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674 D.R. Hines et al.

Fig. 9.4 Optical images of 4 in.

square photomask. Magniﬁed area represented in both clear ﬁeld

and dark ﬁeld

lithography schemes where this is necessary. Photomasks can be designed for either

clear ﬁeld or dark ﬁeld patterning as s hown in the lower panels of Fig. 9.4. A dark

ﬁeld mask has the desired pattern contained in the transparent part of the mask

whereas the clear ﬁeld mask has the pattern contained in the opaque part of the

mask.

For contact, soft-contact, and proximity lithography the mask is designed to pro-

duce a 1:1 image in the photoresist layer [26]. These masks come in contact or close

contact (10–20 µm above the photoresist surface) with the photoresist layer and thus

are susceptible to contamination and wear. A contact mask may need to be cleaned

on a regular basis which exposes the surface to additional damage. Contact masks

are mostly used for research and development and prototyping. For low-resolution

processing, an inexpensive transparency mask can be made using a drawing program

on the computer and a printer loaded with transparency ﬁlm. Photomasks with res-

olution in the 30–40 µm range can be made using a professional graphics (>3000

dpi) quality printer. Standard laser printers (<1200 dpi) can achieve a reasonable

resolution but cannot typically achieve the necessary level of opacity to print usable

masks.

Projection lithography masks (often called reticles) are typically designed to pro-

duce 1:4, 1:5, or 1:10 reduced images in the photoresist layer. They are used with

9 MEMS Lithography and Micromachining Techniques 675

an optical projection system where the image of the mask is projected onto the sur-

face of the photoresist layer through a complex series of optics. Because projection

masks do not come into contact with the photoresist layer, they are much less sus-

ceptible to wear or damage and therefore are more suitable for use in high-volume

production facilities.

There are four variations of projection photomasks. The simplest type uses a

combination of transparent and opaque regions to deﬁne a pattern that can be

imaged onto a photoresist layer. The resolution of such a standard projection mask is

diffraction-limited [27]. To overcome diffraction limitations by very closely spaced

features, optical interference effects can be engineered into the mask. This can be

accomplished by either changing the thickness of a transparent region of the mask

(alternating phase-shift) [28] or by increasing the transmittance of the originally

opaque region of the mask (attenuated phase-shift) [29]. Such phase-shift modiﬁ-

cations in the transmitted light through a photomask cause either constructive or

destructive interference patterns. With proper mask engineering of features on the

photomask, the resulting light intensity pattern imaged into the photoresist layer

can be of higher resolution than was achievable with a standard photomask as is

illustrated in Fig. 9.5.

Fig. 9.5 Transmitted light intensity through (left panel) a phase shift photomask as compared to

that for (right panel) a standard photomask (Modiﬁed from [30])

A standard projection photomask can be further modiﬁed by pixelating an orig-

inally opaque region of the mask. If the pixel size and spacing are below the

resolution limit of the optical projection system then the pixelated region attenu-

ates the intensity of the transmitted light without imaging individual pixels into the

photoresist. These styles of masks are used for grayscale lithography. Due to mask

fabrication limits and resolution limits of a given optical projection system, a ﬁnite

number of discrete pixel sizes and spacings can be realized. Each discrete pixel size

and spacing translates into a speciﬁc intensity of transmitted light from 100% (trans-

parent region, no pixels) to 0% (opaque region). Each intensity level is referred to as

676 D.R. Hines et al.

Fig. 9.6 Cross-section

illustration of (a) standard

projection photomask, (b)

alternating phase-shift

photomask, (c) attenuated

phase-shift photomask, and

(d) grayscale photomask

a gray-level. An illustration of the different types of projection photomasks is shown

in Fig. 9.6.

Photomasks are typically made from soda-lime glass, borosilicate glass, or fused

quartz. Thermal expansion and optical transmission are the primary considerations

for choosing the mask material. Table 9.1 lists the thermal expansion coefﬁcients for

these materials. The transmission properties are shown in Fig. 9.7. The dimensions

of soda-lime glass can change by 1.2 µm across a 5 in. mask for every 1

◦

C change

in temperature. However, for mask fabrication tools and steppers that are housed in

a temperature-controlled environment, this is not typically a great concern and thus

soda-lime glass can be used. For mask fabrication tools that use e-beam lithography,

the mask is in a high vacuum environment, which can affect heat transfer. Therefore,

borosilicate glass is more commonly used with these e-beam mask-writing tools.

In UV lithography applications using wavelengths shorter than 350 nm, and for

applications achieving very high resolution, quartz masks are needed due to thermal

effects and transmissivity.

Table 9.1 Thermal

coefﬁcient for materials used

as photomasks

Material Thermal coefﬁcient, ppm/

◦

Soda lime 9.3

Borosilicate 3.7

Quartz 0.5

Reproduced with permission from [31],

Microlithography: From Computer Aided Design

(CAD) to Pattern Substrate, Cornell NanoScale Science

and Technology Facility

Typically, electronic circuits and completed MEMS devices are fabricated in

stages, requiring several sequential photolithography steps. Each step or layer

requires a separate photomask containing part of the overall circuit or device pattern

that must be carefully aligned to the previously patterned features on the substrate.

An example of a four-layer photolithography process and the associated photomasks

(both clear and dark ﬁeld mask sets) are shown in Fig. 9.8.

9 MEMS Lithography and Micromachining Techniques 677

Fig. 9.7 Transmission of UV light through different mask materials (Modiﬁed from [30])

9.2.2 Optical Projection Systems

Optical projection systems are the tools used to create an image of the pattern con-

tained in the photomask onto the surface of the photoresist layer. They typically

utilize UV light from a mercury arc lamp the spectral emission of which is shown

in Fig. 9.9. Several prominent emission lines are visible in the UV region of the

spectrum at 435.8, 404.7, and 365.4 nm which are referred to as g-, h-, and i-lines,

respectively. Both optical projection systems and photoresist formulations have been

optimized for maximum performance at these wavelengths.

9.2.2.1 Contact Aligner

A contact aligner (used with contact and proximity masks) consists of a UV light

source for exposure, a microscope with X, Y, Z and rotation (R) stages for wafer

alignment and a mask holder. The mask and a photoresist covered wafer are brought

into close proximity with the Z stage and aligned as needed with the X, Y, and R

stages. Once aligned, the wafer and mask are brought into full contact (or close

proximity) and exposed. Contact aligners are typically benchtop units, of which the

MJB3 (shown in Fig. 9.10) and the MA-6 are commonly used examples. A con-

tact aligner transmits a 1:1 image of the photomask onto the photoresist layer. This

678 D.R. Hines et al.

Fig. 9.8 Multilevel mask set for sequential photolithographic fabrication. Mask set examples are

shown for both (a) bright ﬁeld (clear ﬁeld) and (b) dark ﬁeld masks. The ﬁnal patterned substrate is

illustrated in (c)[25] (Modiﬁed from http://www.photronics.com/about/basics.jsp Reprinted with

permission of PHOTORONICS, INC. from Photomask Basics)

allows large area patterns to be exposed into the photoresist in one step. Because

the photomask and photoresist layer are in direct contact, there are no focusing

problems onto the surface of the wafer. Substrates of various thicknesses can be

easily handled and the alignment/exposure is not sensitive to wedge error (substrate

not perfectly ﬂat in relation to the projected image) as the substrate holder can tilt

slightly to provide intimate contact with the mask.

9 MEMS Lithography and Micromachining Techniques 679

Fig. 9.9 Hg emission versus

wavelength (Reprinted with

permission from [32])

Fig. 9.10 Optical lithography systems: Manual contact aligner (left) and stepper (right) (Aligner

reprinted courtesy of SUSS MicroTec Lithography GmbH, stepper reprinted courtesy of EV Group,

Inc.)

Mixing photolithography with other patterning methods is also possible. For

example, nanoimprint lithography features can be etched into the surface of a pho-

tomask allowing simultaneous patterning via both NIL and photolithography. Also,

printing methods can be combined with photolithography by applying a printable

layer or ink to the surface of a photomask. Such combined patterning methods

680 D.R. Hines et al.

require contact to the substrate and therefore are only possible using contact aligner

photomasks.

Contact aligners also possess several disadvantages. Contact between the pho-

tomask and the photoresist layer can cause damage to the photomask and is also

a source of dirt and contamination. Particles trapped between the photomask and

the photoresist layer, or a nonuniform photoresist layer can prevent proper contact

and thus adversely affect the quality of the patterned features in the photoresist.

Resolution is typically less for contact aligners as compared to steppers. Also con-

tact aligners are less automated than steppers resulting in slower and less accurate

operation.

Figure 9.11 contains a diagram showing the main components of a contact

aligner. Some contact aligners have the ability to perform backside alignment. This

allows the bottom surface of a substrate to contain a pattern that is in alignment with

a pattern on the top surface of the same substrate. The aligner uses a second set of

optics to align the previously patterned bottom surface of the substrate to a stored

image of the photomask. Once aligned, a photoresist layer on the top surface of the

substrate can be patterned.

Fig. 9.11 Diagram showing the main components of a contact aligner (Reprinted with permission

from [3])

9 MEMS Lithography and Micromachining Techniques 681

9.2.2.2 Stepper

A stepper (used with projection photomasks or reticles) differs from a contact

aligner in several ways. Primarily, a stepper contains an optical projection system

that can produce an image of the photomask onto the surface of the photoresist layer.

The image at the photoresist surface is typically a 1:4, 1:5, or 1:10 reduction i n the

size of the mask pattern and covers only a small area on the substrate. The lens sys-

tem in a stepper is complex and contains numerous large-size optical elements, only

a few of which are shown schematically in Fig. 9.12.

Fig. 9.12 Simpliﬁed

schematic drawing of optical

components in a stepper

(Reprinted with permission

from [33])

This lens system is designed to correct optical aberrations over a 30 mm circu-

lar ﬁeld of exposure, therefore, alignment is critical and processes are sensitive to

temperature, humidity, and vibrations. For this reason, the lens assembly is housed

in a massive, vibration isolation enclosure equipped with both precise temperature

and humidity control. The alignment of the photomask to the photoresist-coated

substrate is much more computer controlled for the stepper as compared to the

contact aligner. Also the position of the substrate holder is manipulated by high-

precision, computer-controlled micrometers. As can be seen in Fig. 9.10, a stepper is

682 D.R. Hines et al.

Fig. 9.13 Die layout on

exposed substrate

usually a large format machine. The entire substrate is processed by moving (step-

ping) the substrate to an unexposed area and repeating the exposure. Each exposure

of the mask is referred to as a die on the substrate. The stepping/exposing process is

repeated until the entire area of the substrate has been processed in a layout similar

to that illustrated in Fig. 9.13.

9.2.3 Photoresist

Photoresist typically consists of a mixture of organic polymers in a solvent com-

bined with photosensitive additives. Photoresist is designed to change solubility

due to exposure to UV light. The exposed area becomes either more soluble (pos-

itive resist) or less soluble (negative resist) upon exposure. The difference between

these two main types of photoresist chemistries (positive and negative photoresist)

is illustrated in Fig. 9.14.

Fig. 9.14 Patterning of positive and negative photoresist highlighting regions of insolubility that

remain after exposure and developing [32]

9 MEMS Lithography and Micromachining Techniques 683

Photoresist chemistry is designed to have an extremely high contrast; changes in

solubility are nonlinear and exhibit a threshold response resulting in sharply deﬁned

features with steep sidewalls even where small exposure intensity ﬂuctuations exist

within the photoresist layer. An example would be exposing photoresist to a sine

wave intensity pattern. The desired result would be a sawtooth pattern in the pho-

toresist layer rather than a sine wave pattern. In some cases, reﬂection of light from

the substrate surface can produce interference effects within the photoresist layer.

This is more an issue in situations utilizing steppers that employ higher intensity

light sources as compared to contact aligners.

A standing wave proﬁle as a function of photoresist thickness is shown in

Fig. 9.15 which results in a sidewall proﬁle as illustrated in Fig. 9.16. Such side-

wall roughness can be drastically reduced by performing a postexposure bake [34].

Also, antireﬂective coatings (ARC) are sometimes applied to the substrate surface

prior to the application of the photoresist layer [35].

Due to absorption affects, the top surface of the photoresist receives a larger

exposure dose than the bottom surface. This results in a slightly larger exposed

area at the top surface as compared to the bottom surface. This means that more

Fig. 9.15 Energy absorption

of UV light as a function of

depth in a 1 µm thick layer of

photoresist (Reprinted with

permission from [32])

Fig. 9.16 Example of reﬂective standing wave effect of photoresist sidewall proﬁle (a) without

and (b) with antireﬂective coating (Reprinted with permission from B.W. Smith, M. Hanratty:

Multilayer resist Technology, Chapter 10, In J.R. Sheats, B.W. Smith (Eds.): Microlithography

Science and Technology (Marcel Dekker, New York, NY, 1998))