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A1 11 03087603

4- INUS SPECIAL PUBLICATION 400-46

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Semiconductor Measurement Technology.

Automated

Photomask Inspection

Semiconductor Measurement Technology:

Automated Photomask Inspection

Donald B. Novotny

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

and

Dino R. Ciarlo

Lawrence Livermore Laboratory

P. O. Box 808

Livermore, CA 94550

This activity was supported by

The Defense Advanced Research Projects Agency

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1978
Library of Congress Catalog Card Number: 78-606023

National Bureau of Standards Special Publication 400-46

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-46, 38 pages (Apr. 1978) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Stock No. 003-003-01912-7 Price $1.50

(Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

Page

Abstract 1

1. Introduction 2

2. Inspection for Visual Defects 4

2.1 Optical-Overlay Method 4

2.2 TV-Microscope Method 8

2.3 Spatial-Filtering Method 12

2.4 Dual-Beam, Flying-Spot-Scanner Method 13

3. Inspection for Dimensional Defects 14

3.1 Line-Edge Location 14

3.2 Criteria for Microdensitometer Operation 15

3.3 Applications to Mask Inspection 19

4. Experimental Evaluation of Automated Inspection Systems 21

4.1 Test Masks 21

4.2 Mask Inspection 21

5. Conclusions . 27

6. Acknowledgments 27

7. References 28

Appendix 30

iii
LIST OF FIGURES

Page

Figure 1. A schematic drawing illustrating the optical-overlay method 5

Figure 2. Convolution of two identical apertures in one dimension . 5

Figure 3. Illustration of the worst overlap case between the "diode"

apertures and the "pinhole" aperture 7

Figure 4. Block diagram of TV-microscope automatic mask inspection

system 9

Figure 5. Line-edge profiles for coherent radiation showing A, the

physical profile; B, the transmittance of edge A; C, D, and

E are transmittance profiles of edge A obtained by scanning

with coherent Gaussian light spots of 1-, 2-, and 3-pm diameters, respectively. The invariant point is located by T^, = 0.325 for the case where Tq = 0.02 14

Figure 6. Schematic of a microdensitometer-digital computer mask

inspection system 16

Figure 7. The 14 by 14 matrix of the defect mask showing the stepped

locations of the three different defect reticles indicated

by Roman numerals I, II, and III, respectively 22

Figure 8. The run-out errors between the left edges on adjacent dice

of row 12 of the defect mask as a function of column position .... 23

Figure 9. Illustration of the run-out errors from the scan measurements

of the line centers 24

iv
LIST OF TABLES

Page

Table 1. Characteristics of Automated Photomask Inspection Methods

for Visual Defects 3

Table 2. Values of Inspection Parameters for the Optical-Overlay

Method on a 76- by 76-mm Mask 9

Table 3. Critical Parameters for TV-Microscope Mask Inspection Systems with a 20- by 20-mm Image Tube Target 9

Table 4. Scan Times for Two State-of-the-Art TV-Microscope Mask

Inspection Systems 11

Table 5. Comparison of Total Edge Offset for a Line-Width Measurement When the Edges are Located at the Invariant Point and

the 0.5 Transmittance Point, Respectively 15

Table 6. Calculated Values of Lens Parameters Dependent on Numerical

Aperture, for Diffraction-Limited Optics at a Wavelength of

500 nm 16

Table 7. Calculated Maximum Spatial Frequencies for Linear Response

at 500 nm 18

Table 8. Intentional Defects on the Defective Die 22

v
Preface

This study was carried out at the Lawrence Livermore Laboratory and the National Bureau of

Standards as part of the Semiconductor Technology Program in the Electronic Technology Division at the National Bureau of Standards. The Semiconductor Technology Program serves to focus NBS research to enhance the performance, interchangeability , and reliability of discrete semiconductor devices and integrated circuits through improvements in measurement technology for use in specifying materials and devices in national and international commerce and

for use by industry in controlling device fabrication processes. The work was supported by

the Defense Advanced Research Projects Agency (ARPA Order 2397, Program Code 6D10) . The work

at Lawrence Livermore Laboratory was conducted under NBS Order Numbers 503183 and 602156.

vi
AUTOMATED PHOTOMASK INSPECTION

Donald B. Novotny

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

and

Dino R. Ciarlo

Lawrence Livermore Laboratory

Livermore, CA 94550

ABSTRACT

Methods that may be suitable for automated photomask inspection for

visual defects (spots, pinholes, etc.) or dimensional compliance are analyzed and discussed. The analysis of each method includes examinations of the physical principles upon which it is based and the amount

of misalignment that can be tolerated. The size of the minimum visual

defect to be detected was taken as 2 ym. The methods analyzed for visual defect inspection are the optical-overlay; the dual-beam, flyingspot-scanner; the TV-microscope; and the spatial-filtering methods.

For dimensional inspection, an analysis of line-edge location and operating criteria for the microdensitometer are presented. The fabrication

of photomasks with intentionally introduced and controlled defects is

described together with preliminary results of automated inspections of

these photomasks. It was concluded that: automated inspection systems

should be dedicated either to inspection for visual defects or dimensional compliance, not both; and the dimensional tolerances on masks,

both those of the feature dimensions in the die patterns as well as those in the dimensions between the dice, must be significantly smaller

than the size of the minimum defect to be detected.

Key Words: Alignment tolerance; automated photomask inspection;

dimensional inspection; flying-spot scanner; microdensitometer; optical

overlay; photomask; spatial filtering; TV-microscope; visual defect

inspection.

1
1. INTRODUCTION

Both visual and dimensional defects may occur

in photomasks. Visual defects are so named

because they are detected by visual observations with no dimensional measurements; they

consist of pinholes, spots, intrusions, protrusions, etc. in the die pattern geometry.

Dimensional defects are so named because they

are dimensional errors in the die pattern geometry.

Both types of defects cause yield losses in

the production of integrated circuits. Visual observations and measurements through a microscope are generally used to detect these

defects and thereby identify defective photomasks so they are not used in the fabrication

of integrated circuits. These nonautomated

inspection methods are fatiguing, of questionable accuracy, and provide neither 100-

percent inspection of the mask nor a 100-

percent probability of the detection of any

visual defect [1] . Automated inspection of

photomasks can in principle provide 100-

percent inspection and perform this inspection with a greater probability of defect detection in a much shorter time than nonautomated methods and thereby benefit the electronics industry.

A study of the electronics industry throughout the country [2] was conducted to determine the requirements for automated photomask

inspection systems, and to identify methods

or technologies that appear feasible for au- tomated inspection. From this study it was concluded that: for visual defect inspection,

a viable automated inspection system should

be able to detect defects that are 2 ym or larger in dimensions, and for dimensional inspection, pattern registration and critical

dimensions should be measured with uncertainties of a few tenths of a micrometer or less.

A further requirement was that 100-percent

mask inspection for either visual or dimensional defects on a 3- by 3-in. (76- by 76-mm)

photomask should take a maximum time of about

15 min.

Four methods appeared suitable for automated

photomask inspection for visual defects: (1)

the direct comparison of the image of one die

pattern with another on the same mask, which

is called the optical-overlay method; (2) the

comparison of a microscope die pattern image

which is digitized through the use of a TV

tube with information representing the defectfree pattern stored in a computer memory,

which is called the TV-microscope method; (3)

the identification of visual defects by the

optical spatial filtering of the image of the

die pattern in a manner that the defects are easily seen, which is called the spatialfiltering method; and (4) the direct comparison of die patterns by simultaneously scanning, with two small spots of light, two cor- responding positions on different die patterns, which is called the dual-beam, flyingspot-scanner method.

Microdensitometry was the only method that

appeared readily adaptable to automated dimensional inspection. Thus, criteria for the use of the microdensitometer for accurate

dimensional inspection are presented with a discussion of line-edge location. These criteria are also applicable to the opticaloverlay, TV-microscope, and dual-beam,

flying-spot-scanner methods if these methods

are used for dimensional inspection.

All of the analyses were performed with re- spect to an arbitrarily defined "standard"

photomask which is representative of the

masks now used in the electronics industry:

a 76- by 76-mm mask with 3.8- by 3.8-mm die

patterns forming a 20 by 20 matrix. The

size of the smallest defect to be detected

was taken as 2 \im. The calculated inspection times for the respective methods do not

include the loading and system adjustment

times. Table 1 summarizes the characteristics of these methods; the estimated equipment costs listed in this table are primarily based on estimates given by persons working with similar equipment and are intended

only to provide comparisons between the various methods.

In the last part of this investigation, test photomasks with intentionally introduced defects were made. These were intended for use

in determining, for a particular inspection

system, the minimum detectable defect sizes,

inspection speeds, missing defects, probabilities of defect detection, and the numbers of

false defects detected. These masks could

also serve as a means whereby anyone could

evaluate his own automated inspection system

for his particular application.

Such test masks could also be used on a regular basis to verify the performance of operating systems [3]. The results of inspections of these masks with some existing inspection systems indicated that automated inspection methods are limited by the precision

of the die step-and-repeat distance.

2
Table 1 — Characteristics of Automated Photomask Inspection Methods for Visual Defects

^ ^ -, Required Inspection Total ,^7 Estimated

, „ T

. _ ^ . Alignment Method Sampling Inspection ° , . Equipment

n <- -1 T- u or Indexing i 6 Rate, s

' Time, h:min . Cost, kS

Accuracy, um

Optical Overlay 3 0 X 106 0 5 <±1 150

TV-Microscope 1 2 X 106 50 <+0.4 130

Spatial Filtering NA '^0 7 NA 100

Dual-Beam, Flying-Spot

Scanner

3 5 X 106 0 7 <±0.6- 150

Microdensitometer 1 5 X 10^ -^26 4 NA 150

I

I

3
2. INSPECTION FOR VISUAL DEFECTS

2,1 Optical-Overlay Method

Pr-LncipZes of Operation — In this method two die patterns are viewed simultaneously

through two respective microscope objectives

as shown in figure 1. The fields of view of

the two objectives are illuminated continuously by transmitted light and the resultant

images are projected onto the respective linear photodiode array. The sample is scanned

in the x-direction by means of a mechanical

stage and the widths of the inspected swaths

are determined by the long dimensions of the

respective linear photodiode arrays which are each oriented in the y-direction, perpendicular to the stage scanning direction. The

swaths are raster scanned simultaneously by

serially interrogating each diode of each of

the arrays. In this manner pattern information from corresponding coordinate positions

on two die patterns is simultaneously read by

the two diode arrays. This information con- sists of each individual diode exposure which

is the total light energy incident on each

diode during the time between its successive

interrogat ions

.

The photodiodes of each array are interrogated by electronic circuitry. The capaci^

tance for each diode is charged to the video

line potential and left open-circuited until

the next interrogation. During this opencircuited period, the diode capacitance will

be discharged by an amount proportional to the diode exposure providing saturation exposure is not reached. Each time a diode is

sampled, its capacitance is recharged through

the video line. The resulting video signal

for each array is a train of charge pulses,

each having a magnitude proportional to the

exposure on a particular photodiode. This

signal from each device in one of the arrays

is subtracted from the signal of the corresponding diode on the other array. A nonzero difference signal is interpreted as a defect

since it arises from differences in transmission between corresponding locations on two die patterns.

Analysis — The analysis of this method re- quires the calculation of the exposure re- ceived by a diode when two apertures cross over each other as the stage scans. These

apertures are effectively formed on the sam- ple plane by a pinhole defect in the mask and

the projection of a diode through the objective. To calculate the scanning rate, typical photodiode array specifications are com- bined with the exposure calculation. This,

in turn, is used to calculate the inspection

time,, the required light intensity at the sam- ple plane, and the tolerance for the die-todie spacing. Implicit in this analysis is

that the microscope objectives have sufficient resolution to image the smallest defect

to be detected. This required resolution is

determined by the smallest dimension, 6, of

the defect to be detected, and is related to the minimum numerical aperture, NAjjjj^^^, of the

objective by

NA . = 1.2A/6 mm (1)

where X is the wavelength of the light. This

equation is based on the criterion that the

light intensity be zero between the nearest

two points to be resolved [4].

The first step in the calculation of the diode exposure is to calculate the area in common, or the overlap, between the pinhole and

diode apertures as they pass over each other.

In one dimension the overlap between two apertures, each q ym wide, as they pass over each

other, or the one-dimensional autoconvolution,i

C(C) is

C(C) =

S 0 < ? £ q

2q - C, q < C, < 2q, and

0 for all other values of c,.

(2)

where c is the distance between the leading

edge of one aperture of dimension q and the

trailing edge of the other aperture as shown

in figure 2. The autoconvolution function

is proportional to the instantaneous amount

of light incident on a diode. From eq (2)

and figure 2 it may be seen that C(c) is nonzero over a distance of 2q or twice the aperture dimension. This function may be applied in both the x- and y-directions to obtain the two-dimensional convolution of the

apertures. When the two-dimensional convolution is multiplied by the illuminating intensity at the sample plane I[W/cm2], the product is the rate of energy per square centimeter falling on the diode, E(x,y), as a function of the relative positions of the two apertures, or

E(x,y) = IC(x)C(y) (3) ,

The specifications of the typical linear photodiode array in this analysis are:

N = 512 = number of in-line photodiodes,

f = 2 X IqS Hz = photodiode scanning frequency ,
LINEAR PHOTODIODE ARRAYS

PHOTOMASK ON

SCANNING STAGE

L

SCANNED SWATHS

X-DIRECTION

Figure 1. A schematic drawing illustrating the optical-overlay method.

(The small segments in the photodiode arrays illustrate the individual

photodiodes which make up the arrays. Illumination is transmitted through

the mask into the objectives which focus the patterns on the arrays.)

0 2q

Illustration of the physical convolution;

one aperture, represented by the solid

lines, remains stationary while the other

aperture, represented by the dashed lines, moves across it.

The value of the one-dimensional overlap

or convolution C(?) as a function of z,.

Figure 2. Convolution of two identical apertures in one dimension.

5
= 25 ym = edge dimension of each photodiode , L = 13 ram = length of the array, and

Eg = 5.7 ijW's/cm^ = saturation exposure,

or the exposure necessary to com- pletely discharge the diode capacitance .

The optimum stage scanning velocity may now

be calculated by applying the results of the

aperture convolution, the dimensions of a single diode of the array, and the dimensions

of a pinhole of minimum detectable size. To use the photodiode most effectively, the image of the pinhole must exactly fill the diode. This choice of conditions is based on the following: if the minimum defect aperture "underfills" the diode aperture, a more

intense source is required for detection as will be shown later in this analysis; if the

minimum detectable defect aperture "overfills"

the diode, more than one diode is required

for the detection of a minimum size defect

and the utilization efficiency of the diode

is reduced. These conditions require that

the magnification of the objective lens be

M = Djj/q, which is equal to 12.5 for this

analysis

.

The scanning velocity of the stage is determined from the requirement that the stage ad- vance a distance 2q, the nonzero range of the

autoconvolution function, during the time between two successive interrogations of the

same photodiode, namely, t^ = N/f. This

gives a stage scan velocity v^ of

v^ = =2.34 cm/s (4)

where the subscript x denotes the scan direction. Since the width of each swath is given

by L/M, which is approximately 1 mm, 76 non- overlapping scans are required to completely

inspect the mask. Since each scan is 76 mm

long, a total mask inspection time of 4 min,

7 s is needed, plus the time required for

stage reversing and indexing. It is estimated that these additional functions can be

completed in less than 1 min to give a total

inspection time of about 5 min.

It must still be shown that it is possible to attain sufficient light intensity to detect

the minimum size defect. The required intensity at the sample plane is calculated from

the light energy required for a minimum exposure of a photodiode. A diode is exposed by

light energy passing through the "aperture

shutter" formed by the crossing of the two apertures as the stage scans. This energy is

incident on the diode for the duration that

this "shutter" remains open and constitutes

the diode exposure. This diode exposure must

be equal to or greater than the minimum energy, AUjj, which is detectable on the photodiode and is typically 1 percent of the diode

saturation exposure multiplied by the diode area A(j or

AU, = (O.Ol)E A, = 3.56 x IQ"'' yW-s . d s d

The diode exposure must exceed AUj when the

worst possible alignment between the two aper- tures is realized to ensure the detection of

the minimum size defect. The worst alignment

occurs, as shown in figure 3, when the pinhole aperture overlaps half of a diode aperture in the y-direction and the apertures are

in complete coincidence in the x-direction at the time of diode interrogation. Under these

conditions only half a convolution is com- pleted between interrogations and the diode

exposure lasts for a time of only N/2f. From

figure 3 it may be seen that any change in

the alignment in the y-direction for one diode will only serve to increase the overlap

between the pinhole and an adjacent in-line

diode of the array, and that any change in

the x-direction alignment will cause more

than half of the convolution to be completed

during the time between successive interrogations and increase the exposure of the diode.

The best alignment occurs when the ydimension of the pinhole and the diode aperture are in perfect coincidence and, in the

x-direction, the leading edge of one of the

apertures is just touching the opposite edge

of the other. Under these conditions, the

apertures will completely convolve between

interrogation times.

The diode exposure for the worst case is

mathematically written as the integral of

E(x,y) of eq (3), with x = v^t and C(y) = C(q/2), and expressed as

/i\!/2f

V tdt = E I (5) X m

U

where

E = q^N/Sf . m

This expression may be evaluated to obtain

the minimum intensity required for a detectable signal.

The upper limit of the illumination intensity

in the sample plane which will cause diode

saturation is realized when a completely

6
3q -

2q -

q -

q 2q

X-DIRECTION

a. Position of the "pinhole" aperture with

respect to the apertures formed by the inline photodiodes at the position of the

first interrogation.

q 2q 3q

X-DIRECTION

4q

Relative positions of the apertures

shown in (a) at the time of the second

interrogation.

Figure 3. Illustration of the worst overlap case between the "diode" apertures (solid lines)

and the "pinhole" aperture (dashed lines)

.

clear field instead of a pinhole is viewed

during the time between successive interrogations of a diode. This exposure is equal to the area of the aperture in the sample plane

formed by the projection of a single diode

through the objective, namely , multiplied

by the diode interrogation time N/f or, q^N/f

= SEjjj. To avoid diode saturation the maximum

intensity must be less than 100 AUdThis gives the following intensity range

100 AU.

c 8E

AU.

> I > (6)

When this expression is evaluated, the following intensity range is obtained:

5250 mW/cm^ > I > 420 mW/cm^ .

The minimum intensity is quite high in com- parison to the intensity received at the

earth's surface on a clear, bright day, which

is of the order of 100 mW/cm^. However, intensities up to 1 W/cm^ should be attainable

at the sample plane by focusing high intensity sources or a laser beam down to a spot

size slightly larger than the field of view

formed by the projection of the diode array

onto the sample plane.

The calculated minimum required Intensity is

equally valid for the detection of an opaque

spot of q by q dimensions in a transparent

field. The net signal difference between two charge pulses, each from a corresponding diode on each diode array, varies from zero when both diodes receive exactly the same ex- posure, to a maximum that occurs when one diode views a clear field and receives a maxi- mum exposure of SEjjiI while the other receives

zero exposure. The net signal obtained when

a transparent pinhole of minimum detectable

size is "viewed" by one diode while the cor- responding diode "views" an opaque field cor- responds to a net exposure of E^I. The analogous case for the opposite polarity occurs when one diode is exposed to an opaque spot

of minimum detectable size while the other is

exposed to a clear field; for this case the

net differential signal is again E^I, which

is equal to the exposure, 7Ejj,I, on the diode

"viewing" the defect, subtracted from the signal of the diode "viewing" the clear field.

The model used for obtaining the minimum expo- sure can be extended to determine the toler- ance of the die-to-die spacing between the

edges of corresponding geometries on different die patterns to avoid spurious defect detection. Values for this alignment tolerance are obtained by realizing that the total

misalignment must produce exposures less than

those produced by minimum detectable defects.

Such exposures will be obtained if the misalignments Ax and Ay of two edges in each of

these directions are no greater than q/2, or
Ax = Ay < ±q/2 = ±1 ym (7)

Misalignment can also result from a lack of

consistency in line widths on different die

patterns and yaws in the stage motion of the

inspecting stage. These tolerances represent the maximum tolerable misalignment; it

is estimated that the actual misalignment

tolerance should probably be less than 70

percent of these values to prevent copious

spurious defect detections.

From the above analysis it may be seen that

the inspection time, scan velocity, lens magnification, and acceptable intensity range

are functions of the dimensions of the mini- mum detectable defect and the interrogation

frequency of the photodiode. Table 2 summarizes these parameters for 2- by 2-pm and 4-

by 4-ym defects and interrogation frequencies

of 3 MHz and 1 MHz. The scanning times in

this table do not include either indexing

time or time for loading the mask into the

system and its subsequent alignment.

Although this inspection time is much less

than required, the method has a serious

shortcoming, namely, it is insensitive to any defect which is repeated on all die patterns since defects are detected by differences between die patterns.

Appliaation — Recently, an automatic mask inspection system has been reported [5] which,

despite the absence of a description of the

detection method, appears to use the opticaloverlay method to detect defects. In this

system compensation for misalignment is made

electronically through the use of computer

interrogated data. This system is also capable of being programmed to inspect only predesignated areas of the die patterns on the

mask which reduces its inspection time and

increases the probability that a detected defect will be fatal. The quoted inspection

rate is 2 in^/min (12.9 cm^/min) which implies that inspection of a 76- by 76-mm mask

can be done in about 4.5 min, which is in

good agreement with the calculated times.

2.2 TV-Microscope Method

Prinoiples of Operation - This method also

utilizes a microscope and digitization of

the image of the field of view. It consists

of a system which employs a TV camera tube

to view the array of patterns through a single microscope. The photomask is on a movable stage which is stepped across the field

of view of the microscope. The photomask

pattern information is imaged on the tube

target, digitized, and compared with data

that represent the correct pattern and is

stored in a computer memory. This technology

has its primary application in visual defect

detection. It may also be applied to registration inspection provided that the position

of the stage is known to sufficient accuracy.

For this application it is necessary to use either a linear encoder or a laser interferometrically controlled stage. A schematic diagram of the system is shown in figure 4.

Analysis — The required resolution of the microscope objective for this method is the same as that for the optical-overlay method

and is determined by the smallest dimension,

6, of the defect that is to be detected.

This resolution determines the minimum value

of the numerical aperture, NA^-j^j^, of the objective in accordance with eq (1):

NA . = 1.2A/6 min (1)

For 6 = 2 pm and A = 0.55 ym, a numerical

aperture of 0.33 or larger is required.

A microscope objective with the desired magnification, Mobj ' selected from a host

of commercially available objectives provided

the numerical aperture criterion is satisfied.

However, one must also consider the field of

view of the objective because it is this that

determines in part how fast the mask may be

inspected. An objective with a higher magnification produces a larger image at the ex- pense of reducing the size of the field of

view. To utilize the tube target efficiently,

the projected image must fill the target area which is usually square or rectangular. Consequently, the smaller of the target dimensions, e, the side, s, of the square inscribed in the circular field of view of the

microscope objective, and the total optical

magnification of the microscope, M^q^-, are related by

M^ ^ = e/s

tot (8)

The smallest image dimension that must be re- solved by the tube, Rj-, is the product of the

smallest defect dimension, 6, times the total

optical magnification:

R = 6M^ ^ r tot

(9)

The minimum number of scanned lines per millimeter, I, is the reciprocal of this product. Calculated values for comparison of

these parameters are given in table 3 for a 20- by 20-mm target and selected values of 6 and Mqi^-: . For these calculations \ was chosen as 0.55 ym, and the values of s were calculated assuming fields of view of typical

microscope objectives [4].
Table 2 — Values of Inspection Parameters for the Optical-Overlay

Method on a 76- by 76-inin Mask. (Specifications of the

diode array are given in the text.)

Defect ^ , Scanning Minimum

f, MHz M V , cm/s ,2 size, ym x time, mm: sec I, mW/cm^

2 X 2 3 12 5 2 34 4 07 420

4 X 4 3 6 25 4 68 1 02 105

2 X 2 1 12 5 0 78 12 21 140

4 X 4 1 6 25 1 56 3 05 35

TV Camera

1 icroscope

IC MasK

TV

monitor

Digital

computer

I

Teletypewriter

output

A

Figure 4. Block diagram of TV-microscope automatic mask inspection

system.

Table 3 — Critical Parameters for TV-Microscope Mask Inspection

Systems with a 20- by 20-mm Image Tube Targlet.

6 , ym NA . mm objs , mm M

tot

2 0.33 lOX 0.8 25X 50 20

.4 0.16 lOX 0.8 25X 100 10

2 0.33 5X 1.6 12. 5X 25 40

4 0.16 5X 1.6 12. 5X 50 20

9
The resolution of the TV tube and its modes

of operation are dependent on the tube design and construction. Both vidicon and image dissector tubes are compatible with this

technology and each offers different advantages .

The target of the vidicon tube is coated

with a photoconductive coating such as an- timony trisulfide or lead oxide. This target is given a uniform negative charge by

sweeping an electron beam across it; when a light image is projected on the target, the

resultant charge loss on the target coating

is proportional to the light intensity.

This charge pattern is retained on the target; a sweeping electron beam recharges the screen. The beam current necessary to re- charge an area of the screen and the coordinates of the recharged area provide data

that are digitized and read into a computer.

In this image data gathering process, the

charge pattern is erased. Before data on a new target image can be taken, it is necessary for the target to be uniformly charged.

Consequently, the entire target area must be

swept a second time by the electron beam before a new image is projected onto it. The

tube cycle time is the sum of the time necessary to read the charge pattern information and the time necessary to completely

erase the previous image. One property of

the vidicon is that the target charge pattern remains after the target illumination

ceases. Thus, stroboscopic techniques may

be used with this tube.

The image dissector tube target is coated

with a photocathode material, generally some combination of sodium, potassium, cesium,

and antimony. When a spot of light strikes

the target, electrons are released and are focused onto an anode plate containing an aperture. A deflection field is applied,

and photoelectrons from various areas are selectively made to pass through this aperture. The current passing through this aperture is amplified and recorded as a function

of the location of the emitting target area

by a computer. Unlike the vidicon, this tube

has no image storage capability and the image

disappears immediately when the target illumination ceases; consequently, stroboscopic

techniques cannot be used with this tube.

This tube offers the flexibility of randomaccess scanning that can be interfaced with

a computer programmed to recognize particular

features in a scan and automatically analyze

these features in greater detail [6] . Electronic zoom is also possible with this tube

by changing the scan amplitude or frequency.

Image dissector tubes are available with a matrix of 2000 by 2000 resolvable points that

are scanned at the rate of 0.5 microseconds

per point or 2 seconds per complete target

scan. These tubes are not used in conventional raster-scanned TV applications because of

their illumination requirements which are 10^

to 10^ times greater than vidicons.

The rate at which the image information can be read from these targets is a function of

the time required to scan the target. The

standard (U.S.) television picture contains

a nominal 525 lines which are scanned 30

times a second. The advantage of using this scan rate is the economics of system construction; there is a myriad of commercial electronic equipment that is designed to operate

at this scanning rate. The use of a different scanning rate or a different number of

scanning lines is possible, but may involve

the design and construction of special equipment.

The resoliition of the TV tube is a function

of the target coating, the spot size of the

scanning beam for a vidicon or aperture size

for image dissector, and the number of scan

lines. The size of the scanning spot or aperture is a major factor in the ability to read the detail in the target image. Any de- | feet smaller than the spot is read as if it were spread over the same area as the spot, ' with correspondingly less intensity. If two defects in the target image are separated by

a distance of one spot diameter or less, the , tube signal may not drop below the threshold

between the spots and they may not be recognized as separate.

A criterion for defect detection is that the

spacing between successively scanned lines be

equal to or smaller than the tube image of

the smallest defect to be detected. This

spacing can be equated to the spot size. i Since the entire target area is scanned in

525 lines, the spot size or tube resolution,

R^, is given by

Rj. = H/525 , (10) ,

where H is the dimension of the target in the

direction perpendicular to the scan. The val-^ ue of Rj- must be equal to or less than Rj-.

The value of Rj- may be used to estimate the

misalignment tolerance, D. A straight edge

parallel to the tube scanning direction will

be interpreted as a defect if its image appears at a distance perpendicular to the scan- ning direction that is greater than Rt/2 from

where it is programmed to appear. Similarly,

a misalignment of an edge perpendicular to the scan direction that is misaligned by more

10
than Rj-/2 in the scan direction will also be

interpreted as a defect if the tube resolution in each direction is the same. This image misalignment is converted to a feature

misalignment tolerance on the photomask

through the optical magnification of the microscope to give,

D < R /2M

t tot (11)

Scanning the mask on a stage through successive fields of view under a microscope is ac- complished most rapidly by accelerating the

stage through a distance approximately one

half the edge dimension of the field of view

and decelerating the stage through the re- maining distance to the next field of view

so that it stops precisely positioned. The

stage may be designed so that the natural

frequency of the table moving as a rigid

body on its ways as a result of elastic deformation is tuned to the applied torque of

the motor and time of traverse [7]. When

this tuning is accomplished, the stage vibration is negligible when the table stops

[7,8].

The time required to inspect the mask is the

complete cycle time multiplied by the number,

N, of fields of view. The complete cycle

time is the sum of the time it takes to index to the next field of view plus the tube

cycle time, if these steps are done sequentially. A comparison is given in table 4 of

the times required to scan a 76- by 76-mm

field with a vldicon tube with a 10- by 10- mm target area and with an image dissector

tube with a 25- by 25-mm target area. In making the calculations it was assumed that

the optical magnification was 25X, that the

stage acceleration was 4.9 m/s^, and that

the distances for acceleration and deceleration were equal.

The shortest inspection time shown in table 4 is long compared to the time of about 15 min

which is typically allowed for an inspector

to inspect the same mask [2], However, it

should be emphasized that an inspector only

partially inspects the mask and relies on sampling statistics for quality control. The

automated inspection system does a 100-

percent pattern inspection on all of the ar- rays in the mask.

For this system to operate using either a vidicon or an image dissector tube, the signal change on going from a transparent to opaque area must be large enough that an elec

tronic threshold setting can distinguish between them. This places restrictions on both

the signal-to-noise ratio of the TV tube and on the resultant modulation transfer function

(MTF) of the microscope and TV tube. If the

resultant MTF is too low, the system cannot

distinguish between the clear and opaque areas and an erroneously low defect count may

result. If the signal-to-noise ratio is not

sufficiently low, an erroneously high defect

count may result.

Applications — Equipment has been constructed

using this technology and a vidicon tube ca- pable of counting pinholes and spots in masks

which have device geometries with 2.5-Mm line

widths [9,10]. The combined optical and elec

tronic magnification is 250X with a lOX objective. The pinhole and spot counter operates

with a fixed field of view which requires

that the stage holding the mask be indexed in

fixed increments. The equipment is capable

of inspecting masks for wafer sizes up to 2 in. (5 cm) in diameter. In operation, defects are counted as many times as they are intercepted by the TV camera scan line. The

size of the smallest defect counted is a function of the smallest mask geometry. The electronics were designed so that defects are counted which are approximately 80 percent or more of the size of the smallest line width

on the mask. A complete indexing cycle occurs every 8/30 s. The indexing cycle consists of 1/30 s for information reading, 3/30

s for indexing, and 4/30 s for complete tube erasure

.

With such a system, a 100-percent inspection

of a 76- by 76-mm mask with a field of view

of 0.8 by 0.8 mm would require about 39 min.

This time could be shortened to about 13 min

by using stroboscopic "flash on the fly" illuminations of 10 ys for every field of view

in conjunction with a stage velocity of 8 mm/s . This velocity permits a 1/30-s scan

Table 4 — Scan Times for Two State-of-the-Art TV-Microscope Mask Inspection Systems.

Tube s , mm

Scan

t ime , ms Indext ime , ms Eraset ime , ms Total Cyclet ime , ms

N Inspection

time, min

Vidicon 0.4 33 18 33 84 35000 49

Image

Dissector 1.0 2000 30 0 2030 5600 190

11
for information and an additional two 1/30-s

scans for every erasure. This 13-inin inspection time includes about 1 min for stage re- versals and indexing. These modifications

are within the current state-of-the-art using vidicons with more sensitive target coatings and stages that can operate and index

to the required accuracy [8].

The speed of the system using an image dissector tube can also be improved. In principle, it is possible to construct a dissector

tube using a 4 by 4 matrix of apertures, although construction of such a tube would be

very expensive. This tube would scan a single field of view in 1/16 of the time now required. This would decrease the total inspection time per mask to less than 12 min.

The inspection time could be reduced further

by exploiting the random scanning capability

and the programmable features of the image

dissector tube to examine only the critical

portions of the patterns on the mask. These

features also would enable detailed examination of defects as they are scanned which is

necessary for defect identification, but this

is done by sacrificing inspection speed.

These operations are normally not possible

with a vidicon.

2.3 Spatial-Filtering Method

Prinaiples of Operation — This method [11] is

based on the lens property that for coherent

light the Fraunhofer diffraction pattern of

the object, which is represented by the Fourier transform of the object, is produced at

the focal plane of the radiation source. Therefore, if a photomask is placed in front

of a lens and illuminated by a collimated

laser beam, the Fourier pattern of the object

is produced behind the lens at the transform

plane located by the focal length [12]. A

second lens may be used to produce a diffraction pattern of this Fourier transform and

thereby perform an inverse transform recon- structing the original image. Another lens

property exhibited by this arrangement is

that the diffraction pattern is invariant to translating the mask in the object plane,

provided that the rotational orientation and

focal position of the mask are maintained.

The simplest means of applying these physical properties to mask inspection [13,14] is

to construct a spatial filter which is inserted at the Fourier transform plane. If

this filter only blocks the diffraction pattern produced by the periodic die arrays,

any diffraction pattern produced by nonperiodic defects will pass through the filter

and the reconstructed image will consist of

only the defects.

Analysis — This method is substantially different from the previous methods in that each

die is inspected in toto instead of being

scanned. The inspection of each die is manually made by inspecting the filtered and re- constructed image of each die showing only

its defects. Each such inspection is made

for all of the 196 respective dice. A mask

inspection time of about 7 min is obtained

if it is assumed that about 2 s is required

to inspect each die.

Spatial-filtering techniques are sensitive to surface and thickness irregularities in the

glass photomask substrate as well as to changes in the refractive index of the glass. An appreciation for this sensitivity may be

obtained by considering the number of radiation wavelengths in the glass given by ^\ =

5 n/A where X is the wavelength of radiation

in a vacuum, 5 is the thickness of the glass,

and n is the refractive index of the glass.

The variation of the number of wavelengths in

the glass AN;^ is given by

AN, = [CAn + nA5]/A . A

Assuming that n = 1.5, 5 = 60 mils (1524 ym) , and A = 0.5 pm, it is easy to show that either pair of values An = 1.6 x lO"'*, A£, = 0 or An = 0, Ag = 0.17 ym will change AN^ by

1/2 shifting the phase of the emerging wavefront by 180°C. This, in turn, causes un- wanted destructive interference in the recon- structed wavefront precluding the expected interference effects. Although the surface irregularity problems can be alleviated through

the use of a liquid gate, namely, creating an even surface by covering the surface of the

substrate with a liquid film of matching re- fractive index, this creates contamination

and cleaning problems which may be more se- vere than the original defect problems. In addition, it is both difficult and time con- suming to make adequate filters [2]. These

techniques are further hampered by focusing

problems and difficulties encountered in registering the filter with the transformed image.

Apptioations — Watkins [15] modified this

technique by using two apertures to restrict

the illuminated mask surface; the resultant

diffraction pattern is further filtered by a grating prior to reconstruction of the image.

By choosing the aperture separation to be a multiple of the step-and-repeat distance,

the Fourier transforms from two die patterns

can be made to subtract from each other, re- sulting in an image consisting only of the

diffraction patterns produced by random defects and step-and-repeat errors. This tech12
nique is also independent of mask translations in the object plane, but is dependent

on mask and die pattern rotations. It is es- timated [15] that this technique is sensitive to defects of 5-Mm diameter and stepand-repeat errors of 3.8 pm.

A variation of the above techniques, which

is capable of detecting irregularly shaped

defects as small as 2.5 pm, has been described by Axelrod [16]. In this technique

the diffraction pattern is filtered with a filter approximating the form factor or en- velope function for the intensity in the

Fourier transform plane. For a mask with

rectangular features, the width of the diffraction pattern in the transform plane is

determined by the narrowest features. The

diffraction pattern from an illuminated circuit pattern with rectangular geometry will

have the overall appearance of a tapered

cross due to the form factor from the perpendicular lines within one die pattern.

The diffraction pattern from a defect will

extend over a much broader area because of

either the small size of the defect or the

jagged nature of the edges of larger defects.

Thus, a filter consisting of a cross simulating the form factor placed in the transform

plane will suppress regular features and display defects in the reconstructed image.

This technique does not require as stringent registration between the filter with

the transformed image as those techniques

previously described.

A mask inspecting device utilizing the method of Axelrod [16] has been described by

Minami and Sekizawa [17]. This device incorporates a green incoherent light which superimposes the image of the mask pattern over

the image of the reconstructed defects.

This superposition of images enables the

operator to distinguish spots in the reconstructed image produced by the corners of

the mask pattern from defects which are circular in character. The resultant image is

viewed through a microscope and enables the

operator to identify the respective locations of defects in the die pattern. The

analysis of this device [17] indicates that

the translational setting tolerance of the

filter is in excess of 10 mils (0.25 mm) and

that defects as small as O.S-ym diameter

should be detectable. Focusing requirements

are not analyzed.

2.4 Dual-Beam, Flying-Spot-Scanner Method

Pvinoiples of Operation — In this method two spots of light produced by focusing a laser

beam are simultaneously scanned over corre- sponding portions of two die patterns on the same photomask. Differences between the

transmittance of these two light spots are interpreted as defects.

Analysis — A published analysis of this method was based on the detection of a square 2- by 2-ym defect and assumed a Gaussian distribution of the intensities of the scanning

spots [3]. A spot of 3-vim diameter was determined to be sufficient for the detection

of 2-pm defects. The spot edge was arbitrarily defined at the point where the intensity

dropped to a value of 1/e^ of the maximum intensity. The minimum misalignment D, in micrometers, that could be tolerated between

two die patterns and give an adequate signalto-noise ratio without giving rise to the detection' of spurious defects was derived as

D 0.4 W , (12) o

where Wq is the radius of the scanning spot

in micrometers. For a 3-vim spot (Wq = 1.5

ym) this relation predicts a permissible misalignment of 0.6 pm which is comparable to the optical-overlay method. The time re- quired to scan a 50- by 50-mm photomask was

reported to be about 3 min which scales to about 7 min for a 76- by 76-mm mask.

Applications — This system [3] is able to recognize the differences between surface defects on the mask, such as dust, and intrinsic defects. This is done by placing four

small fiber optic bundles in four quadrants

at 45 deg to the principle axes of the mask

and 60 deg to the optical axis of the reduction lens. The four bundles are brought to- gether at a second photodetector ; the output

of this detector is registered if it exceeds

a certain threshold and identifies the defects as surface defects. This technique of

surface defect detection should also be adaptable to the optical-overlay method.

The method can also be used for line-width

measurements through the operation of the

system in a "microdensitometer " mode [3].

This is done in a single spot-scan mode and

the transmittance profile of each of the two opposite edges of a feature, such as a line, are recorded as a function of position. In this system, the edges are located at the

0.5 transmittance points. Measurement errors resulting from the use of this criterion for

edge location are discussed in section 3.

13
3. INSPECTION FOR DIMENSIONAL DEFECTS

Dimensional inspection is the measurement of

photomask die pattern feature dimensions.

These measurements are of two types, referred

to as line-width measurements and run-out measurements. The accuracy of each of these measurements is dependent upon the location ac- curacy of the line or feature edge. The mi-- crodensitometer is designed to measure the

transmittance of a small area as a function

of position and is well suited to dimensional measurements. The application of the

optical-overlay, TV-microscope, and dualbeam laser-scanning methods to dii.iensiona].

inspection essentially consists of operating

any of these respective inspection systems

as a microdensitometer . Because dimensional

inspection is a potentially important application of the microdensitometer, the principles which govern the operation of a microdensitometer in measuring accurate data are reviewed. Criteria for line-edge location

are also presented; these differ according

to whether the microdensitometer is operating with coherent or incoherent illumination.

Since the operation of a dual-beam laserscanning system, with coherent illumination,

was briefly mentioned in section 2.4, lineedge location using coherent illumination is

first discussed.

3.1 Line-Edge Location

A model of a line edge with a sloping (nonideal) physical edge and transmittance profiles produced by scanning coherent light

spots of 1-, 2-, and 3-Mm diameter across

the line edge is used to illustrate the

principles involved in choosing the edge location. Details of this model and associated calculations are given in the appendix.

The physical profile of the line edge is similar tjD those observed for chromium masks in

the NBS line-width measurement program [18];

the edge is inclined at an angle of 55 deg

from the horizontal until the full film

thickness is reached. Gaussian functions were used to represent the intensity distribution of the light spots instead of the actual one-dimensional impulse response functions because mathematically they are much

easier to work with. The calculated transmittance profiles in figure 5 show that an invariant point, i.e., a point that is com- mon to all transmittance profiles regardless

of the scanning spot size, exists at the

transmittance T^ determined from the relat ion

0.25(1 + /T yo (13)

DISTANCE [fim]

Figure 5. Line-edge profiles for coherent radiation showing: A, the physical profile; B, the transmittance of edge A; C, D, and E are transmittance profiles of edge A obtained by

scanning with coherent Gaussian light spots of 1-, 2-, and 3-ym diameters, respectively.

The invariant point is located by T^, = 0.325 for the case where Tq = 0.02.

14
Table 3 — Comparison of Total Edge Offset for

a Line-Width Measurement When the

Edges are Located at the Invariant

Point and the 0.5 Transmittance

Point, Respectively.

Spot Diameter,

ym

Total Line

Offset,

ym

0.24

0.44

0.66

where Tq is the transmittance of the full

thickness of the film. This relation was

first derived by Nyyssonen [19] for ideal

(vertical) physical line edges and is seen

to be applicable to this model with a nonideal edge. From figure 5, measures of the

offset between the invariant point and the

edge positions located by the 0.5 transmittance point are obtained. These offsets

multiplied by 2 represent the total offsets

from the invariant point that would be obtained in a line-width measurement based on

the 0.5 transmittance point. These total

offsets are a function of the spot size used

and are listed for various spot sizes in ta- ble 5. These offsets are representative of

those that may be expected between linewidth measurements made by locating the line

edge at Tq given by eq (13) and measurements

made by locating the line edges at the 0.5

transmittance point a.s described in section

2.4 for the coherent dual-beam, flying-spot

scanner [3]. It may also be seen from figure 5 that a difference of 0.4 ym would result between measurements of the same line

width made on the same microdensitometer

using coherent spot sizes of 1- and 3-ym diameter and locating the edge at the 0.5

transmittance point.

This model does not apply directly to "see through" masks since it does not account for

edge ringing that is observed or other phase

effects that occur with these masks; such effects have been described elsewhere [19,20].

A similar model and calculations can be used

to calculate transmittance profiles for incoherent illumination. The calculated profiles are similar to the coherent profiles

except that the invariant point is located

at a transmittance given by 0.5 (1 + Tq) .

3.2 Criteria for Microdensitometer Operation

The microdensitometer is an instrument that

is designed to measure the optical density

of, or the transmittance through, small areas as a function of position. Figure 6 shows a schematic diagram of a mask inspection system consisting of a microdensitometer

interfaced with a digital computer. An area

of the photomask is illuminated by the system influx optics consisting of a source, a source condenser, a source aperture, an influx ocular, and an influx objective. The

light transmitted through the mask is collected and measured by the system of efflux

optics. The amount of light entering through

the sensor aperture is measured by a photomul

tiplier. The photomultiplier output data are digitized and recorded in a computer memory

along with the corresponding positional infor

mat ion.

The microdensitometer can be used in either

an underfulling or an overfilling mode and

operated in either a sample scanning or an im

age scanning configuration. The operational

mode depends upon the relative selection of

the numerical apertures of the influx objectives, NAj, and the efflux objective, NAg;

overfilling occurs when NAj > NAg, while un- derfilling occurs in the case where NAj < NAg

The sample scanning configuration is achieved

when the source aperture is imaged on the sam

pie and the efflux optics merely serve to gather the transmitted light; the size of the

scanning light spot is smaller than the geometrical projection of the sensor aperture

through the efflux optics onto the sample

plane. In this sample scanning configuration

the resolution is determined by the minimum

or smallest resolvable spot, which is essentially the width, W, of the impulse response

of the lens and is given by the relation

W A/NA (14)

where X is the wavelength of light.

The image scanning configuration is realized

when the illuminating spot on the sample

plane is larger than the geometrical projection of the sensor aperture through the lens

onto the sample plane. In the image scanning

configuration the resolution is limited by

the resolution of the efflux objective.

The following discussion assumes diffractionlimited, aberration-free optics with no defocus . Table 6 summarizes the minimum image

width, depth of focus, and resolution limit

in cycles/mm as a function of NA for light of

500-nm wavelength.

It must be stressed that the microdensitometer must be operated in a manner such that

analytically representative mapping of the

optical density is obtained. The ability to 15
SENSOR

APER

/

URE SENSOR

(Photomulliplier]

SENSOR

CONDENSER

EFFLUX OCULAR

^

EFFLUX OBJECTIVE

XY SCANNING STAGE L_l A

INFLUX OBJECTIVE

, SAMPLE

fa*-

INFLUX OCULAR

D

SOURCE

CONDENSER

SOURCE

APERTURE

SOURCE

DIGITAL

COMPUTER

T

TELETYPEWRITER

OUTPUT

Figure 6. Schematic of a microdensitometer-digital computer mask inspection system.

Table 6 — Calculated Values of Lens Parameters Dependent

on Numerical Aperture, for Diffraction-Limited

Optics at a Wavelength of 500 nm [21]

.

NA

Minimum Image

Width,

pm

Depth of Focus,

ym

Resolution

Limit

,

cycles/mm

0.08 6. 2 39 320

0.10 5 25 400

0.16 3. 1 9.8 640

0.25 2 4 1000

0,30 1. 7 2.8 1200

0.40 1. 25 1.5 1500

0.50 1 1 2000

0.65 0 8 0.6 2600

0.85 0 6 0.35 3500

1.00 0 5 0.25 5000

16
fully characterize the data obtained from a microdensitometer depends on its operation

in either a coherent light regime or an incoherent light regime.

A microdensitometer system ' designed and built

for the accurate measurement of line widths

less than 10 \im has recently been described

[19] . This system operates in an underfilled

mode with NA^^/NAg = 0.6/0.9, uses filtered co- herent light of 530-nm wavelength, scans at a very slow constant speed of a few micrometers

per second, has a scanning stage translational accuracy of 0.01 ym, locates the line edge

at a transmittance given by 0.25(1 + /t^) ^

,

and is capable of line-width measurements

with a three-sigma standard deviation less

than 0.05 ym [18]. This instrument derives

its accuracy from the stage translational accuracy and the high degree of analytical characterization of the data.

Incoherent illumination is more commonly used

for microdensitometer operation. In the incoherent regime, analytical characterization

of the data depends on linear microdensitometer operation. When a microdensitometer is

operating linearly, the line-edge location or invariant point is a function of the background transmittance Tq and is located at a transmittance threshold of T^ = 0.5(1 + Tq) . Swing [21] has derived criteria for the linear operation of a microdensitometer in the

incoherent light regime. Compliance with

these criteria assures that the microdensitometer response is characterized by the linear relation

1(a) = I^(a)T(a)

where

:

1(a) = the spectrum of the image intensity as a function of the spatial

frequency a, I^(a) = the spectrum of the objective intensity, and

T(a) = the optical transfer function of

the system.

The criteria derived by Swing are:

NA

overfilling [21]: ^^1 + ^ , and (15)

t o

NA^ n underfilling [22]: ~ > 1 + — NA^ — K

I o (16)

where Oq and k„ are the resolution limits of

the efflux and influx objectives in terms of

a maximum spatial frequency and are equal to 2NAe/A and 2NAi/A, respectively, and rio is

the upper frequency limit in the sample. The

parameter, rio> is unambiguously defined only

for a sample consisting of a grating with an intensity profile represented by a sine function of frequency, no. This situation does

not occur when the scanned sample is a photomask. Photomasks consist of functional patterns containing line edges with extremely

acute optical density gradients. Such edges

may be thought of as being composed of a superposition of intensity profiles represented by sine functions of different spatial

frequencies, a. The collective group of all

of these spatial frequencies makes up the image spectrum 1(a). The sharper the line edge,

or more acute the density gradient, the more

high frequency components are present in the

spectrum. Consequently, the greater the limiting frequency, rio» that a microdensitometer

responds to linearly, the more accurate is

the measurement of the optical density edge

gradient and the more precise is the edge location.

An image that accurately represents the edge

profile must be measured by the microdensitometer to locate line edges on photomasks

with the precision and accuracy required in

the microelectronics industry. There is little reason to expect that an accurate representation of an edge profile will be measured

if the optics are chosen so that either the

ratio of NAj/NAg for the overfilling case, or the ratio of NAg/NAj for the underfilling

case has a low value. This is because the

nonlinear measurement of the high frequency

components of the edge contribute to the edge

image profile in a way that can unpredictably

distort the image. If this profile is not an accurate representation of the actual edge

profile, the computed edge location will be

in error. Furthermore, nonlinear response

to high frequencies can yield unpredictable

changes in the measured edge profiles with

slight changes in the edge gradients; this

effect precludes accurate edge location.

Consequently, linear microdensitometer operation over the greatest practicable frequency

range must be achieved. Linear operation

cannot be assured unless the criteria of eqs

(15) and (16) are satisfied. In partiaularj

nonlinear response always occurs when the nu- merical apevbures of the efflux and influx

objectives are matched. Matching numerical

apertures is common practice and recommended

by some microdensitometer manufacturers.

17
The maximum spatial frequencies, tIqj fo^ linear response at 500-nm wavelength calculated

from the criteria of eqs (15) and (16) are given in table 7 for various numerical apertures. Caution should be exercised in the

blind application of these values since nonlinear response may exist for special cases even when these criteria are satisfied.

Such a case could result from the use of

large numerical aperture objectives with the

sample resting on a glass platen. The diffraction angles associated with the very high

frequency information are large and at these

large angles, it is possible for total reflection to occur from a glass platen or photomask. This total reflection precludes the

passage of the energy associated with some frequencies below the calculated lens cutoff and has the effect of limiting the numerical aperture of the affected objective.

A final consideration of the optical limitations of the microdensitometer is that of incoherence at the source aperture. The criteria of eqs (15) and (16) are derived on

the assumption of incoherent illumination at the source aperture. This is impossible to achieve exactly, but conditions can be specified for which the illumination can be con- sidered effectively incohevent [23]. The

fundamental criterion that must be satisfied

is

source aperture width , ^, . . . r r— T- > 4 + Q(n + a ) (17) coherence xnterval — o o

where Q is the width of the reduced source aperture projected onto the sample. This criterion must be satisfied for both underfilling and overfilling when image scanning is

employed. When sample scanning is employed,

the optics should be chosen so the size of

the scanning light spot, W, is limited by

the numerical aperture of the influx objective, and its width is given by eq (14).

When this is done, the coherence interval is

effectively the width of the Airy disk. The

light spot can be made no smaller; decreasing

the source aperture width further will change

neither the coherence interval nor the size

of the illumination on the sample. Under

these conditions, this system is independent

of the coherence existing at the source aperture and the source can be a laser, an arc

lamp, an incandescent light bulb, etc. [21];

Table 7 — Calculated Maximum Spatial Frequencies for

Linear Response at 500 nm [21]

,

NA^ n , o , AT A »TA 1 O O NA NA —— , . I L NA cycles/mm cycles/mm

(a) Overfilling (eq (15))

0.85 0.50 1.7 1400 2000

0.65 0.40 1.625 1000 1600

0.50 0.25 2.0 1000 1000

0.40 0.25 1.6 600 1000

0.25 0.16 1.56 360 640

NA^ NA^

NA^

NA^ cycles/mm cycles/mm

(b) Underfilling (eq (16))

0.65 1.0 1.54 1400* 2600

0.40 0.65 1.63 1000* 1600

0.25 0.50 2.0 1000* 1000

0.25 0.40 1.6 600* 1000

0.65 0.85 1.3 780* 2600

See [22].

18
However, the criteria of eqs (15) and (16)

must still be satisfied for linear operation.

Measurements of the coherence functions existing at the source aperture plane of four different raicrodensitometers were made at the

National Bureau of Standards with the aid of

a wavefront shearing interferometer [24].

The coherence intervals varied from 20 to 30

pm; the average was approximately 24 ym. On

those microdensitometers employing a single

element (a microscope objective) on the influx side, the criterion of eq (17) could not

be satisfied. For those instruments in which

an ocular was coupled with the influx objective, this criterion was easily satisfied.

This substantiates the statement by Swing

[21] that the influx optics should employ as large a reduction as practical.

Microdensitometers with low reduction factors may cope with the problem of achieving

1 effective incoherence by operating in a sample scanning configuration using the smallest scanning numerical aperture possible.

This will also reduce the problems with

i flare light, leaving as the only uncertainty

i the actual size of the sampling aperture

j which is difficult to measure and varies as

j the optical density it traverses [21]. This

j introduced uncertainty is less than the one

replaced, and represents a "best" condition

1 for such a microdensitometer

.

I

I Flare light is any unwanted or stray light

i in the image. It may appear in or out of

ij focus and has always been manifested in mij croscope systems. The problems caused by

j flare light worsen as sample frequencies get

I higher and may mask nonlinearities by reducing contrast in the image and smoo thing-out

the telltale signs of nonlinear response

j such as edge ringing. In a microdensitometer

I as much as 1-percent flare light can degrade

j the image of a well-defined edge, whose opti-

[ cal density goes from 0 to 3.0, beyond the

point where useful information can be obj tained from it [21].

It may be concluded that the optical design

of a microdensitometer should be carefully

considered when this instrument is applied to

j photomask inspection. The choice of an operaI tion configuration such as image scanning or sample scanning using either overfilling or

I underfilling must be carefully evaluated with

respect to the particular application for

which the instrument is intended. Difficulties may be encountered when applying this

instrument to scanning highly reflective

chromium masks because stray light may be re-

' fleeted into the microdensitometer and cause

'! flare.

! 1

3.3 Applications to Mask Inspection

A microdensitometer with a stage scanning

speed of up to 30 mm/s has been described

for the automatic inspection of mask registration and critical dimensions [25]. This

system uses both an efflux objective and influx objective with NA = 0.25, locates the

line edge at the transmittance T = 0.5, and

has a stage location precision of 1 ym. This system does not satisfy the various criteria for accurate measurements as discussed

above. Welford [26] has calculated edge profiles for the case of matched influx and efflux numerical aperture objectives. These

calculations clearly show that the edge profile obtained is neither that for the coherent nor incoherent regimes. The low value

of the numerical aperture itself may preclude the observation of significant sample

data and necessary high-frequency edge components .

The full utilization of the optics may not be

realized with this particular system. This

is because the response of the associated

electronics (which includes the detector, am- plifier, and associated circuits) may not satisfy the criterion that the minimum electronic frequency, f, for the band pass of the

electronic system must be greater than the

product of the scanning velocity, v, multiplied by the maximum spatial frequency cutoff, K, for the scanning system [29]; i.e.,

f > VK . (18)

Thus, for example, to realize the full resolution of a microdensitometer equipped with an influx objective of 0.65 NA and using 500-nm

wavelength illumination (which gives a value

of Kq = 2600 mm~-^ from table 7) , and operating at a scanning velocity of 30 mm/s, the

band pass of the electronic system must exceed 78 kHz. This implies that the response

of the detector (l/27rf) must be less than 2 ps. Work on line-width measurements with the

coherent light system [19] indicates that re- sponse times should probably be much less

than those calculated here if accurate lineedge location is to be achieved.

While a microdensitometer is suited for photomask dimensional inspection, it is not well

suited for the inspection of visual defects.

This is illustrated by the following example.

If a microdensitometer equipped with a 0.25-

NA influx objective (kq = 1000 mm~-^ from ta- ble 7) is to be used to inspect photomasks

for visual defects as small as 2 ym, the die

pattern must be scanned with a 2-ym spot in

2-ym swaths and data taken at every 2-ym interval along the scan. If the stage travels
at a velocity of 30 mm/s, a data acquisition

rate of 1.5 x lO'^ words per second is required. This stage velocity also requires a response time (l/2Trf) less than 5 ys. If

these conditions can be achieved, the scan

time per individual die pattern, not including stage indexing time, is about 4 min. The

total time to inspect the 400 die patterns on

the 76- by 76-mm mask is about 26 h.

There exists the possibility of reducing this scan time by image scanning with a rectangular aperture. If a 2- by 100-ym aperture is

used, the inspection time could be reduced by

a factor of 50 to about 32 min. However,

with this approach, serious questions arise

concerning the response time of the photomultiplier and its ability to make accurate

density measurements under these conditions.

Furthermore, the integrated intensity over a 2-ym scan length is changed by approximately

1 percent for every 0.02 ym of misalignment

of any transparent to opaque edge extending

the length of the aperture. This effect,

combined with those from scanning table yaws

die run-out, and other errors commonly found

in photomasks, would lead to the detection

of copious spurious defects.

20
4. EXPERIMENTAL EVALUATION OF AUTOMATED INSPECTION SYSTEMS [28]

4.1 Test Masks

To provide a means for evaluation of various

automated mask inspection systems, test photomasks were designed and fabricated with intentional defects. It was intended that

these masks, if successful, could be used by

anyone to determine such properties as the

minimum detectable defect size, the inspection speed, the number of missed defects,

and the number of false indications for any

particular inspection system or systems.

An MOS metallization pattern of moderate com- plexity was chosen as the basic pattern for

this test mask. The smallest line widths or spaces in this pattern are 0.30 mil (7.6 ym)

.

The width of the basic pattern is 10.5 mils

(0.27 mm) and the height is 7.9 mils (0.20

mm) . Each die consists of a 10 by 10 array

of the basic pattern which makes up the 120-

by 100-mils (3.0- by 2.5-mm) die pattern.

The final mask consists of a 14 by 14 array

of dice on a 76- by 76-mm photomask with a die center-to-center spacing of 150 mils

(3.81 mm) in each direction. Of the 196

dice in the array, 15 contain intentional defects .

These 15 defective dice are inserted into

the array by use of three lOX defect reticles

of the same pattern, but with the intentional

defects located at different random positions

on each die pattern. Each of these defect

reticles contains 36 intentional defects including pinholes, spots, breaks, bridges, intrusions, and protrusions; the distribution

of these defects is given in table 8. Each

of the defect reticles is stepped into essentially randomly selected locations on the

mask as shown in figure 7.

Clear-field test masks were fabricated by

two independent, commercial mask suppliers

using conventional state-of-the-art optical

lithography. One supplier used a step-andrepeat system with a laser controlled stage;

otherwise, the fabrication equipment employed

was the same. Master masks were fabricated

by stepping the defect-free reticle into 181

locations while skipping 15 locations. The

three different defect reticles were stepped

into these skipped locations. The test masks

were made by contact printing the master

masks into both chrome and iron oxide photomask blanks. Examination of these test masks

through a microscope revealed that essentially all of the defects 2.5 ym and larger

printed; nearly all of the smaller pinholes

and about 10 percent of the smaller spots

printed, giving a total of about 510 intentional defects per test mask and an average

defect density of about 18 cm~^.

Run-out errors were measured on these test

masks. These measurements were made along

selected rows and columns of dice and made on scan lines located near the dice edges that were parallel to the scan direction. The locations of these scan lines are indicated by

the arrows in figure 7. The run-out measurements were made from the centers of the first

opaque line located on the leading edge of

the die pattern with respect to the scan direction. These opaque lines were 7 pm wide.

The measurements were made on a commercially

available linear comparator with a stated precision of ±0.25 pm. Line centers were automatically located by the use of a photocell

and signal balancing while a light beam dithered across the line. Thus, the line centers were respectively located at the centers of

each symmetrical line image. Repeated measurements along each scan were within 0.25 pm

in all cases and in most cases were within

0.12 pm.

The run-out errors from two measurement scans,

one across the top and the other across the

bottom of row 12 of one of these masks, are plotted in figure 8. These errors are typical of the run-out errors measured on all of

the masks regardless of whether or not the

masters were made on a laser controlled stepping stage and often exceed the required indexing accuracies listed in table 1. Figure

8 shows that the die run-out errors exceed 1 pm at the location of defect reticle II. It

is suspected that these large run-out errors

are caused by errors in the positioning of

reticle II in the stepping equipment and not

by errors in the stepping stage. Figure 9 illustrates the respective die rotations implied by the run-out errors measured at the

left and bottom edges of die patterns (12,7)

and (12,11). For illustrative purposes the

die patterns are represented as 1.5-pm

squares and show opposite rotations for the same die. Since a die cannot rotate in two opposite directions simultaneously, these phe- nomena are interpreted as caused by line distortions and edge waviness. Such distortions

have a pernicious effect on the capability of

automated inspection systems to detect real

defects

.

4.2 Mask Inspection

Photomasks made from the same runs as those

masks on which the measurements were made.
COLUMN

1 2 3 4 5 6 7 8 9 10 11 12 13 14

T HI' '

HI

in

IT TX II

T

1 TT11

m

m. I I n

14

1 3

1 2

1

1

10

9

8

7

6

5

4

3

2

1

Figure 7. The 14 by 14 matrix of the defect mask showing the

stepped locations of the three different defect reticles indicated by Roman numerals I, II, and III, respectively, (The arrows indicate the scan locations of the run-out measurements.)

Table 8 — Intentional Defects on the Defective Die.

Type Quantity Print Size, pm

Pinholes^ 14 7.6, 6.4, 5.1, 3.8, 2.8, 2.5, 1.9 (two each)

Spotsa 14 7.6, 6.4, 5.1, 3.8, 2.8, 2.5, 1.9 (two each)

Breaks'^ 2 7.6 wide

Bridges'^ 2 7.6 wide

Intrusions'^ 2 Halfway into a clear area

7.6 wide

Protrusions'^ 2 Halfway into an opaque line

7.6 wide

Total per

reticle 36

Round

.

'Made with straight edges.

22
rn—

r

1—I I I I I I

j_j I I I 12 3 4 5 6 7 8 9 10 11 12 13 14

COLUMN

Figure 8. The run-out errors between the left edges on adjacent dice of row

12 of the defect mask as a function of column position ( o : values from the

top scan; O: values from the bottom scan.)

23
DIE (12,7: DIE (12,11

+ 0.25

+ 0.50

I—

+ 1.40

+ 1.80

ROW MEASUREMENTS

+ 0.40 +0.25 + 1.15 +0.40

COLUMN MEASUREMENTS

Figure 9. Illustration of the run-out errors from the scan measurements of the

line centers. (Solid squares indicate the desired dice positions; dashed squares

indicate the measured positions. The arrows indicate the positions of the runout scan measurements; the values associated with the arrows are the run-out errors in micrometers. For illustrative purposes the die patterns are represented

as 1.5-ym squares.)

24
were sent to four independent installations

possessing automated photomask inspection

systems. These masks were packaged and

shipped by mail. This presented a problem

in that the masks were received with considerable amounts of particulate contamination

which necessitated their cleaning before inspection. Since the cleaning thoroughness

could not be assessed, all of the masks were

guaranteed to have more defects than those

intentionally fabricated into them.

The results of these inspections were as follows :

SYSTEM A - This system used the dualbeam, flying-spot scanning technique. During

the inspection the spacing between the two flying spots had to be continually adjusted

indicating that there were run-out errors on

the mask. To alleviate some of these difficulties, the system threshold was set to detect minimum defects of 2.5 \im. Since this

system was already set up to inspect an area

somewhat smaller than that covered by the 14

by 14 mask array, only a 13 by 13 array of

dice was inspected. The total number of defects was given as 540 and the corresponding

defect density was about 24 cm~^. No information was supplied as to whether all of the

intentional defects greater than 2.5 um were

detected. The mask was fabricated with 420

intentional defects in the inspected area;

thus, considerably more defects were detected

by this inspection than were intentionally

fabricated into the mask.

SYSTEM B - This system also used the

dual-beam, flying-spot scanning technique.

Considerable difficulty, primarily attributed

to die orientation and run-out errors, was ex- perienced. Contamination was also a problem.

To avoid the detection of spurious defects

caused by contamination and run-out errors,

the system was adjusted to detect defects

that were only 5 ym and larger. Under these

conditions only the intentionally defective

die patterns were identified. No other information concerning the identity of defects

or defective dice was provided.

SYSTEM C - This system used the spatialfiltering technique with incoherent illumination added to produce a background image of

the entire mask for defect identification

and location. None of the defects on the

mask was found with this system.

SYSTEM D - This partially automated system consisted of an optical comparison technique that alternately displays, in rapid suc- cession, two different superimposed die patterns. This system is similar in principle

to the blink microscope used by astronomers

to detect celestial objects [29] and required

manual observation of the display to detect

mask defects. Only the 15 intentionally defective dice were identified. No problems

caused by run-out errors on the mask were reported. The inspection time was about 15

min.

Although the results reported from these

tests were incomplete, it was clear that dimensional control of run-out, critical dimensions, and line edges is mandatory if masks

are to be inspected by systems directly com- paring one die pattern with another on the same mask. Specifically, the run-out error must be appreciably less than 0.5 ym everywhere. This error includes the cumulative

contributions from table yaws and drifts in

the stepping equipment as well as lack of

geometry uniformity due to chemical processing. While no experimental verification was established to assess the capability of other

types of mask inspection systems to inspect

masks with the type of dimensional errors described above, it appears from the analyses

reported here that the above conclusion is

valid for all types of automated mask inspection systems

In principle, stepping errors can be overcome

by the use of electronic alignment. This involves the continuous shifting of coordinate

positions of the inspection data stored in an interfaced computer while the mask is being

inspected. The increase in the inspection

time that is added by this continuous alignment is expected to be unacceptable. An es- timate of the increase in the inspection time

may be made by considering the following:

There are 1.44 x 10^ separate areas of 2- by

2-ym size to be compared on the entire mask.

If the computer memory consists of RAM memories with serial access or cycling times of

1 ys per bit and each stored word consists of

2 bits (required to store four levels of gray

scale information which may consist of com- pletely clear, completely opaque, and two intermediate gray levels), the total interrogation time for each word is 2 ys . Assuming

that the electronic alignment requires a mini mum of one data interrogation to determine

the misalignment and a second interrogation

to verify the realignment, the total time for

the electronic alignment is equal to the sum

of the cycle time per bit plus the access

time per bit multiplied by the number of bits

per word; for this example, an additional inspection time of 96 min results.

This increase in inspection time could be re- duced by using memories with access and cycle

times of 70 ns per bit which are predicted to
become available in 1978 [30]. The use of

these memories would reduce the additional

time by a factor of 7 x 10"^ to about 7 min

which is an acceptable increase in the inspection time. However, in this model it is

still assumed that only two data interrogations are necessary for complete alignment.

Automated mask inspection devices reported to use such electronic alignment [5] use a mini- mum of one data shift, or two data interrogations, for the alignment in each of the x- and y-directions . Consideration of alignment in both of these directions increases

the additional mask inspection times calculated above by a factor of 2. The calculated

additional times will also be increased if more than two data interrogations per direction are required. There is a possibility of

reducing the additional inspection time by

signal multiplexing, parallel data reading,

and selective data interrogations, although

selective data interrogations have the disadvantage of reducing the detection probabilities of misalignments caused by stage yaws

and thereby increasing the probability of

spurious defect detection.

26
5 . CONCLUSIONS

Four methods were analyzed for automated inspection for visual defects. The results of

these analyses should be interpreted in a comparative sense to indicate which automated mask inspection methods appear most viable in light of the characteristics and

properties considered. The properties and

characteristics that were analyzed were primarily presented as areas that should be considered either by the prospective purchaser

of an automated inspection system or a prospective manufacturer.

A general conclusion that can be made from

this investigation is that an automated mask

inspection system should be designed to do

either visual defect inspection or dimensional inspection, not both. This is based

on the fact that the principles involved in

these two types of inspections are significantly different and accordingly require different designs for each application. The

laser flying-spot scanning system can in

principle be used for dimensional inspection

in a microdensitometer mode; the data collected in this mode must be analyzed for lineedge locations, realizing that the illumination is coherent. The microdensitometer has

utility in dimensional mask inspection, but

its basic design limits its use for visual

defect inspection.

An extremely important conclusion from this

work is that, for visual defect inspections,

the dimensional tolerances on the masks, both

those of the feature dimensions in the die

patterns as well as those in the dimensions

between the die patterns, must be significant

ly smaller than the size of the minimum defect to be detected. This arises from the

basic property of automated inspection systems to interpret the misalignment caused by

the lack of such dimensional control as defects and the unacceptable time added if elec

tronic alignment is used to compensate for

this lack of control. Thus, before successful automated mask inspection systems can be

made using the technologies as they are described here, much more dimensional accuracy

must be achieved in mask making. This re- quires not only better measurement methods

and control, but also better chemical process

ing to assure line and die pattern uniformity

A conclusion that follows directly from the

previous one is that these methods of mask in

spection may be directly applicable to the in

spection of printed circuits and possibly hybrid circuits. The sizes of the tolerable de

fects for these circuits are a much smaller

fraction of the achievable dimensional control than it is for integrated circuit patterns .

6 . ACKNOWLEDGMENTS

The authors would like to acknowledge the

many individuals employed in the microelectronics industry and associated industries

for their helpful discussions during the course of this work. We would like to ex- press a special thanks and appreciation to

the individuals who made the inspection measurements reported in section 4.2. We would

also especially like to acknowledge Mr. Phillip Schultz of Lawrence Livermore Laboratory

for continued assistance and advice during

the analysis phase of this work. Appreciation and thanks are owed to Dr. D. Nyyssonen

and Mr. R. Swing* of the National Bureau of

Standards for many helpful discussions on mi' crodensitometers and related optics and to Dr. W. M. Bullis of NBS for his critical re- view of this manuscript.

27

Now with General Electric Co., Space Division, Valley Forge, Pennsylvania.
7 . REFERENCES

1. Schoonard, J. W. , Gould, J. D., and

Miller, L. A., Experimental Studies of

Visual Inspection, IBM Research Report

RC 3085, October 6, 1970,

2. Ciarlo, D. R. , Schultz, P. A., and

Novotny, D. B. , Automated Inspection of

IC Photomasks, Photofabrication Imagery

55 , J. M, Graf and W. Converse, Eds.,

pp. 84-89 (Society of Photo-Optical In- strumentation Engineers, Bellingham,

Washington, 1975).

3. Bruning, J. H., Feldman, M. , Kinsel, T. S., Sittig, E. K. , and Townsend, R. L.

,

An Automated Mask Inspection System,

IEEE Trans. Electron Devices ED-22 , 487-

495 (1975).

4. Dyer, D. L. , Optical Limits in TV Microscopy, Research/Development _24_, 40-

44 (1973).

5. Sandland, P., Automatic Inspection of

Mask Defects, Developments in Semiconductor Microlithography II 100 , J. W. Giffin and B. Ruff, Eds., pp. 26-35

(Society of Photo-Optical Instrumentation Engineers, Bellingham, Washington,

1977)

.

6. Hyzer, W. G. , Image Analysis, Research/

Development 25, 44-49 (1974).

7. Kossyk, G. J. W. , Laico, J. P., Rongud,

L., and Stafford, J. W. , The Primary Pattern Generator Part II - Mechanical Design, Bell Syst. Tech. J. 49, 2043-2059

(1970)

.

8. Chitayat, A. K. , High Speed Positioning,

Solid State Technology 16_, 42-44 (1973).

9. Hartman, D. K. , Glasbrenner, F. K. , and

Colangelo, D. , Part I, Manufacturing in

Process Control and Measuring Techniques

of Mask Evaluation, AFML-TR-69-334 Technical Report, December 1970. (Available

to qualified requestors from Defense

Documentation Center, Cameron Station,

Alexandria, VA 22314, Accession No. AD

881208.

10. Bourdelais, R. J., Colangelo, D.

,

McFadyen, R. J., and Elliott, J. F. , In- strument for Automatically Inspecting

Integrated Circuit Masks for Pinholes

and Spots, U.S. Patent 3,795,452, March

5, 1974. (Government-owned invention

available for licensing; copy of patent

available from Commissioner of Patents,

Washington, DC 20231.)

11. Goodman, J. W., Introduction to Fourier

Optics, pp. 83-90 and Chapter 7 (McGraw

Hill Book Co., New York, N.Y. , 1968).

12. Goodman, J. W. , ibid., pp. 83-90.

13. Watkins, L. S., Inspection of Integrated

Circuit Photomasks with Intensity Spatial Filters, Proc. IEEE 57^, 1634-1639

(1969)

.

14. Heinz, R. A., Odenweller, R. L. , Jr.,

Oehrle, R. C. , and Watkins, L. S., Tool

and Production Inspection by Optical Spatial Filtering of Periodic Images, The

Western Electric Engineer 27_, 39-56

(1973)

.

15. Watkins, L. S., Application of Spatial

Filtering Subtraction to Thin Film and

Integrated Circuit Mask Inspection, Appl.

Opt. 12, 1880-1884 (1973).

16. Axelrod, N. N. , Intensity Spatial Filtering Applied to Defect Detection in

Integrated Circuit Photomasks, Proc. IEEE 60, 447-448 (1972)

.

17. Minami, M. , Sekizawa, H. , Masuda, H.

,

and Watanabe, T. , A New Mask Inspecting

Device, Proceedings of the Micro-Electronic Seminar INTERFACE '76, Monterey,

California, October 1976, pp. 67-80

(Eastman Kodak Company, Rochester, N.Y.,

1977)

.

18. Jerke, J. M. , Hartman, A. W. , Nyyssonen,

D., Swing, R. E., Young, R. D. , and

Keery, W. J., Comparison of Linewidth

Measurements on an SEM/Interf erometer

System and an Optical Linewidth-Measuring Microscope, Developments in Semiconductor Microlithography II 100, J. W. Giffin and B. Ruff, Eds., pp. 37-45

(Society of Photo-Optical Instrumentation Engineers, Bellingham, Washington,

1977).

19. Nyyssonen, D., Linewidth Measurement

with an Optical Microscope: The Effect

of Operating Conditions on the Image

Profile, Applied Optics 16, 2223-2230

(1977).

20. Nyyssonen, D. , Optical Linewidth Measurements on Silicon and Iron-Oxide

Photomasks, Developments in Semiaonduc-
tor Miarolithography II 100 , J. A. Giffin and B. Ruff, Eds., pp. 127-134 (Society of Photo-Optical Instrumentation

Engineers, Bellingham, Washington, 1977).

21. Swing, R. E., The Optics of Microdensitometry. Optical Engineering _12, 185-

198 (1973).

22. Swing, R. E. , NBS, Private Communication: The criterion for underfilling

and values derived from it are incorrect

as they appear in reference [21] . They

are stated correctly in this manuscript.

23. Swing, R. E., Conditions for Microdensitometer Linearity, J. Opt. Soo. Am. 62 , 199-207 (1972).

24. Nyyssonen, D., Partial Coherence in Im- aging Systems, Opt. Eng. 13, 362-367

(1974) .

25. O'Callaghan, F. G. , Clarke, H. C,

Poindexter, R. W. , Wray, J. D. , and

Benedict, G. F. , Automatic Mask Inspection for Registration Errors and Critical Dimensions, presented at the Society

of Photographic Scientists and Engineers

Winter Sympos iuin on Micro—Photo Fabrica.—

tion, February 6-7, 1975, Palo Alto,

California.

26. Welford, W. T. , Length Measurements at the Optical Resolution Limit by Scanning

Microscopy, Colloquia of the International Commission for Optics, Bruxelles , May

6-9, 1958, Optics in Metrology, P. Mollet, Ed., pp. 85-90 (Pergamon Press, New

York, 1960).

27. Williams, C. S., and Becklund , 0. A.,

Optics: A Short Course for Engineers

and Scientists , p. 335 (John Wiley and

Sons, Inc., New York, N.Y., 1972).

28. Ciarlo, D. R., IC Inspection Test Masks

- Experimental Results, Lawrence Liver- more Laboratory Report UCID 17391, February 14, 1977. (Available from National Technical Information Service, 5285

Port Royal Road, Springfield, VA 22161)

29. Anon. , The Astronomer in the Engine

Room, Time 106, pp. 75-76 (October 27,

1975)

.

30. Altman, L., One-Chip Controllers and 4-K

Static RAMS Star, Electronics 50^, p. 97

(1977).

31. Williams, C. S., and Becklund, 0. A., op

cit.. Chapter 5, pp. 98-129.

32. Goodman, J. W. , op. cit., p. 166 ff.

29
APPENDIX

Details of the Line-Edge Model

This appendix describes the details of the model used to calculate the profiles shown in fig- ure 5. The scanning light spots for the dual-beam, flying-spot scanner described are produced by a laser beam and are coherent. Consequently, the intensity profile of an edge must

be calculated for coherent light by first calculating the amplitudes of the light radiation,

combining these amplitudes by superposition, and calculating the intensities by squaring

these resultant amplitudes [31,32]. The model assumes that the phase is constant and that

no phase changes occur across the edge. Thus, in a one-dimensional analysis the field amplitude of the light P(x)^''^ is found by convolving the amplitude distribution of the Gaussian

light spot g(x)^/2 with the amplitude attenuation of the line edge T(x)^'2 where P(x) is the

transmitted light intensity, T(x) is the transmittance of the light absorbing material on

the photomask, and g(x) is the intensity distribution. Mathematically, this is written

P(x)l/2 /

(A-1)

where

and

g(i;)l/2 = (h/v^)exp - h2c2/2

T(c)l/2 = exp - Kt,

C < 0 ,

0 < c < qg . and

The transmittance function T(c) describes the transmittance of the physical edge profile

shown by line B in figure 5. This material edge profile begins at x = 0, rises at an angle

^ above the horizontal, and extends to x = qg; a film of constant thickness tg and transmittance Tq exists at values of x > q^; no film exists at values of x < 0. The transmittance

of the film is assumed to obey Lambert's law. The film edge shown in figure 5 has (j) = 55 deg and a Lambert's law exponential constant K = (2.303 atan ((>)/2 where a is the absorbency

determined by using the common base 10 instead of the natural base.

The scanning spot diameter R is obtained by assuming a diffraction limited lens with an impulse response given by (sin ax) /ax with a = ttR for the one-dimensional case. This gives a value of R equal to one-half of the characteristic width of this impulse response function

which is the distance between the first node points on each side of the line ax = 0.

To facilitate the mathematics, a Gaussian function, (h//iT)exp - h^x^, which is normalized

such that its integral over all values of x is equal to 1, was fit to the intensity of the

one-dimensional impulse response. The constraints imposed were that the x-coordinates of

the half-maximum points of the Gaussian function and (sin ax/ax) 2 be equal which gives a value ax = 1.38 at the half-intensity point. Using this value, the relation between R and h,

R =

Tr/ln 2 1.38 h

was obtained. Equation (A-1) can be solved explicitly for the intensity P(x) in terms of

the tabulated error function

erf(x) =
and is

P(x)l/2 = (1/2) [1 - erf(hx//2)] + [/k - h^xX /K - h^x hq \

"

2 - erf (

J -erff^ + —-]

\ Tl h I \/2h /if

+

(T^/2)[l - erf(h(q^ - h)//2)] .

Values of P(x) calculated from this equation were used to plot the profiles shown in figure 5,

31
1
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No. SHEET SP 400-46

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Semiconductor Measurement Technology:

Automated Photomask Inspection

5. Publication Date April 1978

6. Performing Organization Code

7. AUTHOR(S)

Donald R. Novotnv and Dino R. Ciarlo

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

National Bureau of Standards Lawrence Livermore Lab.

Department of Commerce P.O. Box 808

Washington, D.C. 20234 Livermore, CA 94550

10. Project/Task/Work Unit No. 4256374

11. Contract/Grant No. ARPA DrHpr ??Q7

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Defense Advanced Research Projects Agency

Arlington, VA 22209

13. Type of Report & Period C 0V c r c d

14. Sponsoring Agency Code

i. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 78-606023

Lawrence Livermore Laboratory work funded by NBS Order Numbers 503183, 602156.

1 16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Methods that may be suitable for automated photomask inspection for visual defects

(spots, pinholes, etc.) or dimensional compliance are analyzed and discussed. The

analysis of each method includes examinations of the physical principles upon which

j it is based and the amount of misalignment that can be tolerated. The size of the

j minimum visual defect to be detected was taken as 2 ym. The methods analyzed for

j visual defect inspection are the optical -overlay; the dual -beam, flying-spot-scanner;

the TV-microscope; and the spatial -fi 1 tering methods. For dimensional inspection,

j an analysis of line-edge location and operating criteria for the microdensitometer

1 are presented. The fabrication of photomasks with intentionally introduced and controlled defects is described together with preliminary results of automated inspections of these photomasks. It was concluded that: automated inspection systems

j should be dedicated either to inspection for visual defects or dimensional compliI ance, not both; and the dimensional tolerances on masks, both those of the feature

j dimensions in the die patterns as well as those in the dimensions between the dice,

must be significantly smaller than the size of the minimum defect to be detected.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Alignment tolerance; automated photomask inspection; dimensional inspection; flyingspot scanner; microdensitometer; optical overlay; photomask; spatial filtering; TVmicroscope; visual defect inspection.

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