Section V

Mask Processing, Materials,

and Pellicles

Rizvi / Handbook of Photomask Manufacturing Technology DK2192_c014 Final Proof page 319 7.3.2005 6:25pm

© 2005 by Taylor & Francis Group.

14

Mask Substrate

Syed A. Rizvi

CONTENTS

14.1 Introduction
14.2 Material

14.2.1 Glass
14.2.2 Chrome Film
14.2.3 Molybdenum Silicide (MoSiO

x

N

y

) Film

14.3 Effect of Substrate on CD Uniformity and Image Placement
References

14.1

Introduction

With the evolution in microlithography and mask design the mask substrate also has
undergone many transitions.

In the early days of the semiconductor industry, the mask substrate (also called

mask blanks) used to be 2 in. 2 in. The mask size has since then been growing, and at
the present the standard size is 6 in. 6 in. The thickness of the substrate has grown from
0.060 in. to the present 0.25 in. Today a 0.25-in.-thick 6 in. 6 in. substrate is referred as
6025 plate. The driving force behind this increase in the substrate size has been, among
other factors, the large chip size and advancement in the step/scan exposure systems.
Larger 9 in. 9 in. substrates have also been made but their use has not been predom-
inant because of the difficulties and expenses involved in tooling and in modifying many
related manufacturing equipment. Today’s exposure systems cover a larger distance
along one of the two axes that could result in the use of rectangular 6 9-in. substrate,
but here also due to tooling difficulties the implementation of rectangular substrate was
not feasible.

The starting material for substrate is glass plate sputtered with chrome-based film

that is then coated with photoresist. The composition of chrome film changes from the
bottom to the top. The bottom portion of the films acts as glue in order to ensure a good
adherence of chrome to glass surface. The top layer of the film acts as an antireflective
(AR) coating to reduce the undesirable reflection that takes place during the exposure
cycle.

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© 2005 by Taylor & Francis Group.

14.2

Material

14.2.1 Glass

The size of the substrate is not the only factor of importance when considering substrate’s
structure. The glass plate also has undergone through a series of transformation and
forms the basis of all substrates. It plays an important role in the structure and workings
of substrates.

In earlier days, the standard material used to be soda lime, which was then replaced by

a superior quality material known as White Crown for its reduced defects. Later when the
thermal expansion of glass during exposure became an issue the White Crown was
replaced with boro-silicate glass, which had a lower coefficient of thermal expansion
(CTE). The next improvement was the introduction of fused silica (also known as quartz),
since the CTE of fused silica was even lower than that of boro-silicate. In addition to its
low CTE, this fused silica material also exhibited better transmission at 365 nm referred as
UV wavelength that was the standard in those days. This transparency of the material
became more important when the industry moved from 365 to 248 nm and now 193 nm
illuminations.

For the upcoming shorter wavelengths of 157 nm, an F2-doped fused silica material

with 76% transmission has been introduced [1,2].

Fused silica has been the material for the leading edge technology for quite sometime,

but with 193 nm exposure over an extended period the material has been found to exhibit
color centers and compaction. The color center formation causes a low level of fluores-
cence at about 400 nm, and compaction causes a small change in refractive index. How-
ever, these changes may affect the optics of the exposure systems but have no detrimental
effect on photomask because the energy involve here is very small [3].

Schott-Lithotec, a supplier of mask blank, has also introduced a promising DUV blank

known as Zerodur

1

with a spec of zero thermal expansion to meet the current require-

ments [4].

There are a number of substrate-related features that affect the CD uniformity that

should be <10 nm.

Plate flatness need to be <1.0 mm. The spec on some of the plates has been quoted as low

as 0.5 mm.

14.2.2 Chrome Film

At present the absorber on the glass is a compound of chrome consisting of Cr, N

2,

O

2

, and

possibly other elements. The composition of the film varies from the bottom to the top
serving different purposes. The bottom layer acts like a glue to improve the adherence of
chrome to glass. The top surface acts as an antireflective coating to minimize the undesir-
able reflection that may take place inside the system.

The top and bottom layers of the film constitute a very small portion of the bulk of the

chrome material that acts as the opaque film. A typical thickness of the film is 100 nm with
an optical density of 3.0, which amounts to 0.1% (or less) transmission.

As regards to AR, a coating based on a three-layer Fabry-Perot structure with reflect-

ance of <1% has also been reported [5].

Smaller chrome thickness (59–73 nm) for improved performance has been explored and

the results are promising [5].

© 2005 by Taylor & Francis Group.

14.2.3 Molybdenum Silicide (MoSiO

x

N

y

) Film

Molybdenum silicide (MoSiO

x

N

y

), commonly known as MoSi film, was first used to

improve the adhesion to fused silica [1]. Now MoSi is the key player in the structure of
embedded attenuated phase shift masks (EAPSMs). These EAPSMs will continue to be
used for the next few more generations.

The MoSi film is sandwiched between the glass and the chrome film. The MoSi has a

tendency to flake and redeposit on the mask during the processing of the substrate and
cause some yield loss. There are also issues with the exposure durability and the chemical
durability of the cleaning process of the film.

In addition to MoSi, there are other promising candidates, such as TaN/Si

x

N

y

, TiN/

Si

x

N

y

, and CrAlO

x

N

y

, that are being looked at [5].

Current status of films: The current status of the chrome and materials for EAPSM film

has also been summarized in Table 14.1 [5].

14.3

Effect of Substrate on CD Uniformity and Image Placement

The homogeneity of glass in terms of its optical properties, e.g., refractive index, trans-
mission, and birefringence, has its direct impact on the CD uniformity [5,6]. A material
developed by Corning quotes a low birefringence of <1 nm/cm and refractive index
homogeneity as <4 ppm.

The inhomogeneity of chrome and other phase-related films on the glass can equally

affect the CD uniformity.

TABLE 14.1

Status of Substrate Films [5]

Absorbers for Binary Masks

Materials for EAPSM Films

Characteristics

Requirements

Fully Met

Requirements

Partially Met

Requirements

Fully Met

Requirements

Partially Met

Optical density > 3

Yes

Phase shift of 180 + 58 at

exposure wavelength

Yes

Reflectivity < 15%

Yes

Yes

Transmission 5–25%

Yes

High conductivity to prevent charging

during the e-beam patterning

Yes

Low roughness, no pinholes,

no particles

Yes

Yes

Ease of etch

Yes

Yes

Chemical resistance (no adhesion

failure or change in optical
properties with cleaning)

Yes

Yes

Uniformity (thickness, transmission,

index of refraction)

Yes

Yes

© 2005 by Taylor & Francis Group.

Today’s plates are considerably thicker compared to the earlier ones, but because of

their increased size the phenomena of ‘‘sag’’ and ‘‘distortion’’ when mounted on the
exposure systems may still occur. Strains suffered by the plates under this condition can
directly contribute to the image placement (IP) errors. A deflection of 0.62 mm can give
40 nm IP error [3].

IP is also affected by the CTE of the glass. Even in fused silica, a change of 0.088C can

change the IP by 10% of the allowed tolerance [3]. Plate flatness needs to be <1.0 mm. The
spec on some of the plates has been quoted as low as 0.5 mm. Out-of-spec flatness may
affect CD uniformity and IPs.

Coated film may cause some stress on the glass, and after the pattern is made some of

the stress is released that can bend the glass causing an IP error.

References

1. J.G. Skinner, Photomask Fabrication for Today and Tomorrow, Short Course 122, SPIE Education

Service Program, 2001.

2. Asahi Glass Brochure/Website/Photomask Substrate.
3. P. Rai-Choudhury, Handbook of Microlithography, Micromachining and Microfabrication, vol. 1, SPIE

Press, 1997, pp. 377–474. Ballingham, Washington, USA.

4. Schott Lithotec Brochure: Mask Blanks, IC Advanced Packaging.
5. R. Walton, Photo blanks for advanced lithography, Solid State Technol., October, 2003.
6. B.B Wang, Residual birefringence in photomask substrates, J. Microlith., Microfab., Miscrosyst.,

1 (1), 43–48 (2002).

© 2005 by Taylor & Francis Group.