Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-07-12
2003-11-04
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06641958
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to phase shift mask blanks and phase shift masks for use in the microfabrication of high-density semiconductor integrated circuits such as large-scale integration (LSI) and very large-scale integration (VLSI) chips, charge-coupled devices, color filters for liquid-crystal displays, and magnetic heads. The invention relates also to methods of manufacturing such phase shift mask blanks and phase shift masks. More particularly, the invention relates to halftone phase shift mask blanks and phase shift masks which can attenuate the intensity of exposure wavelength light with a phase shift film, and to methods of manufacturing such phase shift mask blanks and phase shift masks.
2. Prior Art
The photomasks that are used in a broad range of applications, including the manufacture of semiconductor integrated circuits (IC) and LSI chips, are basically composed of a transparent substrate on which a light-shielding layer made primarily of chromium has been formed in a given pattern. The market demand for ever higher levels of integration in semiconductor integrated circuits has led to a rapid reduction in the minimum feature size of photomask patterns. Such miniaturization has been achieved in part by the use of exposure light of shorter wavelengths.
However, while exposure using shorter wavelength light does improve resolution, it also has a number of undesirable effects, such as reducing the focal depth, lowering process stability and adversely impacting product yield.
One pattern transfer technique that has been effective for resolving such problems is phase shifting. This involves the use of a phase shift mask as the mask for transferring patterns having a small minimum feature size.
As shown in
FIG. 6
, a phase shift mask (halftone phase shift mask) typically has a phase shifter
2
a
that forms a patterned area on the mask and an area
1
a
without phase shifter where the substrate lies exposed to incident light. The phase shift mask improves contrast of the transferred image by providing a phase difference of 180 degrees between light passing through the patterned and unpatterned areas, and utilizing the destructive interference of light at the boundary regions of the pattern to set the light intensity in the interference regions to zero. The use of phase shifting also makes it possible to increase the focal depth at the necessary resolution. Hence, compared with a conventional mask having an ordinary exposure pattern, such as one composed of chromium film, the use of a phase shift mask can improve resolution and increase the margin of the exposure process.
For practical purposes, such phase shift masks can be broadly categorized, according to the light-transmitting characteristics of the phase shifter, as either completely transmitting phase shift masks or halftone phase shift masks. Completely transmitting phase shift masks are masks in which the phase shifter has the same light transmittance as the substrate, and which are thus transparent to light at the exposure wavelength. In halftone phase shift masks, the phase shifter has a light transmittance that ranges from about several percent to several tens of percent the transmittance of exposed substrate areas (unpatterned areas).
FIG. 1
shows the basic structure of a halftone phase shift mask blank, and
FIG. 2
shows the basic structure of a halftone phase shift mask. The halftone phase shift mask blank shown in
FIG. 1
is composed of a substrate
1
which is transparent to the exposure light, and on which a halftone phase shift film
2
has been formed. The halftone phase shift mask shown in
FIG. 2
is composed of a halftone phase shifter
2
a
which forms the patterned areas of the mask, and exposed substrate areas
1
a
on which there is no phase shift film.
Exposure light that has passed through the phase shifter
2
a
is phase-shifted relative to exposure light that has passed through exposed substrate areas
1
a.
The transmittance of the phase shifter
2
a
is selected such that exposure light which has passed through the phase shifter
2
a
has too low an intensity to sensitize the resist on the substrate to which the pattern is being transferred. Accordingly, the phase shifter functions to substantially shield out exposure light.
The above-described halftone phase shift mask include halftone phase shift masks having a simple, single-layer structure. Single-layer halftone phase shift masks known to the art include those described in JP-A 7-140635 which have a phase shifter composed of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxide nitride (MoSiON).
A lithographic process may be used to manufacture such phase shift masks. The lithographic process typically involves applying a resist onto the phase shift mask blank, sensitizing desired areas of the resist with electron beams or ultraviolet light, carrying out development to expose the surface of the phase shift film, then etching the desired areas of the phase shift film using the patterned resist film as the mask so as to expose the substrate. The resist film is then stripped, giving the finished phase shift mask. The etching process employed for this purpose is generally reactive ion etching with a fluorine-containing gas. The material ordinarily used as the transparent substrate is quartz.
Because reactive ion etching using a fluorine-containing gas also etches quartz, overetching during the phase shift film etching step has the undesirable effect of removing some of the quartz in the exposed substrate areas. As a result, the substrate in the exposed substrate areas becomes thinner than the substrate in the phase shifter areas, creating a difference in the light path within the substrate between the phase shifter areas and the exposed substrate areas. This difference in turn causes the amount of phase shift to become larger than that set by the phase shift film and thus to deviate from the intended value.
Also, reactive ion etching normally entails a certain degree of variation within the plane of the substrate. Hence, by the time etching is complete in those areas where the substrate is to be exposed, the amount of phase shift ends up differing between those areas within the substrate plane where the substrate surface becomes exposed a little earlier and those areas where it becomes exposed a little later. As a result, the distribution in phase shift within the plane of the phase shift mask substrate worsens, making it difficult to stably manufacture high-quality phase shift masks having a high in-plane uniformity in the phase shift. An improvement in this regard has been strongly desired in the industry.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide high-quality phase shift mask blanks and phase shift masks manufactured therefrom which have a phase shift layer with a larger etch selectivity versus the substrate so that the substrate is not excessively etched when the phase shift mask is manufactured from the phase shift mask blank, in which the phase shift in the patterned areas has a good controllability, and which have a high in-plane uniformity in the phase shift distribution. Another object of the invention is to provide methods of manufacturing such phase shift mask blanks and phase shift masks.
It has been found that, in a phase shift mask blank having a transparent substrate and at least one phase shift film layer composed primarily of a metal and silicon on the substrate, the use of a phase shift film having a larger etch selectivity with respect to the substrate, and in particular a phase shift film made of molybdenum silicide oxide carbide (MoSiOC) or molybdenum silicide oxide nitride carbide (MoSiONC), makes it possible to prevent excessive etching of the substrate when a phase shift mask is patterned from a phase shift mask blank. As a result, the phase shift in the patterned areas has good controllability, enabling a phase shift mask having a high in-plane uniformity in the phase shift distribution to be achieve
Inazuki Yukio
Kaneko Hideo
Kojima Mikio
Maruyama Tamotsu
Okazaki Satoshi
Millen White Zelano & Branigan P.C.
Rosasco S.
Shin-Etsu Chemical Co. , Ltd.
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