Halftone phase shift photomask and blanks for halftone phase...

Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask

Reexamination Certificate

Rate now

  [ 0.00 ] – not rated yet Voters 0   Comments 0

Details

Reexamination Certificate

active

06764792

ABSTRACT:

ART FIELD
The present invention relates generally to a photomask used for the fabrication of high-density integrated circuits such as LSIs and a photomask blank for the fabrication of the same, and more particularly to a halftone phase shift photomask that enables projected images of minute size to be obtained and a halftone phase shift photomask blank for the fabrication of this halftone phase shift photomask.
BACKGROUND ART
Semiconductor integrated circuits such as LSIs are fabricated by the repetition of a so-called lithography process using a photomask. Possible applications of such phase shift masks as set forth in JP-A 58-173744, JP-B 62-59296, etc. to the formation of microcircuits in particular have already been under investigation. Among others, the so-called halftone phase shift photomask such as one set forth in U.S. Pat. No. 4,890,309, etc. has attracted attention in view of expedited practical applications. For instance, as disclosed in JP-A's 05-2259 and 05-127361, some proposals have been made in respect of arrangements and materials improved in yields and reduced in costs for the purpose of practical applications.
Here a typical halftone phase shift photomask is briefly explained with reference to
FIGS. 7 and 8
. FIGS.
7
(
a
) through
7
(
d
) are illustrative of the principles of halftone phase shift lithography, and FIGS.
8
(
a
) through
8
(
d
) are illustrative of a conventional process. FIGS.
7
(
a
) and
8
(
a
) are each a sectional view of a photomask, FIGS.
7
(
b
) and
8
(
b
) are each illustrative of the amplitude of light on the photomask, FIGS.
7
(
c
) and
8
(
c
) are each illustrative of the amplitude of light on a wafer, and FIGS.
7
(
d
) and
8
(
d
) are each illustrative of the intensity of light on the wafer. Reference numerals
411
and
421
are each a transparent substrate,
422
a 100% light-blocking film,
412
a halftone phase shift film, and
413
and
423
are each incident light. By the term “halftone phase shift film” used herein is intended a film, in a single layer or multilayer form, having functions of substantially inverting the phase of transmitting exposure light with respect to the phase of exposure light passing through the air having the same optical length and attenuating the intensity of the light. According to the conventional process, the 100% light-blocking film
422
formed of chromium or the like is provided on the substrate
421
formed of quartz glass or the like, as shown in FIG.
8
(
a
), thereby achieving a simple arrangement where a light transmitting portion of any desired pattern is formed. The light on the wafer has such a fan-shaped intensity distribution as shown in FIG.
8
(
d
), resulting in poor resolution. With the halftone phase shift lithography, on the other hand, it is possible to achieve improvements in resolution, because the phase of light transmitting through the halftone phase shift mask
412
is substantially inverted with respect to the phase of light transmitting through its opening, so that the intensity of light at pattern boundaries on the wafer is reduced to zero as shown in FIG.
7
(
d
) and, hence, such a fan-shaped intensity distribution is prevented.
Referring to the halftone phase shift photomask in general, the optimum phase difference is 180°, and the optimum transmittance is in the range of 1to 20% (100% for the opening) as determined depending on the pattern to be transferred, transfer conditions, etc. A halftone phase shift mask must be fabricated in such a way as to meet its optimum phase difference and transmittance. Any deviation from the optimum values leads to changes in the correct exposure, etc., ending up with dimensional accuracy drops, a limited tolerance to focus, etc. Thus, the refractive index, extinction coefficient, thickness accuracy and thickness stability of the single layer or multilayer forming the halftone phase shift film are of vital importance.
To ensure the effect of the halftone phase shift photomask, the phase difference and transmittance thereof at an exposure wavelength are of vital importance. To reduce the influences of multiple scattering, etc. between the photomask and lenses in the photolithography process, the reflectance of the mask at an exposure wavelength should preferably be kept low as is the case with conventional photomasks, and most preferably be 20% or less. Regarding the appearance quality, dimensional accuracy, position accuracy, etc. of the pattern formed on the photomask, too, the photomask is required to have similar various properties to those needed for conventional photomasks. To ensure these properties, the optical properties of the photomask, for instance, its transmittance and reflectance at wavelengths at which various inspection and measuring devices are used in the masking process are also of vital importance. To be more specific, the photomask should have a transmittance and reflectance of 50% or less at wavelengths of 488 nm or less.
In this connection, halftone phase shift photomasks may be fabricated by means of PVD or CVD processes. In most cases, however, they are fabricated by a reactive sputtering process wherein main materials are used in the form of targets. The refractive index, extinction coefficient, thickness accuracy and stability of the film are basically determined by the film-formation process. For etching the phase difference shift film at the step of patterning the: phase shift photomask, dry etching such as reactive ion etching is chiefly used. In the process of fabricating phase shift photomasks, washing is repeatedly carried out so as to improve their degree of cleanliness.
For instance, halftone phase shift films comprising any one of tantalum, an oxide of tantalum, a nitride of tantalum and a combined oxide and nitride of tantalum have been proposed as set forth in JP-A's 05-257264, 07-134396 and 07-281414.
In recent years, as the pattern to be formed becomes fine, it is required to make short the wavelength of exposure light used for lithography, and KrF excimer lasers of
248
nm wavelength are now put to practical use for patterns finer than the so-called 0.25 &mgr;m design rule. With expected further size reductions in view, ArF excimer lasers of 193 nm wavelength are under investigation. For halftone phase shift films used for halftone phase shift photomasks, too, the development of materials capable of achieving the optimum phase difference and transmittance for these wavelengths and having stable refractive indices and extinction coefficients are in great demand.
However, conventional halftone phase shift films comprising any one of tantalum, an oxide of tantalum, a nitride of tantalum and a combined oxide and nitride of tantalum, too, have several problems. For instance, a problem with films comprising tantalum or the oxide of tantalum is that when they have a thickness enough to have a phase difference of 180° C. at 248 nm wavelength, their transmittance decreases to 1% or less that deviates the range of 1 to 20% thought of as being the optimum transmittance range.
For instance, the thickness vs. 248 nm wavelength phase difference and transmittance relations of a tantalum nitride film formed on a synthetic guartz substrate by a reactive sputtering process using a pure tantalum target in a nitrogen atmosphere are shown in FIG.
9
. As can be seen from
FIG. 9
, the phase difference increases in proportion to an increasing thickness and, at the same time, the transmittance becomes low.
The tantalum nitride film has a phase difference of 180° at a thickness of 113 nm, but fails to have the transmittance needed for a phase shift photomask, because its transmittance is 0.06% at that thickness.
Films comprising the oxide of tantalum or the combined oxide and nitride of tantalum may have a phase difference of 180° and a transmittance of 1 to 20% as desired, both at 248 nm wavelength. However, these films cannot be inspected with precision by an inspection device for ensuring the appearance quality of a mask or a measuring device because their transmittance exceeds 50% at an

LandOfFree

Say what you really think

Search LandOfFree.com for the USA inventors and patents. Rate them and share your experience with other people.

Rating

Halftone phase shift photomask and blanks for halftone phase... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Halftone phase shift photomask and blanks for halftone phase..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Halftone phase shift photomask and blanks for halftone phase... will most certainly appreciate the feedback.

Rate now

     

Profile ID: LFUS-PAI-O-3219359

  Search
All data on this website is collected from public sources. Our data reflects the most accurate information available at the time of publication.