1. Field of the Invention
The present invention relates to fabrication of a semiconductor device provided with a contact hole.
2. Prior Art
In the case of a lamination-type semiconductor device constituted by laminating a plurality of layers on a semiconductor substrate, the conductivity between interconnect layers is secured by forming a very narrow contact hole on an insulating film and then by burying a conductive material in the contact hole in order to connect interconnect layer patterns of each layer to each other. For example, a conductive material is buried in a contact hole in accordance with the following steps:
(a) An interconnect layer pattern is formed in a lower layer. Then, an insulating film is formed to form a mask pattern of a very narrow contact hole by photolithography;
(b) Then, a very narrow contact hole is formed at a desired position of the insulating film by dry etching by using the mask pattern;
(c) Then, tungsten, a low-resistance, high-melting-point metal, is deposited to bury it in the contact hole and to cover the surface of the circumferential insulating film;
(d) Then, the tungsten film on the surface of the insulating film around the contact hole is removed by etch back to leave tungsten only in the desired contact hole.
The continuity with an upper-layer interconnect pattern is obtained by the contact hole, wherein the tungsten film is buried.
As a contact hole is decreased in diameter, the burying characteristic of a tungsten film by chemical vapor deposition (hereinafter called as CVD) is further deteriorated and voids (nests) more easily remain in the tungsten film in the contact hole. When etching is performed while the voids are produced, as shown in FIG. 11, an over-etched portion 59 is produced at a void portion and a substrate at the bottom of a contact hole is etched.
In this case, the following method is known: a method for improving the burying characteristic by forming a tungsten film under a reaction-rate-determination state at low deposition temperature and pressure. However, under these conditions, a throughput is lowered because of low deposition rate. Moreover, because film stress is increased, film removal occurs and a junction leak current increases when forming a film of a thickness such as 300 nm or more at which a contact hole is sufficiently filled with the film. Furthermore, when a thick film is formed, the wafer is bent, and a stage adsorption error is likely to occur. Moreover, when unevenness of the surface of the wafer excessively increases, it may not be possible to secure a focus margin when performing photolithography.
It is an object of the present invention to improve the burying characteristic by suppressing formation of voids without causing removal of a film made of a conductive material to be buried in a very narrow contact hole or to increase of a junction leak current.
In accordance with one aspect of the present invention, there is a semiconductor-device fabrication method includes a step of forming a contact hole in a semiconductor substrate and a step of forming a conductive contact hole. The step of forming the contact hole is performed by repeating two times or more a burying step of depositing a conductive material to bury the conductive material in the contact hole and an etch-back step of removing the conductive material around the contact hole by etch back.
Preferably, the conductive material may be deposited at a thickness in a range from 25 to 50% of the inner diameter of the contact hole when the burying step is performed first.
More preferably, the conductive material may be deposited at a deposition temperature in a range from 350xc2x0 C. to 430xc2x0 C. after the burying step is performed twice.
Further preferably, the conductive material may be deposited at a pressure in a range from 600 Pa to 6,000 Pa when the burying step is performed first.
The conductive material may be deposited at a temperature equal to or lower than deposition temperature in a last burying step after the burying step is performed twice.
The conductive material may be deposited at a pressure equal to or lower than the pressure in a last burying step after the burying step is performed twice.
The burying step may be performed by chemical vapor deposition process.
The conductive material may be tungsten.
It is an advantage of the present invention that it is possible to bury a conductive material suppressing a void in a very narrow contact hole and obtain a preferable burying characteristic. Moreover, it is possible to form a contact hole having a high conductivity.