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5…Conclusions and Future Perspectives
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Photochemistry in Electronics
Owen J. Guy, Gregory Burwell, Ambroise Castaing
and Kelly-Ann D. Walker
Abstract Photochemistry plays a critical role in modern semiconductor electronics, primarily through the use of photoactive polymers or photoresists in the
lithographic processes used to fabricate semiconductor devices. Photoactive
polymers have been extensively researched in order to develop resists that are
chemically robust and that are able to produce sharp, well defined, high resolution
features through photolithography. This chapter introduces photolithography and
photoresists, and presents review of the photochemistry of some of the more
important commercial photoresists. Miniaturisation of semiconductor devices for
consumer electronics and sensors now places increasing demands on lithography
processes. This has lead to the development of sub-micrometer and now nanometer scale devices. A review of electron beam lithography and other high-resolution lithography techniques concludes this chapter.
O. J. Guy (&) Á G. Burwell Á A. Castaing Á K.-A. D. Walker
College of Engineering, Swansea University, Singleton Park,
Swansea SA3 8PP, UK
K.-A. D. Walker
R. C. Evans et al. (eds.), Applied Photochemistry,
Ó Springer Science+Business Media Dordrecht 2013
O. J. Guy et al.
This chapter outlines some of the most important applications of photochemistry in
electronics. Almost every step in the fabrication of electronic devices involves
photoactive polymers called photoresists. Photoresists, applied as a thin layer on a
semiconductor wafer, are used in the patterning of electronic devices and ‘microchips’ used in every modern electronic appliance. The process by which photoresists are patterned is called photolithography. Photolithography, which is
explained in detail below, relies on a change in solubility of the photoresist
polymer on exposure to light. Selective exposure of the resist and subsequent
selective removal, allows the patterning of millions of micron-sized devices on a
silicon wafer or other semiconductor substrate. Photolithography predominantly
uses UV light to expose the photoresist.
Research on yet smaller (and therefore more dense and efficient) circuitry
continues apace today. Photolithography is still the predominant technology used
in the microelectronics industry, but polymers for extreme UV and electron beam
lithography are becoming increasingly important as the evolution of electronic
devices attempts to conform to Moore’s Law , by packing ever more devices
onto the semiconductor wafer. There have been many developments in photolithography and the polymer chemistry of photoresists, all with the aim of enabling
higher resolution device features to be fabricated. The limits of UV photolithography now appear to have been reached and research is currently focused on a host
of techniques for patterning of features at the nanoscale. Nano-lithography techniques such as electron beam lithography and nanoimprint lithography are now
used in mainstream industrial device processes. These methods utilise photochemistry to achieve nanoscale patterns. A summary of lithographic techniques for
fabricating nanoscale structures is presented in this chapter.
13.2 Photolithography and Photoresists
Photolithography is in essence very similar to the process of photography, which is
discussed in detail in Chap. 11. As in photography, a light-sensitive material is
selectively exposed to light in order to generate a picture, or a pattern. Photolithography uses a set of parameters: the tone, the sensitivity, the resolution and the
contrast, to define the appropriate resist for specific exposure conditions in order to
achieve a desired pattern. The tone determines which areas of the resist will be
removed after exposure to light. Exposure of a positive tone resist enables the
exposed resist to be removed using a chemical development process, whilst
exposure of a negative tone resist makes the resist more difficult to remove with a
developing solution. The sensitivity of the resist represents the amount of light, or
dose, required to completely expose the resist film. The resolution is the minimum
feature size attainable, and the contrast describes how abruptly, or sharply, features
Photochemistry in Electronics
can be defined using a particular resist layer thickness and light dose. A high
contrast allows sharp edges and smaller features, while a low contrast resist, if
combined with precise timing in the development process can allow the formation
of 3D patterns.
Photoresists are photoactive polymers dissolved in a solvent to form a viscous
liquid. The resists are used as protective or mask materials in electronic device
fabrication processes. Photoresists undergo a photochemical reaction and subsequent change in their solubility on exposure to light. Photoresists can be categorised as positive or negative resists. Positive resists become more soluble when
exposed to light whereas negative resists become less soluble.
Resists are applied to semiconductor wafers by dropping the polymer based
liquid resist onto a substrate, which is then spun at 1000–6000 revolutions per
minute (rpm) to form a thin coating. The spin-coated resist layer can be anything
from tens of nanometres to several hundred micrometres in thickness—depending
on the viscosity of the resist and the spin coating parameters (acceleration, spin
speed and spin cycle duration).
The general principle of photolithography is to selectively expose the photoresist to light through a hard-mask (usually made from a patterned chrome layer on
a glass or quartz substrate as shown for example in Fig. 13.1).
Light passes through the transparent areas of the hard mask and selectively
exposes the photoresist layer. The solubility of a positive resist will increase on
exposure to light, and can subsequently be selectively removed using a chemical
developing solution. Negative resists behave in the opposite way, the light exposed
areas becoming insoluble in the chemical developer whilst the soluble un-exposed
areas can be removed. Negative photoresists often rely on a photo-initiated crosslinking reaction to generate an insoluble polymer. Figure 13.2 illustrates this
process for both types of resist.
Fig. 13.1 a Chrome on quartz hard photomask. b Schematic illustration of photoresist (purple
layer) exposure to UV light through a quartz-chrome hard mask. Exposure of positive resist (left)
and negative resist (right) followed by resist removal and subsequent silicon dioxide (white layer)
etching to selectively expose the silicon substrate (blue layer)
O. J. Guy et al.
Fig. 13.2 Microelectronic devices fabricated on a silicon wafer. Devices are patterned using
Novolak based photolithography
Following patterning of the photoresist layer, further process steps such as
etching or metallisation can be carried out. The photoresist acts as a protective
layer or mask, for example, protecting the substrate beneath from an etching
process. In the example illustrated in Fig. 13.1b, the resist (purple) acts as a mask,
initially protecting the silicon dioxide layer (white) from being etched by a
hydrofluoric (HF) acid based etchant. Areas of silicon dioxide covered by the resist
are protected from etching, whilst exposed silicon dioxide is etched away by the
acid. Photoresists are used as protective masks in selective etching of silicon
dioxide, silicon, silicon nitride and several metals. Resists, though partially inert to
many etchants, will however degrade and be removed after time. Indeed, more
aggressive etchants will remove resist layers quickly. The selectivity of the resist
relative to the target etch material is thus critical. The selectivity is the ratio of the
removal (etch) rate of the photoresist layer to the removal (etch) rate of the
material to be etched (silicon dioxide in the above example). The higher this ratio,
the longer the resist will remain intact and more of the target material can be
There are several methods that can be used to increase the endurance of the
Photochemistry in Electronics
1. Baking: Baking the resist induces thermal cross-linking of the resist polymer.
The resist thus becomes harder, less soluble and hence more difficult to remove.
It also increases its resistance to chemical etchants.
2. Resist thickness: Thicker layers of resist can be applied to wafers by varying
the spin coating conditions used to apply the resist to the semiconductor wafer.
Decreasing the acceleration time and/or the spin speed (rpm) and/or the spin
duration, would all result in a thicker resist layer. In addition more viscous
resists can be used to yield much thicker protective layers. SU-8 is a commonly
used epoxy-based negative photoresist. It is a highly viscous resist, which can
form layers around 100 lm in thickness. Standard photoresists such as MicropositTM S1813TM (produced by Shipley a Rohm & Haas company) usually
yield layers around 1 or 2 lm in thickness.
For some semiconductor processing steps such as ion-implantation or deep
etching, the photoresist layer is insufficiently resilient to selectively protect the
substrate material and a more durable mask material must be used. In these cases,
silicon dioxide, silicon nitride or metal, ‘hard masks’, are often utilised. These
materials are first patterned by photolithography using a photoresist to selectively
protect the hard mask from etching. Once patterned, the hard mask provides
greater selectivity to etching than a photoresist layer. Hard masks are also less
penetrable than resist masks with respect to ion implantation. In Fig. 13.2, photolithography processes are used to pattern a hard silicon dioxide mask. This mask
is then used to selectively protect the silicon substrate (blue layer) during further
processing. In processes such as silicon reactive ion etching (RIE), the hard silicon
dioxide mask is consumed during the etch process, but at a lower rate than the
silicon is removed.
13.2.1 UV Light Exposure
The photochemistry of resists predominantly utilises molecules which absorb in
the UV region of the electromagnetic spectrum. UV light is able to cleave bonds
and create radicals which can crosslink polymers or modify functional moieties to
create a change in solubility of chemical species after UV exposure, compared to
pre-exposure. UV photochemistry was selected for lithographic processes for
1. UV light is of shorter wavelength than visible light and thus a higher ultimate
feature resolution can be obtained using UV photolithography.
2. UV light sources can be more effectively controlled (without interference from
visible light sources).
3. Mercury lamp UV light sources give distinct linear light emission output at
three UV wavelengths: 365.4 nm (I-line), 404.7 nm (H-line), 435.8 nm (Gline) . (Emission spectra for mercury lamps are given in Chap. 14).
O. J. Guy et al.
13.2.2 Chemistry of Photoresists
There are many photoresists available commercially. These can be categorised into
positive and negative tone resists. Various formulations of resists have differing
properties (viscosity, ultimate achievable resolution, substrate adhesion etc.).
Several classes of resist have thus been developed for different electronic applications. For example, high-resolution photolithography for patterning of submicron transistors requires a different resist to that needed for deep silicon etching
of micro-electro-mechanical-systems (MEMS) structures.
220.127.116.11 Negative Photoresists
Many negative photoresists rely on cross-linking reactions to create less soluble
areas on exposure to light. Cross-linking refers to bonds formed between polymer
chains creating a more rigid polymer matrix. Cross-links are formed by chemical
reactions that are initiated by light, heat and/or pressure, or by the mixing of a nonpolymerised or partially polymerised resin with various chemicals. Cross-linking
is widely used in polymers or plastics to alter the properties of the polymer to
become harder, more rigid and/or less soluble. It is much more difficult for solvent
molecules to separate polymer chains in the cross-linked matrix, than in a noncross-linked polymer. Cross-linked polymers tend only to swell in response to a
solvent. There are many different cross-linking reactions that can be used in
negative resists, some of which are highlighted below.
Early photoresists used polymers that could be cross-linked using a photoinitiated reaction. One example of a cross-linking resist is based on linking of
poly-isoprene polymer chains via reaction with a bis-azide molecule . The azide
(N3) group undergoes a photochemically initiated breakdown, releasing nitrogen
(N2) and also creating a very reactive nitrene group. Nitrene moieties are formed at
both ends of the bis-azide molecule and the nitrene groups can react with two
poly(cis-isoprene) chains to yield a cross-linked polymer. One of the most common reaction mechanisms is the formation of an aziridine ring (Scheme. 13.1).
Cross-linking of the polymer chains results in the polymer becoming less soluble,
and the areas of polymer resist exposed to light are thus insoluble in the developing solution—constituting a negative tone resist.
Another example of a negative tone photoresist uses poly(vinyl cinnamate). The
first step in the formation of an insoluble polymer is the esterification of poly
(vinyl alcohol) (PVA) with cinnamate groups to yield poly(vinyl cinnamate)
The alkene groups of the cinnamate moieties subsequently undergo a [2 ? 2]
cycloaddition reaction under irradiation (Scheme 13.3). This cycloaddition results
in a cross-linked polymer suitable as a negative tone photoresist .
Photochemistry in Electronics
Scheme 13.1 Cross-linking of bis-azide with poly(cis-isoprene)
Scheme 13.2 Negative tone resist produced by partial esterification of poly(vinyl alcohol) to
yield poly(vinyl cinnamate)
18.104.22.168 Positive Photoresists
Positive resists use an alternative approach to cross-linking. The objective is to use
a photochemical reaction to create a more soluble material from an insoluble one.
O. J. Guy et al.
Scheme 13.3 Cycloaddition cross-linking of cinnamate groups
The approach is often to photochemically alter the functional groups of an
insoluble molecule to convert the molecule into a soluble one. There are many
different examples of this type of functional group conversion, but one of the most
important is the class of polymers known as Novolaks .
Novolaks are phenol-formaldehyde type polymers. Novalaks are synthesised
via a polycondensation reaction which is halted before the polymer becomes fully
cross-linked. As Novalak polymers contain phenol units, they have reasonable
solubility in aqueous base solutions. To act as a positive tone resist, the solubility
of the Novolak polymer in basic solutions must be greatly enhanced. This can be
achieved by using photochemically reactive additives. The additive used is a
diazoanthraquinone, which undergoes a photochemically driven Wolf rearrangement reaction to produce a carboxylic acid (Scheme 13.4).
However, in the presence of suitable additives, the dissolution process can be
greatly enhanced. The additives can be produced photochemically, leading to a
useful photoresist system. In fact, Novolaks have been the ‘workhorse’ photoresists
Scheme 13.4 Wolff
produce carboxylic acid
Photochemistry in Electronics
of the modern microelectronic revolution. The photoresist consists of Novolak
polymer, with a small amount of diazonaphthaquinone dissolved in it. When irradiated, the diazonaphthaquinone undergoes a photochemical rearrangement.
The carboxylic acid produced is highly soluble in the base developing solution
and the dissolution of this carboxylic acid enables the Novolak polymer to be
dissolved much more readily in the base developer. In fact, the solubility of
the Novolak polymer is increased by several orders of magnitude in comparison to
the polymer in the absence of the carboxylic acid.
Novolak based resists that have not been exposed to UV light are therefore
almost insoluble, but the exposed resist is highly soluble. Novolaks are therefore
used as positive tone photoresists. Novolak chemistry has proved very effective for
producing high resolution photolithographic patterns (Fig. 13.2) and is used
extensively industry. An example of a Novolak-based commercial positive resist is
S1813 from Rohm & Haas, which was developed for the microelectronics integrated circuits industry.
13.2.3 Photoresists for Micro-Electro-Mechanical Systems
The microelectronics industry uses both positive and negative tone resists to
pattern features from around 0.5 to several hundred micrometres in size. Smaller
devices require high resolution processing with resists developed specifically for
the microelectronics industry. Resist layers up to a few micrometres thick are
usually sufficient for microelectronics device processing.
However, developments in the MEMS industry, which often relies on deep
etching of silicon to fabricate micro-machine structures, require thicker resist
layers and alternative polymer photoresists.
22.214.171.124 SU-8 Resist
A popular resist for MEMS processing is SU-8 [US Patent No. 4882245 (1989)].
This is a negative tone resist and is commonly applied as a layer, tens of micrometres in thickness. SU-8 was developed by Shell Chemicals and uses epoxybased chemistry. Microchem [www.microchem.com] is one of the companies that
now holds the licence for production and sale of SU-8 photoresists.
SU-8 (Scheme 13.5) is a very viscous polymer that can be spun or spread over a
thickness ranging from 0.1 lm  up to 2 mm and can still be processed with
standard contact lithography. It can be used to pattern high aspect ratio ([20:1)
structures . Its maximum absorption is for UV light with a wavelength of
365 nm. When exposed, SU-8’s long molecular chains cross-link causing reduced
solubility of the exposed resist.