3d detection using laser

8/6/2019 3D Detection Using Laser

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Nano-Level 3-D Measurement System Using

3-Wavelength Laser Light Interference

Seiji Hata 1,Junichiro Hayashi1 , Ichiro Ishimaru 1Shigeaki Morimoto21Kagawa University, 2Ryusyo Industries Co.

E-mail: [email protected]

Abstract-To improve the productivity of very large scale ofLSIs or large LCD panels, the nano-level measurement to inspectthe LSIs is required. To meet with these requirements, a nano-level 3-D shape extraction method has been introduced and themeasurement results are described. To extract a nano-level 3-Dshape, the method using laser interference images is effective.Interference image is produced by light reflected by LSI and byreference mirror. At this time, if the position of reference light ischanged at regular intervals, the brightness of same coordinate ofinterference images change like a sine wave. When positionheights of LSI differ between two coordinates, the brightness ofinterference image differs and the phases of brightness patterndiffer according to height between each pixel. And combiningwith different wavelength laser lights, different brightness is ableto measure more than one wavelength. Look-up table method isused to combine the multi-wavelength laser light measurement.The experiment results show the method is able to measure morethan 10 um height with 10 nm accuracy.

I. INTRODUCTION

The productions of LSI devices are increased every year.

But, there is some problem when the LSI devices are produced.Micro contaminants (1micrometers or less ) adhere on LSIs

surface. Of coerce, the LSIs surface is washed, but microcontaminants are hard to remove by washing. So in current

production lines, detecting foreign-particles on silicon wafer,

and if micro contaminants were found on wafers, these wafers

are scrapped as defectives on device. But when the defective panels are scraped the productivity of the LCDs and plasma

displays are decreasing. To solve the problem, it is required to

develop a measuring system which can detect positions andshapes of particles on LSI wafers and remove them. The size of

particle is bigger than 100nm.

The purpose of this research is to extract the 3-D shape of

micro object surface patterns and to find out positions and

shapes of minute particles. To detect very small foreign- particles, required to measure objects with sub-micron level

accuracy, and performance cannot be achieved by the

conventional methods. So we propose a method using phaseshifts of interference of light to make nano-level measurement

II. OPTICAL SYSTEM

A. Phase Shift MethodPrepare Interference images obtained by Michelson

Interferometer that it was generally used for a sub-micron level

measurement. Fig.1 shows Michelson Interferometer. If the positions of reference mirror in fig.1 is moved at pre-

determined intervals, brightness changes like a sine curve in

same coordinate of interference images (See Fig.2). It is phase

shift method. If heights differ between two coordinates, their phases of brightness curves are different, too. The phase

difference is the function of the height difference between twopoints.

Fig. 1 Phase shift between 2-points

Laser

Camera

Mirror

Object

Fig. 1. Michelson Interferometer

The IEEE International Conference onIndustrial Informatics (INDIN 2008)DCC, Daejeon, Korea July 13-16, 2008

978-1-4244-2171-8/08/$25.002008 IEEE 721


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B. Accuracy of calculatingFig.3 shows that the sine curve is made from brightness

changes of one point in interference images. So the phase ismeasured by this sine curve. Now the phase was taken by

comparing with the correlation of sine curve to brightnesspattern, to detect the reliable phase difference in the inference

images. Because the brightness pattern has noises, so it was

needed to developed measurement system under the noises. Inthis case, the phase measured by using regularized correlation

matching method. This result is showed in fig.4. These plotted

points are measuring brightness value in interference imagesby camera. And the sine curves phase is highest correlation

value in this brightness pattern. As is shown in fig.4, the max

brightness value is matched with sine curve, and minimum

value is similar too. So we say this matching method is

effective in this research.It needs to verify this methods accuracy. So the easyconfirmations were experimented. The two calculated phase

images obtained at two removed intervals phase differences areextracted, these phase differences were compared with really

intervals and measurement errors were evaluated. The two

calculated images are shown in fig.5 and the results are shown

in table.1. We got the errors at the center in calculated imagesand calculated standard deviation inside of 50 pixel radius

circle. So the error deviations are about 0.61.0 degree in

phase. In the 250 pixel radius circle, the max error is about

1.8nm. Its 3sigma is about 6nm.

Table.1 Standard Deviation

Radius (pixel) Standard deviation

(degree)

50 0.6100

100 0.6998

150 0.73187

200 1.0320

250 1.4676

III. 3-DHEIGHT CODE LOOK-UP TABLE

A. Principle of MethodTheoretically, the measurement range using one wave-length

laser is limited only half of laser wavelength. It is difficult to

measure more than 1 um. To solve the difficulty, the

measurement system using combination of multi-wavelength

lasers has been introduced. As is shown in fig.6, the phase shiftw of the brightness change is coded . (code is changes from

0,1,2n,0,1,2n,0,1,2) The bottom of fig.6 shows the

combination of codes differs to each other along more thansingle wave-length range. So if we used three wavelength laserlights, the combination of code differs along 200-300 um..

N

d

SN

Fig. 3 Sine Curve of Brightness Change

Fig. 4 Measured Brightness and Overlapped Sine Curve

Fig. 5 Calculated Phase images (20nm deference)

Fig. 6 Combination of Multi-wavelength Lasers

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First, the code table made under two wavelength laserlights. Fig.7 is code table image. The code number is flowed

diagonal in two wavelength code table. Because, the codenumber (x and y) increases one by one. So code number

reaches max number, next code number is 0. So the heightnumber increases one by one for diagonal. And this height

number difference is the object height.

B. Phase Shift Detection ErrorCode table was made by the method described in A, and the

object was measured according to the movement of thereference mirror. But if the code table is constructed as all the

table column fulfilled, and if there is some errors to detect

phases, it may indicate the differed column, and at that time,

the mistake should be very large. For example fig.8 showsthese measuring cases. Difference of height code number is 3,

but this result difference of it is 3300 (Height code numberchanged from 742, 743, 745, 746.

But the nearly column number 4098 or 4099 is big

difference from really number 744.).

In this result, we can not measure the real height by thiscode table. So the code table should be constructed as it can

contain the margin of measurement error.Fig.8 shows the look up table which every column contains

different values. It causes large measurement errors. To solvethe problems, we propose new code table. In new code table,

the error code which is differed one or two values is included

error margin in common. This method is the height codenumber surrounding same number values. The same height

code is read from the table when the code number values in x

or y axis is differed only one or two. In this method heightcode errors are expected to the decrease. In fig.9 is example of

Fig. 7 Arrangement of Height Code Table

Fig. 8 Measurement Error of Phase Shift

Fig. 9 Example of New Code Table

Plotted surrounding line

Made x-axis and y-axis

Plotted code

Approximate straight line

Fig. 10 Flow-chart of Height Code Table Generation

Fig. 11 First Step of Code Table Generation

Fig. 12 Second Step of Code Table Generation

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new code table. Same code numbers appears 6 or more times.

C. Height Code Table GenerationFollowings explain errors tolerant new code table

generation method. Fig.10 is the flow chart of method in

making new code table. First, the max code number is tripled

the x-axis and y-axis. Because, the new code table must havesame code numbers. So, there were the empty zones in table.

And this means that each wavelength of laser light was divided

three times more accurate than past. Second the measuring

value in the phase images is plotted in this code table. Fig.11

shows this processing. And the code table is approximated thestraight line in plotted phase code number. This processing is

shown in fig.12.Finally the same code number placed to these surroundings

along straight line. The result is shown in fig.9. The table valuemeans the measuring heights with better reliably.

When using the two different wavelength laser lights, thecode table is 2-D table. We made 3-D code table when using

three different wavelength laser lights. This code table image is

shown in fig.13. The code numbers are placed according to W1,

W2 and W3 axis. So, max code number of combined placed bymax code values in each axis.

IV. EXPERIMENTS

Nano-level 3-D measurement system using the combinationof 3 laser-lights interference has been developed as is shown in

fig. 14. The combination of laser light wave-lengths are

488nm(B), 532 nm(G) and 670 nm(R).First, the combination of R and G laser lights measurement

with the 2-D height code table is examined. To make the height

code table, reference mirror is moved by 20 nm steps, and aninterference image is got by each movement. The observed

object is the standard specimen with 52 nm step. The observedimage is shown in fig. 15 . Fig. 16 shows the 2-D height code

table constructed to measure the specimen.

The result of the measurement is shown in fig. 17. In the

image, some optical distortion caused curvature profile. But the

step can be clearly observed. The calculated height of the step

was 54 nm. The certified height of the step is 54 nm. The

accuracy of the measurement is acceptable.

Fig. 13 3-D Height Code Table

Fig. 14 Nano-level Measurement System Using 3 Laser-lights Interference

Fig. 15 Observed Image of 52 nm Step

G

R

Fig. 16 2-D Height Code Table for R and G Laser Lights Combination

Fig. 17 Observed 3-D Shape

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Next, the combination of 3 laser lights measurement with

the 3-D height code table is examined. Fig.18 is the standard

specimen with 4 um step. Fig.19 is the 3-D height code tableused for the3 laser lights, R,G and B, combination

measurement. Fig. 20 shows the result of the measurement of

the specimen. There are some measurement errors at the fringe

of steps, but the shape of the specimen has correctly re-

constructed.

The calculated height was almost just 4 um. It proves that

the measurement system with 3 laser lights combination is able

to 10 times longer length of laser wave length. Also the

accuracy of the measurement can be satisfied.

TAbout 20 times observations are enough to calculate the

3-D shape. It means that the high speed measurement can be

achieved using the method.

V. CONCLUSION

In this paper, nano-level 3-D Measurement system using 3-

laser lights interference. The brightness shift measurement of

laser interference gives the nano-level measurement. Also, the

3 wavelength lasers combination enables to measure more than10 um height measurement. Only 20 times of observation of

interference images are enough to calculate 10 um level height.

It means the method achieves the high speed measurement.

Currently, there are some remained problems to be refined.

In the near future, the refined measurement system will appear.

ACKNOWLEDGMENT

This research was supported by METI, Japan. We thank for

their support.

REFERENCES

[1]Shoji Muramatsu, Yoshiki Kobayashi, Kazuya takahashi, EijiShimizu Development of Template Matching Hardware and ItsSpeed Processing Strategy, Trans EIC,Vol.J83-D-2, No.7,

PP1667-1675,2000.[2]Mitsuji Ikeda, Shoji Yoshida, Keisuke Nakashima, Nagaharu

Hamada, Haruo Yoda, High Speed Template Matching ByMonotonize Normalized Correlation, Trans EIC, Vol.J83-D-2,

No.9, pp1861-1869,2000[3]Shunji Maeda, Takashi Hiroi, Hitoshi Kubota, Automated Usual

Inspection of LSI Wafer Multilayer Pattern Using a Derivative-Polarity Comparisonal Gorithm, Trans EIC, Vol.J82-D-2, No.1,

pp39-52, 1999.

[4] M.Nomura, J.Hayashi, S.Morimoto, S.Hata, I.Ishimaru, Nano-Level

3-D Measurement System Using Combination of 3-Wavelength Laser

Lights, Proc of SICE2008, pp.759-763(2007)

Fig.18 Standard Specimen with 4 um Step

R G

B

66 53

47

Fig. 19 3-D Height Code Table for 3 Laser-lights Combination

Fig. 20 Result of 3-D Measurement

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3d detection using laser

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