Surface Mediated Particle-particle Aggregation During CMP Slurry Delivery and Handling

Transcription

1 Surface Mediated Particle-particle Aggregation During CMP Slurry Delivery and Handling Yongqing Lan, Craig Burkhard and Yuzhuo Li 1 Center for Advanced Materials Processing, Clarkson University, Potsdam, NY, USA 2 St. Lawrence Nanotechnology, Potsdam, NY, USA 2/21/ Levitronix CMP Conference 1

2 Presentation Outline Objectives How and where the aggregation take place? Formation of large particles Kinetic energy and DLVO barrier Importance of surface mediation Surface adsorption and particle-particle interaction Some experimental findings LPC under various pumping conditions Surface defects (LPC and scratch counts) Conclusions 2/21/ Levitronix CMP Conference 2

3 Objectives and Questions Elucidate the mechanism for large particle (LP) formation How are they formed during slurry pumping and handling? Identify the conditions or stress that induce the formation of large particle Where are they formed? Homogeneously across the slurry or some particular spots? Correlate the external stress and type of LPC Not all LPC are born the same. How do they impact on defect count on polished wafers differently? Recommendations How to prevent such defects? 2/21/ Levitronix CMP Conference 3

4 LPC and Defect Count E. Remsen, S. Anjar, D. Boldridge, M. Kamiti, S. Li, T. Johns, C. Dowell, J. Kasthurirangan, and P. Freeney, Analysis of Large Particle Count in Fumed Silica Slurries and Its Correlation with Scratch Defects Generated by CMP, Journal of The Electrochemical Society, 153 (5) G453-G461(2006). 2/21/ Levitronix CMP Conference 4

5 LPC and Slurry Handling - It can come from many sources 2/21/ Levitronix CMP Conference 5

6 Are speeding particles more likely to aggregate? More specifically, how fast the Particles have to travel to posse Enough kinetic energy to Overcome the energy barrier Described by the DLVO theory? Assuming upon collision all kinetic Energy is converted to potential Energy in 100% yield Are the speeding particles behave the same way in a centrifugal pump as in a Bellow pump? Ref: D. Parfitt in Dispersion of Powders in Liquids, 3 rd ed. (App. Science, London, 1976). 2/21/ Levitronix CMP Conference 6

7 Basic Information: Energy Barrier vs. Particle Size D. Parfitt in Dispersion of Powders in Liquids, 3 rd ed. (App. Science, London, 1976). 2/21/ Levitronix CMP Conference 7

8 Homogeneous gelation process where almost all particles in the region are involved Annual Review of Physical Chemistry, Vol. 39, 1988 (Annual Review Inc.) 2/21/ Levitronix CMP Conference 8

9 Product of gelation process. Defect causing LP? 2/21/ Levitronix CMP Conference 9

10 2/21/ Levitronix CMP Conference 10

11 Are speeding particles more likely to aggregate? Some more basic information and calculations: E kinetic =1/2 (mass)*(velocity) 2 = 1/2 mv 2 We shall set the thermal energy of an object equal to its kinetic energy to see how fast it is moving, assuming particles are free from any restrictions from the environment. E kinetic =E thermal 1/2 mv 2 = 1/2 kt v=(kt/m) 1/2 1 micron bead m = 4*10-15kg v = 700 micron/s 100 nm silica m = 5*10-18kg v = 9000 micron/s Oxygen Molec. m = 5*10-26kg v = 270 m/s 2/21/2007 September 28, Levitronix 2006 CMP VMIC Conference 11

12 Review: for 100 nm particles, in order to have enough potential energy to overcome the barrier, they have to travel 50 kt or 0.45 m/s Using Levitronix BPS-4 pump as an example, a portion of the liquid that is near a rotating impeller with a speed of 6000 rpm (100 rps). As the impeller is 68 mm long which gives the linear velocity of the fluid at the edge as high as v = 2pRrpm = 3.14 x m x 100 = 21 m/s. This is significantly higher than what is required to overcome 50kT of energy barrier. So the speeding particles do have enough energy to over come the DLVO barrier to aggregate, right? 2/21/ Levitronix CMP Conference 12

13 Not so fast, We need to remember that in order for two particles to collide with such speed the velocity difference between the two particles has to be greater than 0.45 m/s. If we assume the fluid gap is 1 cm, the shear rate for this system will be = 21/0.01 m x m/s = 2100 /s. In order for two 100 nm particles to collide, the maximum distance between the two planes has to be less than 100 nm apart. The maximum rate difference between the two particles is thus 210 um/s or m/s without considering any thermal contribution. There is not enough energy at all. 2/21/ Levitronix CMP Conference 13

14 What about a bellow pump? 2/21/2007 September 28, Levitronix 2006 CMP VMIC Conference 14

15 Let s consider a typical case with a typical ball checkvalve. In a pump with 1" fittings, the valve ball has a diameter of approximately 20 mm and the travel is also about 20mm. It takes approximately 3-10 ms to shut a valve. Assuming the slowest scenario, the fluid carried by the ball travels at about 20 mm/10 ms = 2 m/s. In a faster senario, the fluid travels at 20 mm/3ms = 6.7 m/s. At the first time of valve closing, the linear velocity difference between the two particles equals to the traveling speed of the ball which is m/s. 2/21/ VMIC 15

16 Aggregation Enhanced at Liquid Air Interface Figure 17. Two video micrographs of monlayers of 3 um particles forming meso structures. (a) cluster s from dimmers to examples containing of 20 particles, (b) larger and more complex structures. WILLIAMS D. F. ; BERG J. C., The aggregation of colloidal particles at the air-water interface, J. colloid and interface sci., 1992, 152 (1), /21/ Levitronix CMP Conference 16

17 Enhanced Aggregation At Solid Interface Figure 18. Small filled circles: number density of the adsorbed particles, Large filled circles: influx density. Open circles: outflux density. Yves Lu thi, Jaro Ricˇka, and Michal Borkovec, Colloidal Particles at Water-Glass Interface: Deposition Kinetics and Surface Heterogeneity, J. colloid and interface sci 206, (1998). 2/21/ Levitronix CMP Conference 17

18 Surface Adsorption on Oxide Wafers Increased with Time 2/21/ Levitronix CMP Conference 18

19 Slurry Processed in Different Containers (Glass vs. Plastic) Glass PP LPC Time (hr) 2/21/ VMIC 19

20 Optical Images of Processed and Unprocessed Slurries U U U P P P 2/21/ Levitronix CMP Conference 20

21 Processed Slurries Using Centrifugal and Bellow Pumps C C C B B B 2/21/ Levitronix CMP Conference 21

22 A Closer View of Processed Slurry with Large Particles 2/21/ Levitronix CMP Conference 22

23 Slurry Processed by Centrifugal and Bellow Pump 2/21/ Levitronix CMP Conference 23

24 Slurries Processed by Centrifugal Pump Yielded Lower Defects Short process Long process LPC = Large particle count on polished wafer, CM = scratch count 2/21/ Levitronix CMP Conference 24

25 Summary There is a difference between speeding particles accelerated by centrifugal pump and impinging particles forced by bellow pumps The former does not lead to high energy collision. The latter does There is a difference between shear induced gelation process and surface induced particleparticle aggregation The former may increase the LPC in slurry and on polished wafer. The latter increases the chance to have scratch 2/21/ Levitronix CMP Conference 25