Development of Post-CMP Cleaners for Better Defect Performance

8/10/2019 Development of Post-CMP Cleaners for Better Defect Performance

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Development of Post-CMP Cleaners for Better Defect Performance

Cuong Trana, Peng Zhang

a, Laisheng Sun

a, Naresh Kumar Penta

b, Uma Rames Krishna

Lagudub, Devon Shipp

b, S.V. Babu

b

aATMI, Inc USA

bClarkson University, USA

In the copper CMP process, organic residues that are related to

Benzotriazole (BTA) adsorbed on copper surface after Cu CMP

process have to be removed during the cleaning. In order to

address this organic defect issue, we present here the study of the

performance of BTA removal by post-CMP cleaners using copper

particles as substrates instead of copper wafers. In this work,

different copper particles including Cu(0), Cu(I) and Cu(II) oxide

particles with high surface area, were chosen to study the removal

of BTA adsorbed on different copper states. TGA and UV-Vis

spectra were used to detect and quantify the BTA removalefficiency. The results by different post CMP cleaners on various

Cu particles will be presented.

Introduction

Post-CMP cleaners are used to remove various defects from the chemical mechanical

planarization (CMP) process. With the shrinking circuitry as the nodes advance to 20nm

and below, post-CMP cleaning faces greater challenge in improving cleaning

effectiveness and electrical performance. Continuous efforts have been made indeveloping post-CMP cleaners with improved cleaning performance to remove particles,

metals and organic matters without damaging copper surface, as well as enabling superiorelectrical performance.Among various defects from CMP process, organic defects have been one of the major

challenges for post-CMP cleaning (1-2). In the copper CMP process, Benzotriazole

(BTA) is used in most slurries to provide the protection to the copper surface duringpolishing. Copper ions can form a cuprous BTA complex (CuBTA) in the form of (3)

passivating films with multilayer polymeric structures (3). These BTA films and residues

are the main source of organic defects that have to be removed during the post CMPcleaning step (1). Therefore, understanding the effect of post-CMP cleaning chemistries

on BTA removal is important for the development of next generation post-CMP products

with better defect performance.

Most prior studies of BTA adsorption and removal on copper surface are carried out oncopper wafers which have very small surface areas, leading to poor reproducibility and

non-conclusive results. In addition, the results were usually confounded by the presence

of both Cu(I) and Cu(II) oxides on copper wafer surfaces. In this paper, we developed themethod of using Cu(0), Cu(I) oxide and Cu(II) oxide particles to investigate the impact of

post-PCMP cleans on BTA removal.

Previously, Cu(I) oxide and Cu(II) oxide particles have been used to investigate the effectof various components in slurries on the performance for chemical mechanical polishing

(4-6). By controlling the surface state of copper oxide, the mechanism of complexing

agents with amine and carboxyl function groups in slurries for copper CMP was

ECS Transactions, 44 (1) 565-571 (2012)

10.1149/1.3694370 The Electrochemical Society

565 )unless CC License in place (see abstractecsdl.org/site/terms_useaddress. Redistribution subject to ECS terms of use (see128.153.13.164Downloaded on 2014-04-08 to IP
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elucidated (4). For the pCMP work, the large surface area and the controlled surface

states of these particles made them ideal candidates for quantitative investigation of BTA

adsorption and removal. In this paper, different copper particles including Cu(0), Cu(I)and Cu(II) oxide particles were chosen , and TGA and UV-Vis spectra were used as

measurement for the BTA removal efficiency.

Experimental

Copper metal particles, Cu(I) oxide and Cu(II) oxide particles were purchased from

Alfa Aesar with purity of > 99% and used without any further treatment. These particleswere characterized by XRD to verify that their purity was suitable for next step

experiments.

The BTA adsorption on copper particles was carried out by dispersing 1% copper

particles in 10 mM BTA solution. The suspension was stirred for 1 hour to allow BTA to

fully adsorb on the surface of copper particles. After centrifuging, these particles werewashed thoroughly by water and centrifuged again to obtain BTA adsorbed copper

particles. Then the particles were transferred to diluted post-CMP formulation solutions.

The suspensions were stirred for certain time and then centrifuged. The supernatantsolutions were used for UV-Vis measurements. The copper particles were washedthoroughly by water and then dried at 75

oC overnight before the TGA measurements.

Commercially available post-CMP cleaner, PlanarCleanTM

from ATMI, along withother post-CMP formulations were used for evaluation of BTA removal from copper

particles. These pCMP cleaners were freshly prepared and diluted to certain ratios before

testing. DI water with different pH adjusted by KOH was also used for evaluation of pHeffect on the BTA removal from copper particles.

Results and Discussion

Characterization of Copper Particles

The SEM images of copper particles were shown in Figure 1. Copper metal

particles had round shape while Cu(I)O and Cu(II)O particles had irregular shapes. The

BET specific surface area and particle sizes of these copper particles were shown inTable 1. The average size of copper metal particles was 1.6 m, smaller than those of

Cu(I)O and Cu(II)O particles. The specific surface area of copper metal particles was

0.69 m2/g. Compared to the surface area of a flat copper surface, 1 g of copper metal

particles has about the same surface area of a copper wafer with the surface area of 6900

cm2. This makes it much easier to use instrumental methods such as UV-Vis and TGA to

detect BTA adsorption and desorption from these particles.

Figure 1 SEM images of Cu metal particles, Cu(I)O particles and Cu(II)O particles




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Table 1. BET specific surface area and particle size of copper metal and copper oxide

particles

Particles Surface area (m2/g)** Particle size (m)*

Cu (-625 mesh) 0.69 1.6

Cu(II)O (-325 mesh) 0.17 12.0

Cu(I)O (-325 mesh) 0.53 9.5

* Particle size measurements were conducted on Malvern Mastersizer 2000 instrument.Copper and copper oxide particles were dispersed in water and sonicated to make

uniform suspensions.

** Specific surface area of copper particles was measured by the BET method.

TGA Study of BTA Removal from the Surface of Copper Particles

The adsorption of BTA molecules on copper surface with different oxidationstates was studied by the TGA method. Figure 2 showed the TGA curves of Cu metal,

Cu(I) oxide and Cu(II) oxide particles after BTA adsorption. For all copper particles, the

weight loss started from about 300oC, while pure BTA molecules started to lose weight at

about 150oC. This temperature difference could be attributed to the binding interaction of

BTA molecules on the surface of copper particles. The weight loss from the TGA curves

in Figure 2 could be used to estimate the amount of BTA adsorbed on copper particles.Higher the weight loss in TGA curves, the more BTA molecules adsorbed on copper

surface. There was about 1.7% weight loss of BTA molecules adsorbed on Cu metal

particles, which is more than 1.0% on Cu(I) oxide and 0.4% on Cu(II) oxide particles.

Figure 2. TGA analysis of BTA adsorbed copper metal particles and copper oxide

particles.

These BTA adsorbed copper particles were subsequently used to evaluate the cleaning

performance of post CMP cleaners. The first step is to investigate the pH effect on BTAremoval using pH adjusted water with nitric acid or KOH. The cleaning efficiency based

on BTA weight loss was estimated from the following equation;




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Table 2 showed the cleaning efficiency of BTA from copper particles. It is clear that

BTA was barely removed from copper surface at pH below 8 regardless of the copper

surface states. When pH of water increased over 10, there was an obvious increase of the

BTA removal for all copper particles. When pH of water increased to 12, BTA cleaningefficiency further increased significantly. This pH effect can be explained by the

deprotonation of BTA molecules which have a pKa of 8.2 at pH above 10.

Among all Cu particles, BTA molecules were more easily removed from Cu metal andCu(II) oxide particles than from Cu(I) oxide particles. In fact, at pH 12, most BTA

molecules were removed from Cu(II) oxide particles while much less BTA molecules

were removed from Cu(I) oxide particles. This is consistent with the previous report offormation of multi-layer cuprous BTA complex on copper(I) surface (3). The cuprous

BTA complex layers cannot be removed with water by simply adjusting to higher pH.

Table 2: BTA removal from the surface of different copper particles by water with

different pH adjusted by KOH or HNO3

pH of water

BTA cleaning efficiency %

Cu Cu(I)O Cu(II)O

4 0 0 0

6 3 2 2

8 3 3 7

10 18 10 43

12 58 15 89

UV-Vis Spectra Study of BTA Removal from Copper Particles by PlanarClean

The second method of evaluating BTA removal was conducted via UV-Vis spectra of the

supernatant solutions. BTA solutions have distinct peaks around 270nm wavelength, asshown in Figure 3. A linear calibration curve was obtained at pH 10, making it possible

for quantitative analysis of BTA concentration in the solution. Even though this method

does not directly probe the BTA concentration on copper surface, it is much moreconvenient and faster than the TGA method. Therefore the majority of subsequent work

is conducted using this method.

x100pH4atlossTGA wt

pHat xlossTGA wt-4pHatlossTGA wtpHat xEfficiencyCleaning =




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a) b)

Figure 3. a) UV-Vis spectra of BTA solutions at different concentrations at pH 10; b)Calibration curve of BTA conc. at pH 10.

Figure 4. UV-Vis spectra of supernatants obtained from 60:1 PlanarCleanTM

solutions

before and after treatment of BTA adsorbed copper(I) oxide particles. The inset is the

enlarged UV-Vis spectra showing the absorption between 400 nm and 800 nm.

TGA results from the previous section showed that it is difficult to remove BTA

molecules from the surface of Cu(I) oxide particles using pH effect alone. In the next setof studies, formulated pCMP cleaners such as the commercially available post CMP

cleaner, PlanarCleanTM

, were tested via UV-Vis method. Figure 4 showed the UV-Vis

spectra of the supernatant of 60:1 diluted PlanarClean

TM

solutions after BTA removalprocess. Diluted PlanarCleanTM

itself showed some absorption at about 270 nm, and its

spectrum was subtracted from the sample signal. As shown in Figure 4, after BTAremoval, the absorption peak at 270 nm was very high, indicating the increase of BTA

concentration in the supernatant. This suggests that PlanarCleanTM

was very effective in

removing BTA molecules from Cu(I) oxide surface. It is also noted that there was anextra adsorption peak located at 600 nm, indicating a new form of Cu complex formed in

PlanarCleanTM

solution during the BTA removal process (4).

Table 3 showed the UV-Vis absorption data of PlanarCleanTM

solutions with different

dilution ratios after the treatment of BTA adsorbed copper particles. With dilution ratio




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increased from 30 to 200, the absorbance at 270 nm decreased accordingly, indicating

that the efficiency of BTA removal from Cu(I) oxide particles decreased as the function

of dilution ratios. The absorbance at 600 nm also decreased with the increase of dilutionratios.

Table 3 The absorption results of PlanarClean solutions with different dilution

ratios after treatment of BTA adsorbed Cu(I) oxide particles.

Dilution ratios

of PlanarCleanTM

pH UV Absorbance

at 270 nm at 600 nm

30:1 12.2

8

0.92 0.062

60:1 11.9

8

0.68 0.043

90:1 11.8

1

0.30 0.025

120:1 11.59

0.21 0.019

200:1 11.45

0.11 0.01

The new Cu complexes at 600nm wavelength seem to be a useful indicator of cleaning

from the surface of Cu(I) oxide particles. Figure 5 showed the absorbance comparison at600 nm between three other post-CMP cleaners with PlanarClean

TM. The results showed

that the cleaner PC-C had the highest absorbance among all cleaners. It also had highest

absorbance at 270 nm indicating the highest BTA removal from copper surface. This is

consistent with the expected behavior of a new type of cleaning agent in PC-Cformulation. In addition, this trend is confirmed by the real tool evaluation data which

indicated that the PC-C formulation had the best cleaning performance among theevaluated cleaners.

Figure 5. Comparison of copper complex formation in different PCMP formulations




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Conclusions

Copper particles had higher surface area than copper wafers that are normally used in

study of BTA removal during post CMP cleaning process. Using copper metal, Cu(I)

oxide and Cu(II) oxide particles, BTA removal from copper surface could be analyzed by

TGA and UV-Vis spectra. The results showed that pH played a very important role inBTA removal from copper particles, and higher pH favors the BTA removal efficiency.

Among different Cu states, BTA on Cu(I) oxide surface is more difficult to be removed

than from Cu(0) and Cu(II) oxide surfaces. Formulated cleans, such as PlanarCleanTM

isvery effective in removing BTA from Cu(I) oxide surface. Further optimization with

new cleaning agents can enhance the BTA removal, as indicated by UV-Vis studies.

Acknowledgements

This paper is made possible through funding from ATMI for the collaboration projectbetween ATMI and Clarkson University. The authors acknowledge Jeff Barnes from

ATMI for various discussions.

References

1. Todd Buley, Yakov Epshteyn, Mike Kulus, Cuong Tran, Kyle Bartosh, Darryl

Peters, Chris Watts, in MICRO Magazine, Wet Surface Technologies,October/November, 2005.

2. D. Peters, E. Walker, K. Bartosh, J. Barnes, C. Tran, and C. Watts, The 2nd

PacRim International Conference on Planarization CMP and its ApplicationTechnology, pp. 68-72 (November 2005).

3. Desmond Tromans,J. Electrochem. Soc.,145, L42 (1998).4. Venkata R.K. Gorantla, Dan Goia, Egon Matijeviand S.V. Babu,J. Electrochem.

Soc.,152, G912 (2005).5. V. Meled, S.V. Babu, E. Matijevi, J. Electrochem. Soc.,156, H460 (2009).6.

Zhenyu Lu, Niels P. Ryde, S. V. Babu, Egon Matijevi, Langmuir, 21, 9866

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Development of Post-CMP Cleaners for Better Defect Performance