulk of the film. The chemical vapor deposition of W by the hydrogen reduction of WF 6 has been extensively studied (6-11) and several authors have studied the reaction kinetics and reaction mechanism (12-16). Studies have shown that for W to nucleate on any substrate one of two things has to happen: a) the substrate must directly reduce the hexafluoride species, or b) the substrate must yield monoatomic hydrogen when exposed to H 2 molecules (15,18). A substrate like silicon reduces WE6 directly and a self-limiting deposition of W is achieved. Once the limiting W thickness is formed on the silicon, further nucleation of W can be achieved by the availability of another reducing agent like H2 or Sill4. A substrate like TiN reduces WF6 to form a titanium subfluoride compound (17) which is not volatile at low temperatures ( 5 min. - 7 min. 0.16 min.
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