28

Tool for Inspecting Masks: Lasertec MD 2500

Makoto Yonezawa and Takayoshi Matsuyama

CONTENTS

28.1 Introduction
28.2 Development Background
28.3 System Outlines and Features
28.4 Application
28.5 Technology

28.5.1 Optics
28.5.2 Stage System
28.5.3 Defect Detection Performance
28.5.4 Inspection Time
28.5.5 Usability
28.5.6 Autoloader
28.5.7 Clean Unit

28.6 Reliability
28.7 Safety
28.8 Conclusions
Reference

28.1

Introduction

Photomasks/reticles are used in the exposure process of semiconductor manufacturing,
where a circuit pattern is transferred on wafers with a stepper. Photomasks/reticles are
required to assure both defect-free quality and quick delivery. To meet these requirements,
Lasertec has developed a new photomask/reticle inspection system MD2500, the newest
model of this series. This chapter describes the outline and features of the new system.

28.2

Development Background

Masks should possess high quality and zero defects and support quick delivery syn-
chronizing to the progress of semiconductor manufacturing processes. Consequently, a
mask defect inspection system is required to have both high defect detection capability

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and performance fast enough to increase productivity, shortening delivery time in mask
manufacturing. In addition, the recent finer design rules of semiconductors have resulted
in complicated mask structure and higher-level requirement. As a result, requirements for
pattern defect inspection are getting severe year after year. As a solution for improvement
of sensitivity in pattern defect detection, some modification and development, including
employment of a shorter wavelength inspection light source, are in progress. Also, for the
purpose of improving yield, resist pattern inspection after development, which was not
performed earlier, is now increasing its importance.

There are other factors affecting productivity and delivery in mask manufacturing.

They include the system operating rates and contamination of masks by foreign particles
during mask manufacturing process. Considering these factors in the development of the
new system MD2500, Lasertec has successfully established highly stable operationality
and superb cleanliness of the inspection system.

Additionally, to satisfy various requirements for mask defect inspection systems,

Lasertec has considerably improved the designs of the optics, stage control mechanism,
and mask transfer system using the technical expertise accumulated through the constant
development of Lasertec MD series models.

28.3

System Outlines and Features

MD2500 is the newest model of the Lasertec photomask/reticle defect inspection system,
MD series. Figure 28.1 shows the external appearance of MD2500.

FIGURE 28.1
A view of the mask inspection system
MD2500.

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This new system is intended for photomasks/reticles used for 100-nm node devices. The

system provides a defect detection sensitivity of as high as 0.20 mm (after-development
inspection, ADI) and inspection time of 18 min (per 100 100 mm), which is less than one
fifth the time by Lasertec conventional systems (100 min). MD2500 supports two defect
inspection methods: (1) die-to-die inspection that compares adjacent chips, and (2) cell-
to-cell (cell shift) inspection that compares adjacent patterns (cells) of the same shape and
same size [1]. The specifications of the model are listed in Table 28.1.

The optics allow particle inspection on a pattern surface and resist pattern inspection

after development (ADI), which leads to improvement of quality and yield in mask
manufacturing. Also, the optics here consists of a line confocal imaging structure, result-
ing in the increased sensitivity and considerable damage reduction on resist at the same
time.

The system employs a clean unit satisfying cleanliness of Class 1. Also, the mask

transfer system of the autoloader is equipped with a multiarticular robot of cleanliness
Class 1, so that it can prevent particles from contaminating masks under inspection. The
autoloader, equipped as standard, operates a multiarticular robot for flexibly supporting
mask cases for steppers, in addition to SMIF pods and such.

Figure 28.2

shows the transfer

mechanism mounted on MD2500, a manual transfer attached on the left side and, an SMIF
pod on the right side.

28.4

Application

The system can provide three types of inspection: resist pattern inspection (ADI), etching
pattern inspection before resist stripping (after etch inspection, AEI), and finished mask
inspection after resist stripping. Thus, a single MD2500 can support pattern defect
inspection in each mask manufacturing process.

Since ADI allows resist pattern conditions to be inspected before etching, we can find a

fatal defect (killer defect) in mask patterns at an earlier stage. The ADI is especially

TABLE 28.1

Machine Specification of the Mask Inspection System MD2500

Item

Specification

Sensitivity

0.20 mm (ADI)

Minimum pattern size

0.375 mm

Image resolution (pixel size)

0.125 0.250 mm

Scan time

18 min (without autoloader handling time)

100 mm 100 mm (6-in. single mask)

Inspection method

Die-to-die/cell-to-cell (cell shift)/mask to mask

Mask type

Binary/alt-PSM/att-PSM (half-tone, tri- tone)/resist

Lens separation

31.0–304.8 mm

Objective lens working distance

7 mm

Macro view

Yes

X–Y stage stroke

314.8 (X) 314.8 (Y) mm

Stage accuracy

+

0.50 mm

Autoloader

Yes (built-in)

Cassette type

Two from SMIF/Canon/Nikon/manual loading table

Cleanliness

Class 1

Footprint

(W) 4000 (D) 3600 (H) 2400 mm (including maintenance area)

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effective for identifying those factors that affect quality and yield, which might not be
found in pattern inspection after etching and subsequent resist separation. In this respect,
ADI makes an important contribution to the improvement of mask quality.

28.5

Technology

28.5.1 Optics

MD2500 optics employ line confocal imaging, so that it can generate images of higher
resolution. This feature gives the system high sensitivity capability to inspect masks.

The reflectance inspection light source is an argon ion laser of 488-nm wavelength.

A reflected light should have such a wavelength that minimizes damage on resist during
resist pattern inspection on masks. At the same time, a wavelength should be short
enough to detect minute defects. As a result, a wavelength of 488 nm is regarded opti-
mum for reflected light at present, this wavelength value being derived from expertise on
Lasertec blanks inspection systems (MAGICS series). Reflected light is separated into 4000
beams by an acousto-optic deflector (AOD) and a diffraction grating, constituting high-
speed multibeam confocal optics (Figure 28.2). With these optical systems, the system
provides reduced amount of heat per unit area on a mask and considerably decreases
damage on resist (by about 30%) compared to the Lasertec conventional system
(MD2100).

The photodetector is equipped with Lasertec original time- delay-and-integration (TDI)

image sensor that has optimized sensitivity spectrum at ultraviolet (UV) wavelength
region, resulting in higher sensitivity of the system. The pixel size is 0.125 0.250 mm
after converting to a size on a mask. Both TDI image sensor and moving stage are used to

Beam expander

Laser

AOD

Relay lenses

TDI sensor

Photomask

Objective lens

PBS

Stage scan

Gratings

1/4

l

plate

Scan direction by AOD

4000 beams make
20 lines of laser illumination

FIGURE 28.2
Fast laser scanning method for line confocal imaging.

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perform line confocal imaging, where the image of a mask pattern can be obtained with
high resolution and high S/N ratio, resulting in the accomplishment of high defect
detection sensitivity of the MD2500.

MD2500 is equipped with two objective lenses, each at the right side and the left side.

The distance between two lenses can be varied between 31.0 and 304.8 mm.

28.5.2 Stage System

The inspection stage consists of an air slider driven by a linear motor. To realize high
accuracy location control, the inspection stage is controlled by a laser interferometer.
Thanks to these features, the inspection system realizes an overall location accuracy of
+

0.5 mm or less. The stage stroke is 314.8 314.8 mm.

The inspection stage is mounted on an active air vibration-free platform. This platform

functions as a vibration absorber against vibration from the floor where the inspection
system is installed, as well as the vibration from the autoloader or moving stage for
inspection, which contributes to the realization of high sensitivity.

28.5.3 Defect Detection Performance

MD2500 obtains mask pattern images with high resolution and high S/N ratio through
the usage of the image sensor whose sensitivity is optimized to UV light employed and
also the line confocal imaging optics. As a result, the system improves its defect detection
sensitivity to as high as 0.20 mm. Also, the system has the capability of inspecting masks
with pattern sizes of as small as 0.375 mm.

With regard to the cell shift defect inspection method, adjacent patterns of the same

shape and the same size are inspected on a single screen of one objective lens, which
minimizes the signal noise during comparative inspection. As a result, the cell shift defect
inspection method has a higher defect detection capability than the die-to-die inspection
method [1]. The cell shift inspection method is very effective for masks having repetitive
patterns.

28.5.4 Inspection Time

MD2500 completes the scanning over a 100 100-mm area in 18 min, which is less than
one-fifth the time of our conventional system (100 min). Such a short inspection time is
accomplished by the newly employed optics, the stage that moves smoothly with high
accuracy and defect detection circuits, which are increased in speed by 10 compared to
our conventional defect detection circuits, and processing the signal in parallel at a high
speed. As a result, this system has the capability of inspecting three masks per hour,
including mask transfer time.

Until now, users have never been satisfied with inspection time. In the course of the

MD2500 development, Lasertec has been realizing that the inspection time is as important
as defect detection capability, and has succeeded in attaining ultrahigh speed inspection
of as fast as 18 min.

28.5.5 Usability

Since the defect inspection system comes equipped with a low-scale macro view for
entire mask observation, an operator can easily find the current observation point by

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an objective lens focused on the mask, and move the mask on the stage to the
desired point easily. This macroview function improves operation efficiency in setup of
inspection conditions (inspection area) and workability for observation/verification
of defects.

Lasertec MD series systems provide capability of constantly displaying pattern images

captured by the right and left object lenses. Display color of the image that reflects the
area where light is bright portion of the mask is set initially to be red for one objective lens
and green for another, respectively. In a superimposed image of right and left lenses, the
bright portion without any defect is displayed in yellow, and a bright portion with a
defect is in either red or green. This color variation on the superimposed image enables
operator to easily locate the defect. Normally, a pattern is displayed on the screen at a
magnification ratio of 750 and can be expanded by up to 6, that is, at a magnification
ratio of 4500.

28.5.6 Autoloader

The transfer system of the autoloader employs a 6-axis polyarticular robot of cleanliness
Class 1, so that it can prevent particle contamination of a mask under inspection and
provide flexible handling, such as rotation, of various mask cases and masks.

The applicable mask cases are an SMIF pod, a stepper case (Nikon/Canon), and a

manual transfer stage. Users can select any two of them to mount on the system.

In the conventional autoloader supplied as optional accessory, rotation and inversion

of masks were handled by independent robots corresponding to each movement,
which resulted in complicated maintenance. MD2500, however, uses the polyarticular
robot to perform all kinds of tasks, which makes maintenance easier, and repair/adjust-
ment time shorter. At the same time, the number of handling times by the robot is
reduced, leading to a lower risk that the mask may be contaminated by foreign materials
or particles. Combined with various interlock sensors, MD2500 provides reliable mask
handling.

28.5.7 Clean Unit

The main unit of the inspection system and autoloader are installed in a clean chamber of
cleanliness Class 1. In the chamber, the airflow speed is adjustable with the maximum
airflow speed of 0.35 m/s, and sufficient positive pressure and ventilatory volume are
secured. The clean unit is also equipped with chemical filters.

28.6

Reliability

Since system uptime strongly affects productivity and delivery in mask manufacturing,
system downtime due to troubles or other causes should be minimized. Reliable system
operation is as important as system performance. MD2500 provides the mean time
between failures (MTBF) of 1500 h or more, mean time to repair (MTTR) of 4 h or less
(after arrival at the site), and scheduled downtime of 4 h or less per month. These figures
mean that MD2500 is a highly stable and highly reliable inspection system.

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28.7

Safety

With regard to safety, MD2500 conforms to SEMI S2-0200, SEMI S8-0701, and CE marking
as standard. Since the clean robot is installed together with the main unit of the inspection
system in the clean unit, the system can transfer masks in clean conditions while the
operator remains in a safe environment.

28.8

Conclusions

In addition to high sensitivity and high accuracy, user demands for inspection systems
include shortening of inspection time and more diversified inspections. As masks become
finer, more advanced technology is required to support those leading edge masks. To
satisfy user demands, Lasertec promises to continue research and development for next
generation inspection systems with such flexible responsiveness to the user’s need,
aiming at higher operationality, higher reliability, and more solid safety while improving
basic performance characteristics of the systems.

Reference

1. Y. Morikawa, et al., Performance of cell-shift defect inspection technique, Photomask and X-ray

Mask Technology IV, 1997, p. 404.

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