This invention relates to a plasma CVD method used in the manufacture of a semiconductor device and to a semiconductor device having a metal film formed by this method, and particularly to a plasma CVD method with which a metal film formed has a smooth surface morphology and the amount of residual halogen element contained in the metal is small and a highly reliable semiconductor device having a metal film formed by this method.
As design rules of semiconductor devices such as LSI devices are established to cover less dimensions reduced from half micron to quarter micron or lower levels and multilayer interconnection structures come to be used in large numbers, aspect ratios of connection holes for connecting interconnection layers together have also been becoming larger. For example, in a semiconductor device based on 0.18 .mu.m rule, because with respect to a connection hole opening diameter of 0.2 .mu.m the thickness of an interlayer insulating film is about 1.0 .mu.m, the aspect ratio reaches 5. To achieve a highly reliable multilayer interconnection structure using connection holes of such small size and high aspect ratio, methods are coming to be employed wherein a metal film of Ti or the like for an ohmic contact and a TiN film or the like as a barrier metal for preventing diffusion of interconnection material are formed thinly and conformally inside the connection hole and then upper layer interconnections and contact plugs are formed by high-temperature sputtering of an Al-based metal or CVD of W.
Normally, Ti metal films and TiN films have been formed by sputtering or reactive sputtering with bulk Ti metal as the target material. Among such methods, collimated sputtering, wherein the component of perpendicularly incident sputtered particles is increased, disclosed for example in Japanese Unexamined Patent Publication No. 6-140359, and long range sputtering, wherein a long target distance is used, have been receiving particular attention. It has been confirmed that with these sputtering methods it is possible to obtain reduced contact resistance and improved barrier characteristics compared with conventional sputtering methods. However, with these sputtering methods, because the component of sputtered particles perpendicularly incident on the substrate is increased, parts where the film thickness is extremely thin are inevitably formed around the shoulders of small connection holes having large aspect ratios and around the edges of the bottoms of connection holes. In this case, when for example blanket CVD of W is carried out in a subsequent step, the process gas WF.sub.6 permeates through the thin parts of the film and problems such as erosion of the base material layer, abnormal growth of W or the Ti metal film or TiN film flaking off occur.
To solve problems of step coverage not solved by these sputtering methods also including collimation and the like, methods of forming conformal Ti metal films and TiN films by CVD methods utilizing chemical reactions taking place at the substrate surface are being expected.
CVD methods for forming Ti-based material layers proposed so far can be generally classified into two types: methods using inorganic metal-halogen compounds such as TiCl.sub.4, reported for example in Proceedings of the 44th Symposium on Semiconductor & Integrated Circuit Technology p.31 (1993), and methods using organic metal compounds such as TDMAT and TDEAT, reported for example in Proc. 11th Int. IEEE VMIC, p.440 (1994).
With the latter methods using organic metal source gases, in many cases a Ti metal film is formed by sputtering and then a TiN film is formed by MOCVD.
The former type of method using an inorganic metal-halogen compound source gas has the merit that it is possible to consecutively form a Ti metal film by H.sub.2 reduction and a TiN film by adding N.sub.2 or the like to this process gas inside the same CVD reactor.
The reaction reducing the metal-halogen compound TiCl.sub.4 using H.sub.2 molecules is the endothermic reaction given by the following formula (1), and thermodynamically is difficult to sustain (AG is standard heat of formation). EQU TiCl.sub.4 +2H.sub.2 .fwdarw.Ti+4HCl .DELTA.G=393.3 kJ/mol (1)
For this reason, formation of Ti metal films by plasma CVD wherein H.sub.2 is dissociated in a plasma and reduction by H atoms and activated H species is used has been receiving attention. This reaction is the exothermic reaction shown by the following formula (2). EQU TiCl.sub.4 +4H.fwdarw.Ti+4HCl .DELTA.G-478.6 kJ/mol (2)
With this formation of a Ti metal film by plasma CVD, the reaction proceeds easily and a film can be formed at a relatively low temperature. In particular, plasma CVD reactors having a high density plasma source such as ECR-CVD reactors, inductively coupled plasma CVD reactors and helicon wave plasma CVD reactors are advantageous in film formation rate and uniformity.
However, even when plasma CVD is carried out using one of these high-density plasma CVD reactors, depending on the film-forming conditions the Ti metal film does not grow uniformly on the substrate and granular Ti metal grows nonuniformly.
This problem will be described with reference to FIGS. 1A and 1B.
These figures are schematic sectional views showing a substrate under processing having a connection hole 3 formed in an interlayer insulating film 2 on a semiconductor substrate 1 made of silicon or the like, and illustrate a problem arising when a Ti metal film 4 is formed on this by plasma CVD. That is, when the reduction of TiCl.sub.4 by H atoms and activated H species is not carried out well, as shown in FIG. 1A the surface morphology of the Ti metal film 4 obtained is deteriorated by grains of nonuniformly grown Ti metal.
When a TiN film 6 is then formed on this, because as shown in FIG. 1B the TiN film 6 grows with the same surface shape as the Ti metal film 4 below it, this film also is granular, and at parts where adjacent TiN grains make contact with each other the film cannot grow any further. In particular, a large gap 7 sometimes forms at the corner of the bottom of the connection hole 3.
When in a subsequent step a layer of a refractory metal such as W is formed by CVD with WF.sub.6 or the like as a process gas on a substrate on which is formed this kind of gap 7, the WF.sub.6 passes through the gap 7 and erodes the semiconductor substrate 1 below and destroys shallow junctions (not shown). Also, when the connection hole 3 is filled in by an Al-based metal, the Al-based metal passes through the gap 7 and reacts with the semiconductor substrate 1 made of silicon or the like to form alloy spikes. In either case, serious defects such as increased leak current at the interlayer connection result.
Also, when the reduction of the TiCl.sub.4 by H atoms and activated H species is incomplete, precursors TiCl.sub.x (where x is an integer below 4) and chlorine atoms are taken into the Ti metal film being formed, and residual chlorine in the Ti metal film increases. As a result, an Al-based metal layer or the like formed in a later step is corroded and there is a possibility of this leading to increased contact resistance and, in extreme cases, disconnection.