Control of implantation area in direct-current plasma immersion ion ...

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IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 31, NO. 3, JUNE 2003

Control of Implantation Area in Direct-Current Plasma Immersion Ion Implantation (DC-PIII) Ricky King-Yu Fu, Pai Peng, Xuchu Zeng, Member, IEEE, Dixon Tat-Kun Kwok, Member, IEEE, and Paul K. Chu, Fellow, IEEE

Abstract—In plasma immersion ion implantation (PIII) of planar samples such as silicon wafers in the PIII—ion-cut as well as separation by plasma implantation of oxygen (SPIMOX) processes, the only important ions are the ones arriving at the top surface. Ions implanted into the other surfaces are, in fact, undesirable as they reduce the efficiency of the power supply and plasma source and give rise to metallic contamination. We have demonstrated direct-current PIII (DC-PIII) by using a grounded grid to separate the vacuum chamber for planar sample implantation. The advantages include lower equipment cost, higher power and time efficiency, larger impact energy, and last but not least, smaller instrument footprint. In this paper, we investigate the control of the implantation area by adjusting the radius of the extraction hole, the distance between the conducting grid and the sample, and the radius of the wafer stage. Theoretical simulation is conducted using particle-in-cell and experiments are also carried out. Our results indicate that the implanted area increases with the radius of the extraction hole and wafer stage, but decreases with a larger distance between the grid and sample. The are the largest, followed effects of the extraction hole radius by the placement of the sample to the conducting grid . The wafer stage poses the least influence in this respect, but a proper wafer stage dimension improves the lateral implant dose and incident angle homogeneity. Our simulation and experimental results suggest optimal ratios of these parameters for each wafer size. Index Terms—Direct-current plasma immersion ion implantation (DC-PIII), implant uniformity.

I. INTRODUCTION

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EAM-LINE ion implantation is the traditional means to fabricate commercial silicon-on-insulator (SOI), but the need to scan the ion beam makes it a fairly costly technique especially for large wafers [1]. Plasma immersion ion implantation (PIII) has been demonstrated to be an excellent technique to treat large and irregularly shaped components as well as planar samples in many microelectronic applications [1]–[8]. Typically, PIII experiments are conducted in the pulsed mode with pulse durations ranging from several microseconds to several tens of microseconds. However, pulsed-mode PIII has several drawbacks. First, stray ions impacting the edge and bottom parts of the wafer stage do not contribute to the useful dose, but they can sputter metallic ions from these surfaces and reduce Manuscript received June 25, 2002; revised January 26, 2003. This work was supported by Hong Kong RGC CERG # CityU 1013/01E or 9040577. The authors are with the Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong (e-mail: paul.chu@ cityu.edu.hk). Digital Object Identifier 10.1109/TPS.2003.811645

the efficiency of the power modulator. Second, the duty cycle in pulsed-mode PIII is typically quite low. This, however, can be beneficial if sample cooling is not adequate, but with sufficient sample cooling, the efficiency is not optimized. Third, the finite rise and fall times of the voltage pulses can introduce low-energy ion implantation. These low-energy ions broaden the implant in-depth distribution and introduce unnecessary damage that may affect the yield of the SOI fabrication process. In particular, a broad oxygen distribution may make precise control of the thickness of the silicon layer in the SPIMOX process difficult. Steady-state directed-current PIII (DC-PIII) is a novel technique especially suitable for the processing of planar objects like silicon wafer. This process boasts high quality, high throughput, and low-instrument footprint, making it more acceptable to the semiconductor community than pulsed mode PIII [9], [10]. The cost of the equipment can also be reduced, as only a dc power supply is required in lieu of the expensive power modulator used in conventional PIII. In DC-PIII, the target is biased using a negative dc voltage while the vacuum chamber is separated into two parts with a conducting grid made of a compatible material (such as silicon coating) to avoid contamination, as shown in Fig. 1. The purposes of introducing the conducting grid are to confine the plasma inside the top chamber, to stop the expansion of ion sheath beyond the grid to avoid plasma extinction, and to enhance stability enabling long-pulse and dc operation. As the plasma is confined above the grid, the loss of ions and electrons in the plasma is subsequently small [11], and so the efficiency of the plasma source is enhanced. The use of a variable extracting hole in the grid can efficiently control both the amount of the “drifting out” ions and implantation area, alleviate ion loss, and minimize contamination due to sputtering of the side and bottom of the exposed sample stage. Besides, as there is no voltage rise or fall time in DC-PIII, the low-energy ion component can be dramatically reduced. Normal angle implantation across the entire wafer can be more easily achieved because in steady-state DC-PIII, the ion sheath has already propagated to the grid and ions only track the electric field between the top surface of the sample and grid. All in all, more precise control and better efficiency can be accomplished using DC-PIII. This paper focuses on the determination and control of the implantation area by adjusting the radius of the extraction hole , the distance between the conducting grid and the sample , and the radius of the wafer stage . We theoretically and experimentally determine their relationship and contributions.

0093-3813/03$17.00 © 2003 IEEE

FU et al.: CONTROL OF IMPLANTATION AREA IN DC-PIII

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The centered difference approach [17] is used to approximate the potential and

Therefore, (1) can be rewritten in a 2-D coordinate system as

1

Fig. 1. Schematic diagram of the DC-PIII system in the City University of Hong Kong. , , and are the variables. The plasma is generated by the inductively coupled RE coils and the simulation region is shown.

rH

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II. NUMERICAL SIMULATION AND EXPERIMENT The simulation is conducted using particle-in-cell (PIC) [12]–[16] modeling with the following five assumptions. 1) The potential above the grid reflects the plasma potential and positive ions diffusing into the lower part of the grid according to the density gradient are considered as a sheet of particles/ions just below the conducting grid. 2) Ions are accelerated by the electric field as soon as they diffuse through the grid, and initially, there is no plasma in the lower part. 3) New ions extracted from the grid hole will be placed at the same location. 4) The space charge density is approximately equal to zero in the lower part during the dc mode. 5) Secondary electrons created during implantation are immediately absorbed by the chamber walls and grounded grid. As the lower part of the chamber has a cylindrical symmetry, the simulation region can be reduced to a plane shown in Fig. 1. The extension of the potential beneath the grid can be solved by Laplace’s equation in two-dimensional (2-D) cylindrical coordinates (1)

The trajectories and motions of ions are governed by Newton’s equation of motion in cylindrical coordinates. The interior dimensions of the instrument are based on our semiconductor DC-PIII apparatus. The radius and height of the vacuum chamber are 38 and 100 cm, respectively. The 5.6-cm-thick silicon wafer chuck is supported by a metal rod 0.65 cm in radius and of variable length connected to a high negative distance between the grid voltage. The radius of the grid and sample , as well as the radius of the wafer , can be varied. Hydrogen ions are chosen for the simulation and experiments since a high hydrogen implant dose will create blistering on the silicon surface enabling easy determination of the implantation area. Hydrogen implantation was carried out using 20-kV target bias at a working pressure at 3.5 10 torr for 3 min. The implantation dose based on previous secondary ion mass spectrometry (SIMS) analysis was in the range of 1 to 2 10 cm and to create surface blistering, the silicon wafer was annealed in air at 550 C for 1 h. III. RESULTS AND DISCUSSION Different placements of the sample holder geometry affect the local electric field and the ion trajectories. Fig. 2 shows the potential contour plot for a 100- or 150-mm wafer stage at 20-kV or 20 cm. Comparing Fig. 2(c) and applied voltage for 2(d) to 2(a) and 2(b), flatter contours are observed when the sample stage is closer to the grid. We can also change the potential contours by using different sample stage geometry, for instance, by increasing the dimension of the wafer holder using a guard-ring [18] and adopting a beveled edge on the bottom of the sample chuck [19]. In general, the incident angle of the ions does not depend on the charge state and mass of the ions but is inherently determined by the local electric field in the lower part of the chamber, the tangential angle of the change of the axial potential, and the change of the radial potential (2)

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Fig. 2. Evolution of the potential contour lines in DC-PIII. (a) 100-mm wafer holder for wafer holder for = 20 cm. (d) 150-mm wafer holder for = 20 cm.

H

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In steady-state DC-PIII, since the ion sheath has already reached the grid and ions from the top chamber are pulled immediately through the grid hole by the electric field exerted by the sample chuck, flat field contours reduce nonnormal angle incidence while not distorting and broadening the depth profile. This is crucial for ion-cut/layer transfer and SPIMOX for precise control of the thickness of SOI as well as reduction of the damaged zone. Five representative ion trajectories are displayed in Fig. 3 to describe the influence of the implantation area using different instrumental setups. The ion trajectories exhibited in Fig.3(a) and 3(b) are derived using a 150-mm stage, 30-cm grid radius, and different values of . The incident ions impact mainly the midplane of the stage, and some ions overlap, cross each other, and impinge into the other side of the stage. Even by raising the stage to be closer to the grid as depicted in Fig. 3(b) to achieve disk-like shape potential contours for a larger implantation area, overlapping of the impinging ions cannot be over-

H = 70 cm. (b) 150-mm wafer holder for H = 70 cm. (c) 100-mm

come. Fig. 3(c)–3(e) shows the results of smaller grid size. Although the situation is improved, some of the ions are still implanted into the edge of the stage or supporting rod. Such phenomena are undesirable because of the introduction of sputtered contamination [18]. Therefore, to accomplish 100% top surface implantation and control the implantation area, a proper relationship among the radius of the wafer stage , radius of the , and distance between the grid to the top of the stage grid is necessary. In accordance with our previous works [20], the ratio for long pulse/DC-PIII 1 4 2.5 2. optimal Accordingly, a disk-like chamber, unlike a cylindrical chamber commonly used in pulsed mode PIII, is more suitable for long pulse/DC-PIII. Our work shows that varying the grid hole and the distance has a large impact on implant uniformity and area. Two series of experiments were carried out to investigate and . Silicon wafers of 150-mm the relationship between diameter were implanted using hydrogen DC-PIII at a hydrogen


Fig. 3.

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Simulated trajectories of ions implanted from the grid into the wafer stage at

pressure of 0.35 mtorr for 3 min. The samples were subsequently annealed in air at 550 C for 1 h to achieve surface blistering and reveal the implanted area. During annealing, the implanted hydrogen atoms coalesce along the implant projected range where there is a high density of defects after implantation to form buried microcavities. Upon annealing, the internal pressure causes local surface exfoliation manifesting in surface blistering [21], [22]. Surface blistering only takes place at the hydrogen implanted region and there are only very minor effects of implantation area caused by lateral hydrogen diffusion. The surface bubbles about 1 m in diameter are visible under the naked eyes and easily observed using scanning electron microscopy (SEM), as shown in Fig. 4. In the first set of samples implanted using a constant , the implanted area

020-kV sample voltage.

increases with a larger grid hole diameter as shown in Fig. 5 and the trend is quantitatively in agreement with our simulation results (the error bars in the figure indicate the unclear blistering boundaries). Another set of samples implanted using a constant show similar quantitative agreement with our theoretical and are the two main data as shown in Fig. 6. Hence, parameters determining the implantation area in DC-PIII. will change the potential contours in the lower Varying will affect the numbers of hychamber while changing should drogen ions diffusing from the top chamber. Hence, be below 25 cm with respect to the placement of the sample to the conducting grid in our equipment. In order to improve the lateral implant uniformity, a guard-ring-type extension can be added to the sample chuck to flatten the electric field in the

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IV. CONCLUSION Our theoretical simulation and experimental results show that , , and , the implanted area can be by properly adjusting conveniently controlled in DC-PIII. The novel technique of employing a grounded conducting grid to separate the chamber into two parts is demonstrated. Hydrogen implantation induced surface exfoliation shows that hydrogen ions not only can be extracted from the upper chamber through the conducting grid to implant onto the silicon substrate, but also the implanted area can be effectively controlled. Therefore, DC-PIII is an economical and efficacious technique for the treatment of planar materials and components such as SOI. Fig. 4. SEM micrographs of the surface blisters showing the pit (A) and fragment (B) after hydrogen PIII and annealing.

Fig. 5. Experimental and simulated implanted areas in DC-PIII obtained by adjusting the radius of grid hole at a constant distance between the grid and sample .

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Fig. 6. Experimental and simulated implanted areas in DC-PIII obtained by adjusting the distance between the conducting grounded grid to sample using a constant grid hole size .

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vicinity of the wafer to achieve primarily normal incidence. In conclusion, with respect to the control of the implanted area, our results show that the effects of the extraction hole radius are the largest, followed by the placement of the sample to the conducting grid , and has the smallest effects.

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[17] D. T. K. Kwok, P. K. Chu, and C. Chan, “Ion dose uniformity for planar sample plasma immersion ion implantation,” IEEE Trans. Plasma Sci., vol. 26, pp. 1669–1679, Dec. 1998. [18] P. K. Chu, R. K. Y. Fu, X. C. Zeng, and D. T. K. Kwok, “Metallic contamination in hydrogen plasma immersion ion implantation of silicon,” J. Appl. Phys., vol. 90, pp. 3743–3749, 2001. [19] X. B. Tian, R. K. Y. Fu, L. P. Wang, and P. K. Chu, “Influence of target geometry on maximum electric field in plasma immersion ion implantation,” in Proc. 28th Int. Conf. Plasma Science (ICOPS)/13th Int. Pulsed Power Plasma Science (PPPS-2001), Las Vegas, NV, June 17–22, 2001, Paper P4J18, p. 561. [20] D. T. K. Kwok, X. C. Zeng, C. Chan, and P. K. Chu, “Direct current plasma immersion ion implantation using a grounded conducting grid,” J. Appl. Phys., vol. 87, pp. 4094–4097, 2000. [21] M. K. Weldon, V. E. Marsico, Y. J. Chabal, A. Agarwal, D. J. Eaglesham, J. Sapjeta, W. L. Brown, D. C. Jacobson, Y. Caudano, S. B. Christman, and E. E. Chaban, “On the mechanism of the hydrogen-induced exfoliation of silicon,” J. Vac. Sci. Technol. B, vol. 15, pp. 1065–1073, 1997. [22] J. Grisolia, G. Ben Assayag, A. Claverie, B. Aspar, C. Lagahe, and L. Laanab, “A transmission electron microscopy quantitative study of the growth kinetics of H platelets in Si,” Appl. Phys. Lett., vol. 76, pp. 852–854, 2000.

Ricky King-Yu Fu was born in Fujian, China, on April 17, 1974. He received the B.S. degree in materials science and technology and the M.S. degree in plasma physics from the City University of Hong Kong, Kowloon, in 2000 and 2002, respectively. He is working toward the Ph.D. degree at the same university. His current research interests include plasma processing, surface modification, semiconductor processing, and plasma source ion implantation configurations and applications.

Pai Peng was born in Jilin, China, on July 19, 1976. He received the B.S. degree in materials processing engineering and the M.S. degree in welding robotics from Harbin Institute of Technology, Harbin, China, in 1999 and 2001, respectively. He is working toward the Ph.D. degree at the same university. He is currently a Research Staff Member of the Plasma Laboratory, the City University of Hong Kong, Kowloon. His research interests include robotic off-line programming, application of artificial intelligence, plasma processing, and surface modification.

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Xuchu Zeng (M’97) was born in Sichuan, China, on February 12, 1964. He received the B.S. degree in electrical engineering from Xian JiaoTong University, Xi’an, China, in 1985, the M.S. degree in plasma physics from Southwestern Institute of Physics, Chengdu, China, in 1991, and the Ph.D. from the City University of Hong Kong, Kowloon, in 2001. He was an Assistant Engineer from 1986 to 1991, and Engineer from 1992 to 1993, and a Senior Engineer from 1994 to 1995, all with the Southwestern Institute of Physics. From June 1995 to June 1997, he was a Research Staff Member of the Plasma Laboratory, City University of Hong Kong. He is currently Manager of Technology, Sales, and Marketing with Chengdu PulseTech Power Company, Chengdu, China. His research interests include plasma immersion ion implantation, equipment design, as well as applications.

Dixon Tat-Kun Kwok (M’98) received the B.Sc. degree in physics and the Ph.D. degree in solid state physics from King’s College London, University of London, U.K., in 1988 and 1993, respectively. From April 1994 to March 1995, he was a Postdoctoral Fellow at the Surface Physics Lab, Fudan University, Shanghai, China. From April 1995 June 1996, he was a Research Associate with the Physics Department, Hong Kong University of Science and Technology. He is currently a Research Fellow in the Department of Physics and Material Science, City University of Hong Kong, Kowloon. He has authored published papers on topics related to photoluminescence and photoabsorption of point defects in silicon, reflectance difference spectroscopy, and plasma immersion ion implantation.

Paul K. Chu (M’97–SM’99–F’03) received the B.S. degree in mathematics from The Ohio State University, Columbus, in 1977, and the M.S. and Ph.D. degrees in chemistry from Cornell University, Ithaca, NY, in 1979 and 1982, respectively. He joined Charles Evans and Associates, Redwood City, CA, in 1982, and started his own business in 1990. He is currently Professor (Chair) of Materials Engineering with the Department of Physics and Materials Science, City University of Hong Kong, Hong Kong. He also holds concurrent Professorships at Fudan University, Shanghai, Peking University, Peking, China, Southwest Jiaotong University, Chengdu, and the Southwestern Institute of Physics, Chengdu, China. His research activities include plasma-processing technology, microelectronics processing, and materials characterization. He is author/co-author of 450 publications, has seven U.S. patents, and serves on the Editorial Board of Nuclear Instruments and Methods in Physics Research B Dr. Chu is a Fellow of the Hong Kong Institute of Engineers (HKIE) and an Executive Member of the International Plasma-Based Ion Implantation Committee. He also serves on the Engineering Panel of the Hong Kong Research Grants Council (RGC).