7

Conventional Optical Masks

Syed A. Rizvi

CONTENTS

7.1 Introduction
7.2 Classification of Optical Masks

7.2.1 Conventional Optical Mask
7.2.2 Advanced Optical Masks

7.3 Making Conventional Masks

7.3.1 Starting Material
7.3.2 Mask Writing
7.3.3 Mask Processing
7.3.4 Mask Qualification

7.3.4.1

Mask Inspection and Repair

7.3.4.2

Metrology

7.3.5 Pellicles

7.4 General Comments

7.1

Introduction

This chapter is a prelude to

Chapter 8

that addresses Advanced optical masks. In order

to fully appreciate the innovation and technologies that led to the development of
the advanced optical masks, it is necessary to know something about the conventional
optical masks behind the backdrop of which these technologies and innovation took
place.

Unlike

Chapter 6,

which gives an overview of optical masks in general, this chapter

focuses exclusively on conventional optical masks, whereas Chapter 8 provides a detailed
description of advanced optical masks technologies.

Chapter 1

in this book has given an overview on ‘‘all types’’ of masks. The connotation

behind this ‘‘all types’’ of mask requires knowledge of the evolution of masks in the
semiconductor industry.

In the early 1960s, when the semiconductor industry was in its infancy, the masks

consisted of glass plates with emulsion patterns, which were not much different from the
black and white negatives used in photography in its early days. Later on, the emulsion
patterns on glass plates were replaced by chrome pattern on glass (CoG) plates because
the chrome material was less prone to damage when mask and wafer had to be brought
into intimate contact during exposure.

Rizvi / Handbook of Photomask Manufacturing Technology DK2192_c007 Final Proof page 157 7.3.2005 6:18pm

© 2005 by Taylor & Francis Group.

But as the requirement on mask quality became more stringent even the CoG masks

could not meet the specifications, since the contact printing on wafers could still cause
damage to the masks, although to a lesser degree.

The answer to mask damage was sought in ‘‘off-contact,’’ also known as ‘‘proxim-

ity’’ printing, where the mask is brought in close proximity of the wafer without
making a real contact between the two. This approach was a form of shadow casting
of the mask features on the wafer that required a tradeoff between mask’s life and
resolutions. With continuous demands for higher resolutions the practice could not be
continued.

The next step was then to project the image of the entire masks on the wafer through

some forms of 1:1 optics, where the desired resolution could be obtained without the
mask coming into direct contact with wafers. Early systems had utilized refractive optics
consisting of set of lenses, but to project the image of the entire mask and still attain high
resolution required large-diameter and almost perfect lenses that were not available in
those days. In fact very few such machines were built and their use was very limited. The
1:1 projection printing really took off when Perkin-Elmer introduced its first ‘‘all reflect-
ive’’ scanning-mirror optics, which is capable of imaging the entire mask on the wafer and
also attaining desirable resolutions. Use of mirror also did away with chromatic aberra-
tion encountered in lens optics. The scanning mirror technology lasted almost a decade
until wafer steppers were introduced into the fabs. Wafer stepper technology was essen-
tially an extension of the fully matured technology of mask steppers that were routinely
applied for making masks in those days.

The masks consisted of an array of dies that were made by stepping de-magnified

images of reticle across the entire mask area. Typically, the ratio of the reticle image to the
image of die on wafer used to be 10:1. In case of wafer stepper also, the practice of 10:1
reduction continued for sometime. In today’s technology, most machines utilize stepping,
as well as scanning, mechanism for image transfer with 5:1 and 4:1 reduction ratios.

The evolution of optical lithography also caused changes in the design and structure of

optical mask. Although the transformation from emulsion mask to chrome mask could be
viewed as significant change in mask structure, the most radical change in mask making
came about with the introduction of PSM and OPC that are referred as advanced optical
masks in this book. The masks that do not employ these techniques are referred as
conventional masks, which form the subject of this chapter.

Beyond the optical masks there are a number of nonoptical masks over the horizon

belonging to various nonlithographic techniques referred as next generation lithog-
raphy.

Although advanced optical masks remain as the focus for many chip manufacturers,

the masks that do not incorporate OPC or phase shifting in their structure continue to be
used by those manufacturers for their noncritical levels and claim a major share of the
mask market. These masks are to be regarded as the basis on which the advanced mask
technology is shaped and is labeled by the author as ‘‘conventional masks.’’

7.2

Classification of Optical Masks

The masks can be classified in a number of ways, and the most common set of the criteria
for classification is shown in

Figure 7.1.

The classification of optical masks is discussed in

the following.

© 2005 by Taylor & Francis Group.

7.2.1 Conventional Optical Mask

This is the basic mask structure and represents a generic mask, since it consists of
CoG mask. The CoG mask can be of two types. It can be a light field mask with CoG
background, or it can be dark field mask which means features are etched into the chrome
film covering the entire mask.

7.2.2 Advanced Optical Masks

The masks with OPC are essentially CoG masks as mentioned earlier, except that these
have some ‘‘very small features’’ (called serif, scattering bars, assist, etc.) added to the
already existing features to offset the diffraction effects that occur as the industry moves
towards the smaller features. Hence, in principle, the processing of OPC mask is not
different from the processing of conventional mask except that OPC masks require more
refinement and precision in their making.

The structure of phase shift masks may be just CoG masks, or some other phase shifting

material (e.g., MoSi) on glass. There are also phase shift masks where the features are
simply etched into glass to induce phase shifting during the printing. These masks have
no chrome or any phase shift material on them. However, during the processing of these
masks, chrome film can be employed as will be mentioned later in

chapter 8.

7.3

Making Conventional Masks

Conventional masks form the basis for all optical masks, since fabrication of all other
types of optical masks incorporates the basic steps employed in the making of conven-
tional masks.

7.3.1 Starting Material

The starting material in mask making continues to be glass plates coated with a film
of chrome that is covered with another film of some kind of photosensitive or electron

All masks

Optical

NGL

Conventional

Advanced

EPL

EUV

IPL

X-ray

FIGURE 7.1
Classification of masks.

© 2005 by Taylor & Francis Group.

beam-sensitive material known as photoresist or simply resist. The final material is
known as mask blanks.

7.3.2 Mask Writing

The mask blank is then mounted on the stage of an electron beam writer or laser writer
depending upon the type of the resist it is coated with. The beams are scanned and
manipulated to selectively expose the resist to form desired patterns on the resist film
after the plates are developed.

7.3.3 Mask Processing

Develop: The exposed plates are developed using the chemicals that can dissolve out the
exposed resist as in the case of positive resist, or by using the chemicals that can dissolve
out the unexposed resist in the case of negative resist. In either case, a resist pattern can
become visible after the develop cycle. This resist pattern is formed on the chrome surface
exposing parts of the chrome to be etched in the next step.

Etch: The plate is then subjected to the etch process, where the exposed chrome is

etched away leaving the glass surface behind. The unexposed chrome remains protected
under the resist. The etching can be done by wet chemicals or dry plasma process.

Resist removal and final cleanup: In the next and final step, the resist is removed and the

plate is sent through a cleanup operation. The resist can be removed using wet chemicals,
as well as oxygen plasma that is regarded as the dry process.

7.3.4 Mask Qualification

7.3.4.1 Mask Inspection and Repair

Once the mask is cleaned it is ready for inspection and repair. There are a number of
machines in the market that can do inspection. The machines for repairing opaque defects
simply zap the defect with the high power laser, whereas clear defects are repaired
by spot deposition of gallium or any material with the properties similar to that of
gallium.

7.3.4.2 Metrology
Two important parameters to be measured before the mask can be qualified are size of
any prescribed critical feature known as critical dimension (or simply CD) and the
position of features referred as image placement (IP) measurement.

For CD metrology, there are many systems in the market, but only few of them can

measure IP. Leica IPRO is one of them.

Thin film measurement is also important, and that is typically employed for measuring

thicknesses of chrome and resist at the beginning of the process, at the time of fabrication
of mask blanks.

7.3.5 Pellicles

The final step in the mask fabrication flow is the mounting of pellicle on mask to protect
its surface from particles in the ambient that may fall on the mask surface and contam-
inate it.

© 2005 by Taylor & Francis Group.

7.4

General Comments

The discussion here describes the fabrication of conventional optical masks, and the
process is generic in nature. Deviation form the process can occur especially when making
the state-of-the-art optical masks, which has been explained in detail in

Chapter 8.

The

content of this chapter due to its basic nature sets the stage for Chapter 8.

© 2005 by Taylor & Francis Group.