Chemically Vapor Deposited Tungsten for Semiconductor Metallizations

Abstract

The properties of CVD tungsten films, produced by the hydrogen reduction of WF6 or WCl6, have been investigated to determine the suitability of the films for use as silicon integrated circuit metallization.s. Both low pressure and atmospheric pressure flow system have been investigated.The tur~gsten films show excellent adhesion to silicon substrates, have resistivities between 6 and 15 #ohm-cm depending on thickness, and are easily etched into submicron patterns. In addition, data on the contact resistance to silicon and MOS properties of tungsten films from both deposition reactions have been measured.The use of chemical vapor deposition (CVD) to deposit thin tungsten films for integrated circuit metallizations has been investigated for several years (1, 2). Usually the tungsten is deposited using the hydrogen reduction of tungsten hexafluoride at temperatures between 600 ~ and 800~ The process is easily controlled and produces excellent tungsten films which are suitable for some integrated circuit applications (2).However, the corrosive nature of the reactants and products tends to degrade the thermally oxidized silicon wafers used for the integrated circuits. This fact has generated additional interest in the use of tungsten hexachloride, which is less corrosive toward SIO2, and has been used to deposit tungsten films in a similar manner to the hexafluoride (3, 4) although less information is available on their subsequent use for integrated circuit metallizations.Consequently we have compared the two reactions for application to silicon integrated circuits, extending previous studies to include the use of CVD tungsten as a gate metallization for unipolar devices. ExperimentalTungsten films were deposited in both atmospheric and low pressure (1-10 Torr) flow systems from the hexafluoride and at atmospheric pressure from the hexachloride. Schematic diagrams of the deposition systems are shown in Fig. 1 and 2. The systems were rf heated using graphite or molybdenum susceptors. The temperature was controlled by a thermocouple embedded in the susceptor. The low pressure system proved somewhat unsatisfactory for two reasons. Firstly, at low pressure good thermal contact between the susceptor and the substrate is difficult to achieve and an inevitable uncertainty exists as to the substrate surface temperature in a fast flow system. Secondly, the use of rf heating at low pressure resulted in a plasma in the deposition chamber, the effect of which could result in ion bombardment damage to sensitive substrates.The tungsten hexachloride saturator and the gas lines leading from the saturator to the reaction chamber were heated to 150 ~ 4-5oC. Chemicals used were: hydrogen, purified using a palladium diffuser; argon, purified using a titanium chip furnace; helium (99.-9999%); hydrogen chloride (99.99%); tungsten hexafluoride (99.5%); and tungsten hexachloride (c9%). The major impurity in the tungsten hexachloride was tungsten oxychloride (WOC14) which was removed by sub]imation at 100~ The thickness of the deposited tungs...

Process diagnostics and thickness metrology using in situ mass spectrometry for the chemical vapor deposition of W from H2/WF6

“…The weight of this self-limiting W layer resulting from the WF 6 reduction in the Ar atmosphere corresponds to a film thickness of 26 nm 16 in excellent agreement with measurements performed by Melliar-Smith et al 18 and Yu and Eldridge. In the absence of H 2 , only reactions ͑1͒ and ͑2͒ may occur, and the weight of the W seed layer was found 0.0040Ϯ0.0003 g, which is within the two-sigma uncertainties of the values obtained from both the HF and H 2 data.…”

Structure of LPCVD Tungsten Films for IC Applications

“…The sheet resistance was measured with a four-point probe and was uniform over the sample within better than 5% for all except the very thinnest films. The resistivity was 13.9 ~12-cm for films about 162 nm thick and decreased to 10.5 ~l-l-cm for films about 430 nm thick, in reasonable agreement with the resistivities reported in the literature (6,7,8). The actual resistivity may be less than that indicated, since surface roughness may increase the apparent film thickness measured with a profilometer on a step etched through the tungsten.…”

The Kinetics of LPCVD Tungsten Deposition in a Single Wafer Reactor

“…(*Total) = (*) + (WF.~*) (*Total) = constant (*WF~) = constant, because it is the MASI Therefore (*), the empty site concentration is also a constant. When invoking (15), the rate equation becomes r = k3(WFs*)Kg'/~H2'/2(*) [16] r = k"(H2) 1/2 [17] where k" = k3(WF.~*)(*)K91/2 A similar rate equation would result from the assumption that any of reactions [2a] through [7a] could be rate limiting. The resulting equations are all zero order in tungsten hexafluoride and one-half order in hydrogen.…”

“…Other authors report an activation energy of.67,000-69,000 J mol-' (2,9,11,14,15). Most authors also observed the reduction reaction to be zero order with respect to tungsten hexafluoride and one-half order with respect to hydrogen (2,11,14,15,16). All of the authors reporting a mechanism assumed the rate limiting step to be hydrogen dissociation or HF desorption without rigorously evaluating those mechanisms.…”