Photolithography

Course :IC Fabrication & VLSI

Course Code : ECL 3130
B.Tech 6th Sem ECE
Session : Jan-May 2022

Dr.Anil Kumar Bhardwaj

 Overview
• Introduction

Source: Fundamentals of Semiconductor Fabrication SM SZE

Source : Canon : Canon Technology | Canon Science Lab |
Semiconductor Lithography Equipment (global.canon)
 Introduction
• Lithography is the process by which circuit or device patterns are
transferred from layout to Si wafer.
• Several methods can be used to make circuit patterns on wafers.
• The most common process is to make the master photo mask using
electron beam exposure system and replicating its image by optical
printers.
 Introduction
• Lithography is the process of transferring patterns of geometric shapes
in a mask to a thin layer of radiation-sensitive material (called resist)
covering the surface of a semiconductor wafer.
• Figure 1 schematically the lithographic process employed in IC
fabrication. As shown in Figure 1(b), the radiation is transmitted
through the clear parts of the mask and makes the exposed
photoresist insoluble in the developer solution, thereby enabling the
direct transfer of the mask pattern onto the wafer.
• After the patterns are defined, an etching process is employed to
selectively remove masked portions of the underlying layer.
 Parameters of Lithography
• The performance of a lithographic exposure is determined by three
parameters: resolution, registration, and throughput.
• Resolution is defined to be the minimum feature dimension that can be
transferred with high fidelity to a resist film on a semiconductor wafer.
• Registration is a measure of how accurately patterns on successive masks can
be aligned or overlaid with respect to previously defined patterns on the
same wafer.
• Throughput is the number of wafers that can be exposed per hour for a
given mask level and is thus a measure of the efficiency of the lithographic
process.
(a) Lithographic process flow chart. (b) Optical replication process.
Various ways in which dust particles can interfere with photomask patterns.
 Optical Lithography
• The vast majority of lithographic equipment for IC fabrication is optical
equipment using ultraviolet light (λ = 0.2 μm to 0.4 μm) or deep
ultraviolet light.
• There are basically two optical exposure methods: contact printing and
proximity or projection printing.
 Shadow printing and Projection printing
• In shadow printing, the mask and wafer may be in direct contact, as in
contact printing, or in close proximity, as in proximity printing.
• Contact printing yields very high resolution (~ 1 μm), but suffers from
major drawback caused by dust particles or silicon specks accidentally
embedded into the mask, thereby causing permanent damage to the
mask and defects in the wafers.
• Proximity printing is not as prone to particle damage. However, the
small gap between the mask and wafer (typically 10 μm to 50 μm)
introduces optical diffraction at the feature edges on the photomasks
and the resolution is typically degraded to the 2 to 5 μm regime.
• The minimum line-width or critical dimension ( CD) that can be printed in
shadow printing is roughly given by:

• λ is the wavelength of the exposure radiation and g is the gap between

the mask and the wafer and includes the thickness of the resist.
 Shadow printing and Projection printing (contd..)

• In order to circumvent problems associated with shadow printing,

projection printing exposure tools have been developed to project an
image of the mask patterns onto a resist-coated wafer many
centimeters away from the mask.
• The small image area is scanned or stepped over the wafer to cover
the entire surface depicts the various ways to project and scan the
image.
• The resolution of a projection system is given by:
Equation (1)

• λ the exposure wavelength, k1 is the process-dependent factor, and

Projection printing is the most used technique in modern optical lithography
equipment. This method does not use contact to project patterns on a wafer’s
surface. Instead, it uses a large gap between the mask and the wafer. It
employs a well-designed objective lens that effectively collects diffracted light
and projects it onto the wafer's surface
 Printing(a) Annual-field wafer scan. (b) 1:1 step-and-repeat.
(c) M:1 reduction step-and-repeat. (d) M:1 reduction step-and-scan
 Shadow printing and Projection printing (contd..)

• The numerical aperture is given by where n is the index of refraction in

the image medium( usually air, n=1), and θ is the half-angle of the cone
of light converging to a point at the wafer

• Also shown is the depth of focus (DOF), which can be expressed as

Equation (2)

• where k2is the process-dependent factor

• Equation 1 indicates that resolution can be improved by either
reducing the wavelength or increasing NA or both.
• Equation 2 indicates that DOF degrades much more rapidly by
increasing NA than by decreasing λ
Depth of focus (DOF),
 Lithography (contd.)

• The high pressure mercury lamp is widely used in exposure tools

because of high intensity and reliability.
• The mercury arc spectrum is composed of several peaks.Slide 15
• The terms G-line, H-line and I-line refer to the peaks at 436 nm, 405
nm, and 365 nm, respectively.
• I-line lithography with 5:1 step and repeat projection can offer a
resolution of 0.3 um with resolution enhancement techniques.
• Advanced exposure tools such as the 248 nm lithographic system using
a KrF excimer laser, the 193 nm lithographic system using ArF excimer
laser, and 153 nm using F2 excimer laser have been developed for mass
production of 180nm, 100 nm and 70 nm respectively.
high-pressure mercury-arc lamp spectrum
 Masks
• Fabricated by e-beam direct write using a electronic database
generated by the CAD tools
• There are several substrate (transparent) types
Quartz, low expansion glass, sodalimeglass
• There are also several Opaque materials used to block light
Chrome, emulsion, iron oxide
• Often, a master is made on quartz; then the pattern is transferred to
less expensive L.E. glass where it is step and repeated to create several
dies
Two polarities of masks are common
• Light field, LF(mostly clear)
• Dark field, DF(mostly dark)
 Defects due to Masks
• Mask defects can be introduced during the manufacturing of masks or
during subsequent lithographic processes.
• Yield is defined as the ratio of good chips per wafer to the total
number of chips per wafer. As the first approximation, the yield, Y for a
given masking level can be expressed as,

• where D0 is the average number of “fatal” defects per unit area, and AC
is the defect sensitive area ( or “critical area”) of the IC chip.
• If D0 remains the same for all mask level, then the final yield becomes
Yield for a 10-mask lithographic process with various defect densities per level.
 Photoresist
• A photoresist is a radiation-sensitive compound.
• For positive resists, the exposed region becomes more soluble and
thus more readily removed in the developing process.
• The net result is that the patterns formed in the positive resist are the
same as those on the mask.
• For negative resists, the exposed regions become less soluble, and the
patterns engraved are the reverse of the mask patterns.
• A positive photoresist consists of three constituents: a photosensitive
compound, a base resin, and an organic solvent.
• Prior to exposure, the photosensitive compound is insoluble in the
developer solution.
• After irradiation, the photosensitive compound in the exposed pattern
areas absorbs energy, changes its chemical structure, and transforms
into a more soluble species.
• Upon developing, the exposed areas are expunged.
 Photoresist (contd)
• Negative photoresists are polymers combined with a photosensitive
compound.
• Following exposure, the photosensitive compound absorbs the
radiation energy and converts it into chemical energy to initiate a chain
reaction, thereby causing crosslinking of the polymer molecules.
• The cross-linked polymer has a higher molecular weight and becomes
insoluble in the developer solution.
• After development, the unexposed portions are removed.
• One major drawback of a negative photoresist is that the resist absorbs
developer solvent and swells, thus limiting the resolution of a negative
photoresist.
 Photoresist (contd)
• It exhibits a typical exposure response curve for a positive resist.
• The resist has a finite solubility in the developer solution even prior to
exposure.
• At a threshold energy, ET, the resist becomes completely soluble.
• Sensitivity of PR is defined as the energy required to produce complete
solubility in the exposed region.
• ET therefore corresponds to the sensitivity of the photoresist.
• Another parameter, ϒ is the contrast ratio and is given by:

• E1 is the energy obtained by drawing the tangent at ET to reach 100 % resist

thickness.
• A larger ϒ implies a more rapid dissolution of the resist with an incremental
increase of exposure energy and results in a sharper image.
• The image cross section depicted in Figure illustrates that the edges of the
resist image are generally blurred due to diffraction.
Exposure response curve and cross section of the resist image
after development for positive photoresist (left) and negative
photoresist (right).
 Photoresist (contd)
• Image cross section of Positive Photoresist illustrates the relationship
between the edges of a photomask image and corresponding edges of the
resist images after development.
• The edges of the resist image are generally not at the vertically projected
positions of the mask edges because of diffraction.
• The edge of the resist image corresponds to the position where the total
absorbed optical energy equals the threshold energy ET.
• Figure (b) illustrates the exposure response curve and image cross section for
a negative resist.
• The negative resist remains completely soluble in the developer solution for
exposure energies lower than ET .
• Above ET more of the resist film remains after development.
• AT exposure energies twice the threshold energy, the resist film becomes
essentially insoluble in the developer.
• The sensitivity of a NPR is defined as the energy required to retain 50% of the
original resist film thickness in the exposed region.
• Contrast ration is same except ET and E1 are interchanged.
Positive and Negative Photoresist
 Pattern Transfer
• illustrates the steps of transferring IC patterns from a mask to a wafer.
The wafer is placed in a clean room that typically is illuminated with
yellow light as photoresists are not sensitive to wavelengths greater
than 0.5 μm.
• For satisfactory adhesion of the resist, the surface must be changed
from hydrophilic to hydrophobic.
• For the adhesion promoter is required .
• Most common adhesion promoter is HMDS ( Hexamethyl disilazane)
• After the application of adhesion layer the wafer is held on a vacuum
spindle and 2 to 3 cm3 of liquidous resist applied to the centre of the
wafer.
• The wafer is then rapidly accelerated to a constant speed, which is
maintained for about 30 seconds.
• Spin speed ( 1000 to 10,000 rpm ) to coat a uniform film about 0.5 to
1um.
• The thickness of the photoresist is correlated with its viscosity.
Optical lithographic transfer process
 Pattern Transfer

• Step 1 Application of photoresist

• Step 2 Spinning of wafer to uniformly apply resist.
• Step 3 Soft Bake of wafer ( 60 to 120 C for 60-120 s)(90-120deg cel)
• Step 4 Wafer alignment to the mask
• Step 5 Development of the photoresist
• Step 6 Post Baking ( to increase the adhesion of the resist) (100 -180 C)
• Step 7 Etching of the exposed layer.
 Liftoff
• A related pattern transfer process is the liftoff technique.
• A Positive PR is used to form the resist pattern on the substrate.
• The film is deposited over the resist and the substrate. The film thickness
must be smaller than that of the resist.
• Those portions of the film on the resist removed by selectively dissolving the
resist layer in an appropriate liquid etchant so that the overlying film is lifted
off and removed.
• The liftoff technique is capable of high resolution and is used extensively for
discrete devices such as high power MESFETs.
• In ULS dry etching is preffered technique.
Lift-off process for pattern transfer.
 Resolution enhancement techniques
• Optical lithography has been continuously challenged to provide better
resolution, greater DOF and wider exposure latitude in IC processing.
• Can be resolved by reducing wavelength and developing new resists.
• Phase shifting Mask (PSM) and Optical proximity correction
• PSM
• OPC uses modified shapes of adjacent subresolution geometry to
improve imaging capability.
• Example, a square contact hole with dimensions near the resolution
limit will print nearly as a circle.
• Modifying the contact-hole pattern with additional geometry at the
corners will help to print a more accurate square hole.
 D=y/2(n-1)

diffraction

intensity

The principle of phase-shift technology. (a) Conventional technology.

(b) Phase-shifting technology.
 Next Generation Lithographic Techniques
• Electron beam lithography
• Extreme UV lithography
• X-ray lithography
• Ion beam lithography
Thank You