1.  Field of the Invention
The present invention relates to a method for preparing a thin film and more particularly to a phase-shifting photomask blank which permits the improvement of transferred patterns in their resolution by making a difference in phase between exposed light rays passing through a mask and a phase-shifting photomask prepared from the phase-shifting photomask blank as well as a method for producing the photomask blank and the photomask and an apparatus for manufacturing the phase-shifting photomask blank.  In other words, the present invention relates to a so-called half-tone type phase-shifting photomask blank, a phase-shifting photomask as well as a method for producing them and an apparatus for manufacturing the phase-shifting photomask blank. 
2.  Disclosure of the Related Art
In case of a phase-shifting photomask 208 as shown in FIG.  21(B), a circuit pattern 230 to be transferred onto a semiconductor substrate has in general been formed by making use of a phase-shifting photomask which is produced by etching the surface of a phase-shifting photomask blank 200 which consists of a quartz substrate 201 and a phase-shifting film 204, as shown in FIG.  21(A), to remove a desired portion of the film 204 and to thus form openings 210 through which the surface of the quartz substrate 201 is exposed and phase-shifting portions 205, i.e., the unetched portions of the film 204 remaining on the substrate. 
The phase-shifting portion 205 has light-transmission properties to the exposed light rays and the thickness thereof is designed in such a manner that a phase of the exposed light rays transmitting through the opening 210 has a phase difference, by 180 degrees, from that of the light rays transmitting through the phase-shifting portion 205.  Therefore, when exposing a wafer as shown in FIG.  21(C) to light rays, the light intensity observed on the wafer at the boundary between the opening 210 and the phase-shifting portion 205 becomes zero.  For this reason, the circuit pattern transferred using the phase-shifting photomask 208 would have a high resolution. 
As the phase-shifting film 204, there have in general been used a monolayer or multi-layer film and it has been desired for the phase-shifting portion 205 to have a light-transmittance to the exposed light rays in the range of from 4% to 40% in order to obtain an appropriate quantity of exposed light rays during lithography operations and to control the thickness, as determined after development, of a resist film applied onto the wafer. 
Up to now, when forming a monolayer phase-shifting film 204 having a desired thickness on a substrate on which a film is deposited (hereinafter simply referred to as xe2x80x9cfilm-forming substratexe2x80x9d) in order to form a phase-shifting photomask blank as shown in FIG.  21(A) using a film-forming device wherein a reactive gas is supplied to a film-forming chamber and a thin film is formed on the film-forming substrate according to the reactive sputtering technique while passing the substrate over a sputtering target, a film is formed by passing the substrate over the target only one time by controlling the deposition rate of the film on the substrate and the conveying speed of the substrate in such a manner that a desired film thickness can be obtained by a single pass (see, for instance, Japanese Un-Examined Patent Publication (hereinafter referred to as xe2x80x9cJ.P. KOKAIxe2x80x9d) No.  Hei 9-17928).  The disclosure of the above publication is hereby incorporated by reference herein. 
Such a conventional technique will hereinafter be described with reference to the attached FIG.  1.  FIG.  1 shows a part of a film-forming chamber 11 of a film-forming device.  A mixed gas comprising a sputtering gas and a reactive gas is supplied to the film-forming chamber through a gas-supply port 14 after vacuum exhaustion through an exhaust port 15.  A DC voltage which is negative with respect to the grounding voltage is applied to a sputtering target 12 (MoSi) to thus form a discharge zone in the proximity to the sputtering target and as a result, the target 12 begins to discharge sputter materials.  If a film-forming quartz substrate 101 is continuously conveyed along a conveying path 18 positioned at a predetermined distance from the target 12, a desired film is formed on the substrate. 
As has been discussed above, when forming the monolayer film having a desired thickness on the film-forming substrate using the film-forming device wherein the reactive gas is supplied to the film-forming chamber and the film is formed on the film-forming substrate according to the reactive sputtering technique while passing the substrate over the sputtering target, the thin film has conventionally been formed by passing the substrate over the target only one time by controlling the deposition rate of the film on the substrate and the conveying speed of the substrate in such a manner that a desired film thickness can be obtained by a single pass.  In this case, however, the thickness distribution of the monolayer film has a tendency as shown in FIG.  1.  More specifically, FIG.  1 shows the deposition rate distribution of the target component 19-1 (or the thickness distribution of the target component formed on a static substrate).  The distribution of the target component along a conveying path 18 is not uniform and therefore, the higher the deposition rate of the target component along the conveying path 18, the larger the amount of the unreacted target component if it is assumed that the frequency of the reactive gas incident upon the substrate 101 is approximately constant along the conveying path 18 as shown in FIG.  1 (see line 19-2).  Accordingly, the content of the reaction product in the resulting film is low and a non-uniform film is formed on the conveyed substrate.  If an opening 17-1 positioned between shielding plates 17 is divided into regions A, B and C arranged along the conveying path 18, a non-uniform film 31 shown in FIG.  2 is formed on the substrate 101 by a single forward movement of the substrate 101, in a horizontal direction, over a target 12.  The non-uniform film is thus formed on the substrate and this accordingly results in various drawbacks.  For instance, the resulting film shows optical characteristics which are widely different from those expected for a uniform monolayer film and exhibits resistance to chemicals considerably inferior to that of a uniform film.  Moreover, this film cannot optically be handled as a monolayer film and this in turn results in such disadvantages that this makes the calculation of the optical constants quite difficult and that the design and the quality control of films become difficult.  In order to eliminate such drawbacks, it has been devised to reduce the lengths of the opening 17-1 of the shielding plate and that of a chimney 13 along the substrate-conveying direction (this is referred to as xe2x80x9copening lengthxe2x80x9d) so that only the region B (for instance, the region whose deposition rate is not less than 90% of the maximum level thereof) in which the deposition rate of the target component is approximately constant contributes to the film-formation, or so that even a part of the region A contributes to the film-formation.  However, the former suffers from a problem of reduction of the utilization efficiency of the target 12 (the ratio of the amount of the target capable of being used in the film-formation to the total consumed amount thereof) and productivity, while the latter suffers from a problem of, for instance, the reduction in the utilization efficiency of the target according to the reduction in the opening length, the deterioration of the resistance to chemicals due to the non-uniformity of the film composition and complication of optical characteristics. 
Accordingly, it is generally an object of the present invention to solve the foregoing problems associated with the conventional sputtering techniques and specifically to provide a substantially uniform thin film which can ensure optical characteristics quite similar to those expected for a uniform monolayer film and resistance to chemicals almost comparable to is that observed for a uniform monolayer film without impairing the utilization efficiency of a target and productivity of films and which can optically be handled as a substantial monolayer film and which makes the calculation of optical constants of the film, the design and the quality control thereof easy, in particular, a phase-shifting photomask blank and a phase-shifting photomask excellent in resistance to chemicals and optical characteristics. 
Another object of the present invention is to provide a phase-shifting photomask which can be prepared from the foregoing phase-shifting photomask blank. 
A further object of the present invention is to provide a method for preparing such a phase-shifting photomask blank and such a phase-shifting photomask. 
Still another object of the present invention is to provide a device for manufacturing such a phase-shifting photomask blank. 
According to a first aspect of the present invention, there is provided a method for preparing a thin film which comprises supplying a reactive gas to a film-forming chamber and depositing a film on a film-forming substrate by the reactive sputtering technique while passing the substrate over a sputtering target, wherein the film is formed while passing, at least 4 times, the substrate over the target and establishing sufficient lengths of the openings of shielding plate and chimney to thus form a substantially uniform film on the substrate without impairing the utilization efficiency of the target and the productivity of the film.  The method makes it easy to produce, in particular, a desired phase-shifting photomask blank and a desired phase-shifting photomask. 
According to a second aspect of the present invention, there is provided a phase-shifting photomask blank which comprises a thin film formed on a substrate according to the reactive sputtering technique in which a reactive gas is supplied to a film-forming chamber and a film is formed on a film-forming substrate while passing, at least 4 times, the substrate over a sputtering target. 
According to a third aspect of the present invention, there is provided a phase-shifting photomask which comprises a pattern consisting of light semitransparent portions and light transmitting portions transparent to light rays having an intensity substantially contributing to light-exposure, the pattern being produced by subjecting the light semitransparent film of the foregoing phase-shifting photomask blank to a pattern-forming or patterning treatment to thus remove a part of the light semitransparent film. 
According to a fourth aspect of the present invention, there is provided a device for manufacturing a phase-shifting photomask blank in which a film is formed by supplying a reactive gas to a film-forming chamber and depositing a thin film on a film-forming substrate using the reactive sputtering technique, while passing the substrate over a sputtering target, and which is designed so that even regions whose deposition rate of the target component is not more than 90% of the maximum level thereof also contribute to the film-formation by sufficiently enlarging an opening length along the substrate-conveying direction.