Ž .Spectrochimica Acta Part B 56 2001 2307�2312

Sensitive detection of trace copper contamination on asilicon wafer by total reflection X-ray fluorescence

using W�L� or Au�L� excitation source�

Takashi Yamadaa,�, Masaru Matsuoa, Hiroshi Kohnoa, Yoshihiro Morib

aRigaku Industrial Corp, 14-8 Akaoji, Takatsuki, Osaka 569-1146, JapanbAd�anced Technology Research Laboratories, Nippon Steel Corp., 3434 Shimata, Hikari, Yamaguchi 743-0063, Japan

Received 19 December 2000; accepted 14 May 2001

Abstract

Wafers, intentionally contaminated with copper in a range of 109 to 1010 atoms�cm2, were measured using totalŽ .reflection X-ray fluorescence TXRF instruments equipped with a W or Au target as the excitation source. The

Ž .results were compared with vapor phase decomposition-atomic absorption spectrometry VPD-AAS . Linearity ofTXRF reached the lower range of 109 atoms�cm2 both with W�L� excitation and with Au�L� excitation.1 1Deviations of the results of Au�L� excitation in a lower range of 109 atom�cm2 are significantly smaller than those1of W�L� excitation. � 2001 Elsevier Science B.V. All rights reserved.1

Ž .Keywords: Wafers; Copper; Total reflection X-ray fluorescence TXRF

1. Introduction

In the semiconductor industry, copper is one ofthe principal metal impurity contaminants to bedetected in silicon wafers. At present, the level ofcopper impurities is in the order of 1010

atoms�cm2. Total reflection X-ray fluorescenceŽ .TXRF has been widely used for determining the

� This paper was presented at the 8th Conference on TotalReflection X-Ray Fluorescence Analysis and Related Meth-ods, Vienna, Austria, September 2000, and is published in theSpecial Issue of Spectrochimica Acta Part B, dedicated to thatconference.

� Corresponding author. Fax: �81-72696-9155.

� �level of impurities 1 , and vapor phase decompo-Ž .sition VPD is often used to enhance the sensi-

� �tivity of TXRF 2,3 . It is preferable, however, tomeasure the concentration on a silicon waferwithout VPD treatment if the sensitivity of theTXRF instrument is sufficient for the level re-quired.

Ž .The lower limit of detection LLD is used inthe specification of a TXRF instrument. As is wellknown, LLD is derived from the following equa-

� �tion 4 ,

3 I' B Ž .LLD� C 1IS

0584-8547�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 5 8 4 - 8 5 4 7 0 1 0 0 3 4 4 - 5

( )T. Yamada et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 2307�23122308

where I is the intensity of background, I is theB S

intensity of the signal, and C is the concentrationof the element. In most cases, I is derived byB

measuring a blank wafer; I is an intentionallyS

contaminated standard wafer. The values of LLDfor available TXRF instruments are currentlylower than 1010 atoms�cm2 for transition metalsfor 1000-s measurements. However, direct quanti-fication of samples of a concentration lower than1010 atoms�cm2 are not usually made directly butwith samples of a higher order of concentrationof 1012 to 1013 atoms�cm2. Then, LLD is derivedfrom samples at such a higher order of concentra-

Ž .tion using Eq. 1 . Mori reported the experimen-tal quantification of samples of the order of 1010

11 2 � �to 10 atoms�cm 5 but there have been noreports of experiments using samples of a concen-tration lower than 1010 atoms�cm2.

In addition to the difficulty of measuring weakX-rays from the sample, the escape peak in thespectrum may cause an undesirable effect on the

� �analysis of copper with W�L� excitation 5 .1

Appearance of an escape peak in a spectrum is aninevitable phenomenon for a solid state detector.Au�L� excitation is suitable for the measure-1

ment of copper because its escape peak is notnear the copper signal as shown in Fig. 1.

In this study, we prepared several levels ofsamples, intentionally contaminated with copperin a range of 109 to 1010 atoms�cm2 and per-formed actual TXRF measurements on them withW�L� and Au�L� excitation. From these mea-1 1surements, we estimated the lowest level possiblefor using TXRF to detect copper contaminationon a wafer.

2. Experimental

The instruments used were the RigakuŽ . Ž .TXRF300 W and the Rigaku TXRF300 Au .

Targets were rotating anodes, and the appliedpower was 30 kV at 300 mA for the W target and35 kV at 225 mA for the Au target. The Au targetwas operated at a voltage and current differentfrom those for the W target in order to reducecontamination on the Au target by W vapor. TheAu target can be operated for 4000 h of alterna-

Ž .tive operation of full power 9 kW, 50% of timeŽ .and a minimum power 0.2 kW, 50% of time with

Fig. 1. Comparison of spectra taken with W�L� excitation and Au�L� excitation.1 1

( )T. Yamada et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 2307�2312 2309

a 30% decrement in X-ray intensity before polish-ing of its surface for the recovery of the intensity.

Ž .Based on Eq. 1 , the values of LLD of thoseinstruments for nickel were evaluated to be 2�109 atoms�cm2 for the W target and 3�109

atoms�cm2 for the Au target using a standardsample of 5�1012 atoms�cm2 of spin coatednickel. Both instruments have a double multi-layermonochromator to achieve high-energy resolution

� �of the excitation line 6 and are equipped with anx�y-� stage.

The immersion in an alkaline hydrogen perox-Ž .ide solution IAP method is effective in making a

� �low-concentration sample 7 . Using the IAPmethod, we prepared contaminated 8-inch wafersat six levels of copper concentration in the rangeof 109 to 1010 atoms�cm2. Deviation in the uni-formity of the concentration of copper on thesurface is presumed to be 10�20 in C.V.% based

� �on reported results from Fe and Ni 8 .We prepared a pair of identical samples for

each level and measured one of the pair withTXRF and the other with VPD-AAS, using a

� �solution of 2% HF and 2% H O 10 . The in-2 2strument used for VPD was the SES VRC-300T;the instrument for AAS was a Perkin-ElmerSIMAA6000. The LLD of the VPD-AAS methodfor copper is 0.13�1010 atoms�cm2 when the

LLD is represented as the concentration of thecontamination on an 8-inch wafer.

Multi-point mapping of five positions on a waferŽ . Ž . Ž . Ž . Ž .at 0,0 , 50,0 , �50,0 , 0,50 , and 0,�50 were

performed for each level of wafer by TXRF mea-surements. The same and the optimum azimuthwere used in multi point mapping measurementsbecause the background signal must be kept at aminimum at all points. This is especially impor-tant in measuring low-concentration samples.While an r-� stage could not achieve this adjust-ment, an x�y-� stage could. Both TXRF instru-ments used in this study have an x�y-� stage andsatisfactorily performed this adjustment. An azi-muth angle of ��35�, which was theoreticallyand experimentally derived to be the optimum

� �value 9 , was used for the W�L� line. An angle1of ��15�, which has been experimentally de-rived, was used for the Au�L� line.1

3. Results and discussion

Fig. 2 shows the spectra of six concentrationlevels found using W�L� excitation for 500 s at1a glancing angle of ��0.09�. The spectrum rep-resented for each level is that of the median offive points. The big tail from the giant peak of

Fig. 2. Measured spectra of wafers of different concentrations taken with W�L� excitation. Escape peaks of W�L� exist near1 1the Cu�K� line. The densities shown are the results of VPD-AAS analysis.

( )T. Yamada et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 2307�23122310

Fig. 3. Measured spectra of wafers of different concentrations taken with Au�L� excitation. No escape peaks of Au�L� are near1 1the Cu�K� line.

9.67 keV lasts to 7 keV. Escape peaks appear at7.93 keV in all spectra. The values of the densi-ties shown in the figures are the analyzed valuesof AAS. The peaks of Cu�K� are clearly visiblein the spectra of 1.87�1010, 1.27�1010, and0.81�1010 atoms�cm2.

Fig. 3 shows the spectra taken with Au�L�1

excitation for 500 s at a glancing angle of ��0.08�. The background changes gradually andmonotonically; no escape peaks exist in this en-ergy range. These spectra are the medians ofeach level as were used in Fig. 2. A large peak ofZn�K� at 8.6 keV, shown in one of the spectra,is considered the result of unintentional contami-

Fig. 4. X-Ray intensities of five-point mapping taken with W�L� excitation. The filled circles indicate the medians of the mapping1results. A linear correlation exists between X-ray intensity and the analyzed results of VPD-AAS.

( )T. Yamada et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 2307�2312 2311

Fig. 5. X-Ray intensities of five-point mapping taken with Au�L� excitation. A linear correlation clearly exists between X-ray1intensity and the analyzed results of AAS.

nation by human handling of the wafer. Cu�K�peaks are visible in the spectra from 0.81�1010

to 1.87�1010. It is easier to distinguish theCu�K� peak from the background than the peakin Fig. 2.

All the X-ray intensities measured with W�L�1and Au�L� are plotted in Figs. 4 and 5, respec-1tively. These values have been obtained by apply-

ing the analytical software as installed in theTXRF instruments. The filled circles indicate themedians for each level and the open circles indi-cate the data of other points. The x-axis repre-sents the analytical results of VPD-AAS. Al-though the sample with the lowest concentrationŽ 10 .0.058�10 is less than the LLD of VPD-AASŽ 10 .0.13�10 , we plotted the nominal data to help

Ž .Fig. 6. Coefficients of variation % for five-point mapping. The deviations are smaller with the results of Au�L� excitation than1those with the results of W�L� excitation in a lower concentration range of 109 atoms�cm2.1

( )T. Yamada et al. � Spectrochimica Acta Part B: Atomic Spectroscopy 56 2001 2307�23122312

in the investigation. In addition, we must bear inmind that each VPD-AAS value has at least anerror bar of �0.13�1010.

Using W�L� , signals of Cu�K� were de-1tected at all the points except two points of 0.81�1010. The filled spots of the medians in Figs. 4and 5 indicate that linear correlation exitsbetween TXRF and VPD-AAS in both figures.The deviation of the spots seems to be smaller inFig. 5 than in Fig. 4 in the lower region of theorder of 109. We plotted these deviations in the

Ž .coefficient of variation C.V.% in Fig. 6. Thisclearly shows that deviations of the W�L� exci-1tation and the Au�L� excitation are different in1the lower region of the order of 109. The devia-tion is small from the order of 1010 through thelower region of the order of 109 by Au�L�1excitation and remains small from the order of1010 to the higher region of 109 by W�L� exci-1tation. We consider that there is no effect of theescape peak on the result of Au�L� excitation,1while results of W�L� excitation are influenced1by the escape peak in the lower range of theorder of 109.

4. Conclusion

Using an excitation line of high-energy resolu-tion and an x�y-� stage, we were able to performactual measurements of copper contamination on

a wafer in a range lower than 1010 atoms�cm2

both with W�L� excitation and with Au�L�1 1excitation. From this we determined that a linearcorrelation exists between TXRF and VPD-AASin the range from 1010 to 109 atoms�cm2 andcould perform quantitative analysis of copper evenif the concentration was in a range of the order of109. When Au�L� excitation was used in TXRF,1deviations of the analyzed results in a lower rangeof 109 atom�cm2 were significantly smaller thanthose of W�L� excitation, enabling a more reli-1able analysis.

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