ISSN 1063-7397, Russian Microelectronics, 2007, Vol. 36, No. 1, pp. 40–48. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © S.V. Zelentsov, N.V. Zelentsova, A.N. Kolesov, L.A. Bogatyreva, I.A. Mashtakov, 2007, published in Mikroelektronika, 2007, Vol. 36, No. 1, pp. 45–56.
Photolithography plays a central role in microelec-
tronics manufacturing, accounting for the greater part
of process costs. It is a major determinant of the mini-
mum feature size and active-layer thickness attainable.
In fact, we can say that photolithography has been the
engine of progress in microelectronics.
In selecting a photoresist, one has to strike a balance
between the conﬂicting requirements for sensitivity to
actinic radiation, etch durability, low defect density,
high contrast, high tolerance to lithographic-parameter
variations, etc. .
As the industry moves deeper into the submicrome-
ter region, it becomes increasingly apparent that wet
processing (resist removal, etching, etc.) should give
way to plasma and ion techniques. The problem thus
arises of how to enhance the durability of photoresist
masks under dry etching without compromising the
lithographic performance of the photoresist.
This paper provides a review of the main strategies
to enhance photoresist-mask durability under plasma
etching and/or reactive ion etching (RIE), describing
the physical and chemical processes involved. Section 2
presents the most common approaches. Section 3 is
speciﬁcally concerned with surface imaging owing to
2. THE MOST COMMON APPROACHES
As can be seen from the research literature and
patent speciﬁcations, the most popular methods used to
enhance the dry-etch durability of photoresist masks
may be classiﬁed into four groups:
(1) modiﬁcation of photoresist chemical composi-
tion to enhance dry-etch durability;
(2) changeover to a multilayer structure;
(3) postexposure photoresist modiﬁcation (physical
(4) ﬁnished-mask modiﬁcation (physical or chemi-
This classiﬁcation, deﬁned on a structural and func-
tional basis, sets the stage for a consistent treatment of
2.1 Modiﬁcation of Photoresist Chemical Composition
to Enhance Dry-Etch Durability
Introducing suitable functional groups into the poly-
mer is probably the simplest way to enhance the dry-
etch durability of a photoresist.
Experiments have revealed that plasma-etch dura-
bility depends on the ratio of the number of aromatic
groups to the total number of functional units .
Highly unsaturated materials should display good resis-
tance to plasma etching; accordingly, a photoresist is
modiﬁed to contain double bonds or cyclic structures,
which ensures a large carbon content as a proportion of
carbon and hydrogen combined. The rate of erosion
under oxygen-plasma etching can also be reduced by
The most widely known form of the above strategy
is incorporating aromatic groups: phenyl, naphthyl, or
anthranil ones. These have a larger C/H ratio than ali-
phatic compounds, deactivate molecules readily, and
are more resistant to attack by free radicals from the
plasma (aromatic groups also inhibit oxidation associ-
ated with hydrogen detachment by plasma free radi-
However, phenyl groups cannot be used in photore-
sists that are to be exposed at a wavelength of 193 nm,
due to their strong absorption of the radiation. Acrylic
polymers have been proposed instead; their plasma-
etch durability is improved by incorporating bicyclic
groups to form a copolymer .
As an example, the polymer reported by Lee et al.
 was synthesized by addition vinyl polymerization of
derivatives of tetracarbocyclic norbornene;
Enhancing the Dry-Etch Durability of Photoresist Masks:
A Review of the Main Approaches
S. V. Zelentsov, N. V. Zelentsova, A. N. Kolesov, L. A. Bogatyreva, and I. A. Mashtakov
Nizhni Novgorod State University, Nizhni Novgorod, Russia
Received January 26, 2006
—A review is presented of the main approaches to the enhancement of photoresist-mask durability
under dry etching. The physical and chemical processes involved are described.
PACS numbers: 81.40.Wx