A Concept of Ultraviolet Lithography System and Design of its Rear

Part using Artificial Intelligence for Starting Design

Irina Livshits and Nenad Zoric

International Research Lab “InformationTechnologies in Optical Design & Testing”,

ITMO University, Birgevaya line, bld.14-16, Saint Petersburg, Russian Federation

Keywords: Lithography, Optical Design, Projection Lens, UV, Artificial Intelligence, Starting Point.

Abstract: This paper describes a concept for designing a projection lens in lithographic optical system for 365 nm.

Our approach for meeting this objective is to use the starting design obtained by artificial intelligence mode

in Synopsys software. The proposed method describes the steps of getting a desired starting point of the

optical system and the optimization problems in the optical system with a high numerical aperture.

1 INTRODUCTION

Optical lithography is a photographic process of

using an optical image and a photosensitive film to

produce the patterned silicon wafers in

semiconductor manufacturing. The industry of

integrated circuits mostly is using this technique in

manufacturing process. In figure 1 it can be seen one

type of projection optical lithography system. The

source of ultraviolet light is the laser which shines

through the illuminator, which expands,

homogenizes, and conditions the beam in the

condenser. Further, the light goes through a photo -

mask, and the projection lens to the wafer which is

coated with a photosensitive film (Rothschild,

2005). Many technologies have been proposed to

improve the process of the optical lithography, but

so far none has succeeded to replace lithographic

systems (Harriott, 2001).

The driving forces which are pushing

lithographic systems beyond the limits are

decreasing wavelength and increasing numerical

aperture, while the solution space is limited by

several conflicting constraints such as diffraction

limited performance, reasonable overall dimensions,

minimum number of optical elements, availability of

material, limits on the angles (Ulrich, 2000).

Lithographic objectives are famous by its high

quality, and by many challenges in optimization of

the projection optical system (Levinson, 2005).

Hereby, we proposed a simple concept in developing

of the ultraviolet (UV) lithographic optical system

which can simplify the work of the optical designer

at the early stage of design.

Figure 1: Schematic of the lithographic optical system.

We divide the total lithographic lens into two

parts:

 Condenser (front) lens with removed back exit

pupil, which could be understood as a reversed

lens with the removed forward entrance pupil;

Livshits, I. and Zoric, N.

A Concept of Ultraviolet Lithography System and Design of its Rear Part using Artiﬁcial Intelligence for Starting Design.

DOI: 10.5220/0005688500820086

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 84-88

ISBN: 978-989-758-174-8

 Projection (rear) lens with the removed forward

entrance pupil (Figure 1).

Figure 2: Schematic of the reference lithographic system.

Both parts can be designed separately as two

objectives with removed entrance pupil and

constraint of telecentric chief ray. An each part of

the lithographic optical system must be optimized as

good as possible in order to achieve diffraction

limited optical system when we connect designed

parts.

The desired specifications for proposed optical

system we used from a reference UV lithographic

system. Figure 2 shows our reference UV system: a

lithographic objective for 365 nm with aberrations

corrected up to diffraction-limit. The system is

defined by next characteristics : F number is 1.2, the

Gaussian image height is 9 mm, image distance is

22 mm and the magnification is - 0.2. The

spectrum range of lithographic lens is ultraviolet

(UV) with wavelengths: 362 nm, 365 nm and 368

nm; principle color is 365 nm. In our case we have

split the total lithographic system at the place of

aperture stop (APS) where the chief ray angle has a

minimum.

The starting point for a projection (rear) part of

the lithographic system was obtained by the artificial

intelligence (A.I.) mode in Synopsys software, and

optimized by the merit functions for transverse and

OPD aberrations.

2 STARTING DESIGN OF THE

PROJECTION LENS

In order to keep the specification of a total optical

system we have derived the specifications of the

projection part. Focal length 100 mm was chosen for

the projection part in order to keep the total length of

a reference system. We have calculated the field

angle using formula (1)

ὡ = Arctg (y’/f’) (1)

where ὡ is the chief ray angle, y’ is gaussian

image hight and f’ is a focal length of the projection

lens. The image distance we kept 22 mm the same as

in a reference litographic system.

Successfully choosing the starting system at the

early stages of development significantly shortens

the overall planning time (Livshits, 2007). In the

period of the wide applications of computers in

optical design, the speed of the ray tracing itself

increased thousand times, but the speed of new

schemes creation is not so fast, approximately 2-3

times. The main reason is hidden in the unsuccessful

starting point selection, If selected scheme doesn’t

have enough correction features, no computer can

add them without optical designer. In order to obtain

good starting point for projection lens we were using

artificial intelligence (A.I) mode in Synopsys

software.

Artificial intelligence capability is the expert

systems program within Synopsys software. A

general set of requirements may be input, and Macro

will find the 10 best designs that most closely match

them when the scale and aperture are adjusted as

well as possible. Applied algorithm is one that

employs a tree-structured logic wherein decisions

are derived from the responses of a number of

experts (their experience) in a particular field to a

lengthy debriefing (Dilworth, 2013).

DSearch macro (Design Search) is searching

through lens space in order to find an attractive

starting point. We give it the desired system

parameters and the number of elements we want,

along with some target quantities to define the

design goals. It constructs a series of candidate

lenses, with initial dimensions assigned according to

either a binary search scheme or randomly,

depending on user input.

Table 1: Specifications for the starting point of design.

Specification Value

Object distance Infinite

Object height Infinite

Marg. ray height 41.67 mm

F/Number 1.2

Chief ray angle 5.69 degrees

Focal length 100 mm

Gaussian image height 9 mm

Image distance 22 mm

The default option assigns element powers

according to the bit in a binary number that is

incremented at each cycle. Thus, if you request, a

four-element lens, the first lens would have all

negative elements taken from the binary number

(0000). The next try would have one positive

element, from the number 0001.

In Table 1 are presented the specifications for the

starting point of our projection lens. It is desired that

a bulge of the lithographic optical system possess

A Concept of Ultraviolet Lithography System and Design of its Rear Part using Artiﬁcial Intelligence for Starting Design

more positive lenses. In order to achieve this goal

we have been starting with the smaller system, with

shorter focal length and smaller marginal ray height.

In DSearch macro we have defined characteristics of

our system:

TIME

DSEARCH 1 QUIET

SYSTEM

OBB 0 5.69 5

UNI MM

WAVL .368 .365 .362

END

GOALS

ELEMENTS 7

FNUM 1.2

BACK 2.1

STOP FIRST

STOP FIXED

GLASS POS

O S-FSL5Y

GLASS NEG

O PBM2Y

END

SPECIAL

ACM 3 1 1

ACC 5 1 1

M 0 100 A P HH 1

END

Figure 3: Starting points obtained by DSearch macro.

Section OBB 0 5.69 5 defines field angle 5.69 and

marginal ray height 5. Next section WAVL .368

.365 .362 defines wavelengths; number of lenses

ELEMENTS 7; Fnumber FNUM 1.2; back focal

length, BACK 2.1. We demand aperture stop at the

first surface STOP FIRST, STOP FIXED; section

which defines minimum and maximum lens

thickness ACM 3 1 1, ACC 5 1 1; and special

section where we define constraint for telecentricity

SPECIAL M 0 1 A P HH 1. In this particular case,

estimated time is around 90 minutes, while DSearch

macro is searching for 10 best candidates for starting

design. We have defined in macro 7 elements

(lenses) in order to decrease number of lenses in our

design comparing it to the reference projection lens.

Obtained 10 starting designs for projection part are

shown in Figure 3. The positive lenses are marked

by green, while the negative lenses are marked by

yellow color. It can be seen that we reach our goal

to get more positive lenses in the system bulge. The

logical decision related to decreasing of the aperture

is to have more positive lenses in a bulge. By

comparing the transversal aberrations of all ten

candidates and number of positive lenses in a bulge

of the systems the seventh design search (DS7) was

chosen as the most appropriate.

Figure 4: Starting point for projection lens obtained by

DSearch macro.

DS 7 has one negative lens closer to the aperture

stop; it is shorter than others obtained starting points

with much better sagital transversal aberration in

range 0.005 mm. On another hand, we obtained the

optical system having the short focal length 9 mm

and thin lenses. In the next step we have scaled focal

length (f) to 100 mm. In this way we reached the

desired characteristics of the optical system shown

in Table 1. The chosen starting design DS7 is shown

in Figure 4, having 6 positive elements of total 7

elements. After the chosen lens DS7 is scaled to

focal length 100 mm, the transversal aberrations are

in range 0.05 mm.

3 OPTIMIZATION OF THE

PROJECTION LENS

The issue of optimization of the lithographic

objectives is probably the most difficult one in the

optical system optimization. Usually the modern UV

lithographic objectives have more than twenty

components having aspheric surfaces. That results in

more than one hundred optimization variables. The

PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology

most important constraints are the magnification,

total track and telecentricity. (Mack, 2006)

Considering all these issues, the optimization of the

projection lens should be easier than the

optimization of the total lithographic system because

of the less optimization variables and less number of

the lenses. However, the design of optical system

with high aperture is challenging work. Increasing

NA has meant increasing the acceptance angle of the

lens (Kawata, 1989). This process has had to

overcome significant challenges in optical design

and fabrication because the lens must be near

aberration-free and the image size must be kept

large, ~4 mm x 26 mm. Despite these difficulties,

the NA of projection systems has grown steadily,

from 0.5 in around 1990 to over 0.8 in 2004, with

plans to exceed 0.9 in the future.

Figure 5: Steps of the optimization in design of projection

lens.

Our referent lithographic system having NA 0,52,

for 365 nm, was chosen in order to be designed just

by the lenses, avoiding utilization of the mirrors

(Born, 1999). The starting design DS7 of

lithographic projection lens obtained in Synopsys

was optimized in Synopsys by using a merit function

for transversal and OPD aberrations as well in

Zemax by using a default merit function for the best

focus, RMS Spot Radius, Centroid RA 18x18 having

the constraints for a telecentric chief ray. Figure 5

shows the few steps in the process of optimization.

It can be seen that we have been keeping logic of

projection lens having a bulge and positive lenses

which decreasing numerical aperture.

In Table 2 are explained the few applied steps, in

Synopsys software, during process of the

optimization of the starting design. The final design

of the projection part and its MTF (modulation

transfer function) are shown in Figure 6.

Developed projection part is optimized close to

diffraction limit for central field angle (Figure 6.)

Longitudinal aberrations are in range 0.00112 mm,

while distortion has to be more decreased up to zero.

Total length of developed projection lens system is

310 mm what is one disadvantage compared to the

reference projection part of total system. It could be

improved in further work, where we intent to

develop the total lithographic lens by connecting a

projection part of system with the condenser part.

Table 2: Optimization steps of the projection lens.

Optimization

step

Description

1. step

Starting design from 0.05 transv.

aberration, aperture stop was

added, design was optimized with

default merit function, 3*6 full

grid, constraints 35 mm max

thickness, biggest weights for third

order aberrations

2. step

New merit function with additional

rays for wave front aberrations- full

grid, bigger weights for astigmatic

curve

3. step

Merit function with more

additional rays in field zone for

OPD (optical path difference)

4. step

Merit function for OPD, changing

weights for particular aberrations

in order to get better MTF on axis

and transverse aberration scale

Figure 6: Designed projection lithographic lens with its

MTF.

4 CONCLUSIONS

We have presented a method for designing

projection part of lithographic lens using artificial

intelligence for starting points which should to

simplify the early stage in design of the lithographic

optical system. By using Dsearch macro we showed

the way how to obtain desired starting design with

more positive lenses in a bulge of the lithographic

system. Starting design of projection lens was

A Concept of Ultraviolet Lithography System and Design of its Rear Part using Artiﬁcial Intelligence for Starting Design

optimized close to diffraction limited optical system,

and the main steps in applied optimization were

explained. In additional, designed projection part it

can be more optimized in order to reach diffraction

limited system. Proposed method can be easily

adopted for the starting design of any optical system

having more positive lenses. Further work will be

design of front (condenser) part using artificial

intelligence for starting design, and final design of

total lithographic optical system.

ACKNOWLEDGEMENTS

The research leading to these results has received

funding from the People Programme (Marie Curie

Actions) of the European Union's Seventh

Framework Programme (FP7/2007-2013) under

REA grant agreement no. PITNGA-2013-608082

‘ADOPSYS’.

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Livshits I. L., Sal'nikov A. V., and Cho U., 2007.

Choosing the starting system for designing objectives,

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Ulrich W., Beiersdoerfer S., Mann H.J., 2000. Trends in

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PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology