Plasma immersion ion implantation doping using a ... - IEEE Xplore

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 39, NO. IO, OCTOBER 1992

Plasma Immersion Ion Implantation Doping Using a Microwave Multipolar Bucket Plasma Shu Qin, Nicol E . McGruer, Member, IEEE, Chung Chan, Senior Member, IEEE, and Keith Warner

Abstract-Using plasma immersion ion implantation, silicon has been doped with boron in a high-voltage pulsed microwave multipolar bucket plasma system. Diborane gas (1%) diluted in helium is used as an ion source. A sheet resistance of 57 Cl/o and an implanted dose of 1.9 x 10'5/cm2 are obtained in 10 min when the target potential is pulsed to -10 kV with a 1% duty cycle. The boron profile in the silicon substrate is different than that predicted for a conventional 10-keV ion implantation. Silicon p-n junctions fabricated by this technique are of good quality.

I. INTRODUCTION HE PLASMA immersion ion implantation (PIII) technique [ l ] , [2] has been proposed as a low-cost, low-energy implantation method with applications in semiconductor manufacturing including shallow junction doping and trench sidewall doping [3]-[5]. It is a nonline-of-sight doping process capable of high average current at very low implant energies ( < 10 keV compared with > 10 keV of conventional ion implantation techniques). The process is performed by repetitively applying a large negative voltage pulse to a silicon wafer immersed in a plasma of dopant ions. Ions are accelerated by the target potential and are implanted in the target. In comparison with dc or RF plasma sources used in plasma immersion ion implantation, microwave plasmas, such as the MMB (Microwave Multipolar Bucket) and ECR (Electron Cyclotron Resonance) plasmas [6]-[8], are considered better for PI11 doping because of their higher ion density, lower ion energy, and lower contamination levels. The PI11 technique was first used for surface modification to improve the mechanical performance of machine parts [l], [2]. Recently, some results of PI11 doping for semiconductors have been reported [3]-[5]. Mizuno et al. [3] report doping the sidewall of a trench structure using an ECR microwave plasma system. A B2H6/He ( 5 % ) plasma was used at a pressure of 0.5 mtorr. An RF source was used to induce a self-bias of typically -700 V. The reported surface boron concentration of up to 1022/cm3 may indicate deposition rather than implantation. Kitagawa et al. [4] used an ac (20 kHz) biased ECR micro-

T

Manuscript received August 1, 1991; revised April 28, 1992. The review of this paper was arranged by Associate Editor R . B. Fair. The authors are with the Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 021 15. IEEE Log Number 9202307.

wave plasma to dope silicon substrates with boron. The peak-to-peak voltage was 500-1000 V. B2H6/H2 (0.5%) was used as the gas source. Results of a sheet resistance of 0.5-10 kQ and a junction depth of 0.1 pm after annealing are reported. The group of UC Berkeley [5] used a pulsed or dc biased ECR microwave plasma system to dope silicon using BF, gas. Typical pressure and target potentials were 3 mtorr and -5 kV, respectively. With SiF4 PI11 preamorphization followed by a -2-kV BF, PI11 at a dose of 1.2 X 1015/cm2 and a rapid thermal anneal at 1060°C for 1 s, 80-nm pf-n junctions were obtained. Plasma immersion ion implantation is fundamentally different than conventional ion implantation. One practical difference is that it is more difficult to determine the impurity dose during the process because there are many unknown factors including excitation rate, ionization rate, and ion-impact secondary electron emission. Another difference is that PI11 processing will yield a different impurity profile in the silicon substrate because the ions have a relatively wide energy distribution caused by the collisions in the plasma sheath [9]. Ion energies generally become lower and become distributed over a wider range of energies as the pressure is increased. In this paper, a microwave multipolar bucket (MMB) plasma system is used to dope silicon with boron to form p+-n junctions. B2H6 diluted in helium (1%) is used as the ion source, and the silicon wafer is biased by a negative high-voltage pulse of up to -15 kV. Based on the measurements, the boron dose is determined and the boron profile in the silicon substrate is derived. Characteristics of diodes fabricated by this PI11 technique are reported. 11. MICROWAVE MULTIPOLAR BUCKETPLASMA The MMB plasma system is shown in Fig. 1. The system consists of four parts: a chamber with multipolar magnetic field structure, a microwave source, a vacuum and gas handling system, and a high-voltage pulse generator. The aluminum chamber is 30 cm high and 36 cm in diameter. The outside wall of the chamber is surrounded by a ring of parallel rows of permanent magnetic bars (1000 G at the pole) which are arranged such that successive north and south poles face the plasma in a multipolar magnetic bucket structure. The benefit of this structure is that higher ion densities and better radial ion density uniformities are obtained in the chamber because

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QIN et al.: PLASMA IMMERSION ION IMPLANTATION DOPING

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TABLE I PI11 PROCESS CONDITIONS SOURCE

Specifications of wafer: (100) n-type crystal Si wafer, resistivity p = 2-5 Q ( N ~ 2 x 1015/cm3).

-

QUARTZ WINDOW

MAGNETIC BAR

Fig. 1. Microwave multipolar bucket (MMB) plasma system.

. cm

Implantation conditions: 1% B2H6in helium Gas: Gas flow rate: 14.6 sccm 50 mtorr Pressure: Microwave power: 600 W High-voltage pulse: voltage = - 1 to - 10 kV pulse frequency f = 500 Hz pulsewidth rp = 20 ps 2 to 20 min depending on dose Time: stainless steel, area = 45.24 cm’, not cooled or Wafer holder: heated Annealing conditions: Temperature: 1100°C 50 min Time: 5 % 0, in N, for first 2 min, then pure NZ Ambient:

I-

O

I

U CH.Z

Fig. 2. Typical voltage and current waveforms of the pulse. (Pressure: 50 mtorr, microwave power: 600 W, pulse: f = 500 Hz, rp = 20 ps. Y-axis: channel 1 is HV pulse, 5 kV/div; channel 2 is target current, 0.5 A/div. X-axis: time, 5 ps/div.)

the electrons are reflected back into the plasma by the magnetic mirror instead of being lost to the chamber walls. The microwave source (ASTeX Model S-1000) includes a magnetron and a three-stub tuner. The magnetron can supply up to 1000 W of continuous wave power at a frequency of 2.45 GHz. A pulse generator (Model Velonex 570) is used to apply the high-voltage pulse to the silicon wafer. 111. EXPERIMENTS n-type silicon wafers were immersed in a B2H6/He (1 %) plasma and biased with a high-voltage pulse. A dynamic sheath [lo] around the wafer is generated by the applied negative high-voltage pulse, and positively charged ions are accelerated by the electric field in the sheath and implanted into the wafer. The total target current per pulse, which consists of the ion currents of boron, hydrogen, and helium and of the secondary electron emission current, is measured with a pulse current transformer. Fig. 2 shows typical waveforms of the high-voltage pulse and corresponding target current. Channel 1 indicates the pulse potential and channel 2 indicates the target current, which reach steady-state values of - 10 kV and 1.1 A, respectively. Before implantation and annealing, the samples were cleaned by a standard pre-oxidation cleaning procedure. Different thicknesses of S i 0 2 were grown on some wafers in order to study the depth of implanted boron. After stripping any Si02 layers, a relatively long anneal (50 min

at 1100°C) was performed to facilitate dose measurements. The anneal used 5 % dryooxygen for the first 2 min in order to grow a thin ( - 100 A ) layer of silicon dioxide to ensure that most of the implanted boron atoms diffused into the silicon substrate during the annealing process. At each step, ellipsometry measurements were made to monitor the silicon surface. A four-point probe was used to measure the sheet resistance, and a groove sectioner was used to measure the p-n junction depth. The process conditions are shown in Table I. VI. RESULTSAND DISCUSSION A . Implantation into Bare Silicon Substrates Several bare silicon wafers were implanted for different times and at different target potentials under the process conditions in Table 1. After implantation and post-implantation cleaning, no deposited films were observed by ellipsometry measurements. A bare silicon wafer processed in the plasma for 10 min without applying pulses remained n-type and showed little change in sheei resistance after annealing. A silicon wafer with a 945- A thermal Si02 film was implanted using -10-kV potential pulses for 10 min. The sheet resistance of the substrate did not change, and no change in the thickness of the S O 2 layer was observed by ellipsometry, indicating that sputtering during the implant is minimal. These controls provide evidence that the observed doping is due to implanted ions. Fig. 3 shows the sheet resistance as a function of implant time when the pulse potential is - 10 kV. Good doping uniformity was observed on the silicon wafer. When the target potential was - 10 kV and the implant time was 20 min, the sheet resistance values varied less than 1 % over a 5-point measurement on the 4 cm x 4 cm silicon substrate.

B. Boron Dose We have calculated the boron dose from sheet resistance (R,) and junction depth (x, ) measurements assuming

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IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 39. NO. IO. OCTOBER 1992

5

10

15

1 20

IMPLANT TIME (MINUTE)

Fig. 3. Sheet resistance as a function of implant time. (V: - 10 kV, pressure: 50 mtorr, microwave power: 600 W , anneal: 7' = 11OO"C,t = 50 min.)

a Gaussian distribution of dopants. For example, a bare silicon wafer implanted by -10-kV pulses for 10 min yields Rs = 57.0 Q / O ,x, = 1.46 pm. From this it is found that the boron dose remaining after the anneal is 1.70 X lOI5/cm2. A SUPREM-I11 [ l l ] simulation of the process with a 10-keV conventional ion implantation predicts that 10% of the implanted dose will be lost during the anneal, requiring a pre-anneal boron dose of 1.87 x 1015/cm2 for a 1.70 X lOI5/cm2 post-anneal dose. In the PI11 process, the working gas B2H6 was diluted with 99% helium for safety. Several ions including B+, H ' , He+, etc., contribute to the total ion current to the target. In order to determine the boron dose from the process parameters we would need to know the percentage of B+ ions in the plasma and the secondary electron emission coefficient of the target for all ions. The boron ion current during the pulse can be calculated as IBoron

=

Dose A q f . t * t p

where A is target area, q is the electron charge, f is the pulse frequency, t is the implant time, and tp is the pulsewidth, assuming one boron atom/ion and singly ionized ions. The percentage of the total current attributable to boron ion current is 0.21 % from (1) and the boron dose figures. The largest parts of the total current are secondary electron emission, helium ion current, and hydrogen ion current. The implanted helium and hydrogen ions diffuse much faster than boron in silicon when the temperature is 1100°C so very little will remain after annealing. They would be expected to have a much larger effect on short diffusion cycles than on those used in these experiments. C. Boron ProJle in Silicon In the PI11 process, the target is immersed in the plasma. A dynamic sheath [lo] is formed when the high voltage pulse is applied to the target. The ions in the sheath are accelerated by the potential and implanted into the target.

For our system, the dynamic sheath thickness is up to 2.5 cm [lo] when the target potential is -10 kV and the pulse width tp is 20 p s . The sheath is much wider than the ion-neutral mean free path A,, which is 0.124 cm for helium when the pressure is 50 mtorr, so there are many collisions in the sheath. The ions have a distribution of energies less than 10 keV [9], which leads to an impurity profile that is different than that seen in conventional ion implantation. Fig. 4 shows boron profiles in the silicon substrate before annealing determined by SIMS (Secondary Ion Mass Spectrometry) measurements (measured by Charles Evans and Associates) for a sample implanted with - 10-kV target potential. The primary ion bombardment for the SIMS analysis was with 3.0-keV 0: incident at an angle of 42", measured from the normal. The sample was implanted for 10 min at 50-mtorr pressure and 600 W of microwave power. The boron profile of a conventional 10-keV ion implantation process with a dose of 1.87 x 10k5/cm2simulated by SUPREM-I11 is also graphed in Fig. 4 for comparison. For the PI11 doping technique, a narrower profile and a higher concentration peak are obtained because of the large number of low-energy ions. From the boron profile of Fig. 4, we obtain a rough estimate of the energy distribution of the implanted boron ions. We assume that the boron profile in the silicon substrate is the superposition of profiles of ions with different energies and that the boron profiles are Gaussian, so that

where S is the dose, Rp is the projected range, up is the projected straggle, and i is the index for the energy of the ions. Rpland up,are the constants [12] for the energy E,. The dose components S, are determined by least squares fitting. The energies used are 0.63, 1.25, 2.5, 5, and 10 keV. The values of the corresponding dose components S, are 0.73, 0.26, 0.41, 0.39, and 0.28 X lOI5/cm2, respectively. The fitted curve is shown with a dashed line in Fig. 4. This indicates that most of the boron ions have lower energies than 10 keV when the target potential is - 10 keV because of the collisions in the sheath. Multiply charged ions may be responsible for the relatively large number of ions that have energies near 10 keV despite the short mean free path. The total boron dose computed from the fit is then dose

=

c Si

=

2.08 x lOI5/cm2

1

which is 22 % greater than the post-anneal dose calculated from the sheet resistance measurements described in Section III-B. Implantation through oxides of different thicknesses followed by sheet resistance measurements provides an independent determination of the pre-anneal boron profile. After implantation, the Si02 layers were removed,

QIN e! al.: PLASMA IMMERSION ION IMPLANTATION DOPING

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source diffusion process was fabricated for comparison. The wafers were processed together for all steps except for the doping step. The area of each diode is 2.4 X cm2. The final p-n junction depths for the implanted and diffused diodes are 1.27 and 1.62 pm, respectively. The measured I- V characteristics for implanted and diffused diodes in Fig. 5 show low reverse currents for both diodes.

SIMSPROFILE

ACKNOWLEDGMENTS The authors wish to thank Dr. F . Sinclair of Eaton Corporation for SIMS analysis and advice. REFERENCES

DEFTH (MICRON)

Fig. 4. Boron profile in silicon before annealing. Shown are the SIMS profile, the concentration points derived from sheet resistance measurements of implants through different thickness SiO, layers, the points on the SIMS profile used for the least squares fit (LS POINT), the concentration curve given by (2) in the text (LS FIT CURVE), and the concentration from a SUPREM-I11 simulation of a conventional 10-keV ion implantation. ( V : - 10 kV, implant time: 10 min, pressure: 50 mtorr, microwave power: 600 W.)

FORWARD IMPLANTED DIODE

100

10-’0 10-11

0

1

2

3

4

I

5

VOLTAGE (V)

Fig. 5. Comparison of implanted and diffused diode I-V characteristics. (Diode area: 2.36 x IO-’ cm2.)

and the wafers were annealed using the process previously described. From the measured sheet resistance and junction depth after annealing, the post-anneal boron dose was determined. From this information and assuming that the boron profile in the Si02 layer is identical to the profile in the silicon substrate [ 131, the pre-anneal boron profile can be estimated. The boron profile based on this method is also shown in Fig. 4 for comparison.

D. Characteristics of p-n Junctions Fabricated by PIII Devices including diodes, MOS capacitors, and PMOS transistors were fabricated using a simple PMOS aluminum gate technology. Before ion implantation, a layer of 164 A of thermal S i 0 2 was grown on the silicon surface. The implantation conditions were the same as in Table I. Boron was implanted for 10 min, corresponding to a postanneal dose of 2.11 X lOI4/cm2. The initial S O 2 layer was removed after the implant, and a 1170-A Si02 gate layer was grown by TCA dry oxidation at a temperature of 1100°C for 30 min. A wafer doped using a BN solid

[ l ] J. R. Conrad and C . Forest, “Plasma source ion implantation,” presented at the IEEE Int. Conf. on Plasma Science, Saskatoon, Canada, May 19-21, 1986. [2] J. R. Conrad, J. Radtke, R. A. Dodd, and F. Worzala, “Plasma source ion implantation technique for surface modification of material,”J. Appl. Phys., vol. 62, no. 1 1 , pp. 4591-4596, Dec. 1987. [3] B. Mizuno, I. Nakayama, N. Aoi, M. Kubota, and T . Komeda, “New doping method for subhalf micron trench sidewalls by using an electron cyclotron resonance plasma,” Appl. Phys. L e f t . , vol. 53, no. 21, pp. 2059-2061, NOV. 1988. [4] M. Kitagawa, N. Matsuo, G . Fuse, H. Iwasaki, A. Yoshida, and T. Hirao, “Plasma ion-doping technique with 20 kHz biased electron cyclotron resonance discharge,” Japan. J . Appl. Phys., vol. 27, no. 11, pp. L2139-2141, Nov. 1988. [5] N . W. Cheung, “Plasma immersion ion implantation for ULSI processing,” Nucl. Insr. Mefh., vol. B55, pp. 811-820, 1991. [6] R. Mallavarpn, J. Asmussen, and M. C. Hawley, “Behavior of a microwave cavity discharge over a wide range of pressure and flow rates,” IEEE Trans. Plasma Sci., vol. PS-6, no. 4 , pp. 341-354, Dec. 1978. [7] M . Pichot, A. Durandet, J. Pelletier, Y. Arual, and L. Vallier, “Microwave multipolar plasma excited by distributed electron cyclotron resonance: Concept and performance,” Rev. Sci. Instrum., vol. 59, no. 7, pp. 1-4, Jcly 1988. [8] J. Hopwood, D. K. Reinhard, and J. Asmussen, “Charged particle densities and energy distributions in a multipolar electron cyclotron resonant plasma etching source,” J . Vac. Sci. Technol., vol. A8, no. 4, pp. 3103-3112, Jul./Aug. 1990. [9] S . Qin, C. Chan, and N. McGruer, “Energy distribution of boron ions during plasma immersion ion implantation,” Plasma Source Sci. Technol., no. 1, pp. 1-6, 1992. [lo] S . Qin, C. Chan, N. McGruer, J . Browning, and K. Wamer, “The response of a microwave multipolar bucket plasma to a high voltage pulse,” IEEE Trans. Plasma Sci., vol. 19, no. 6 , pp. 1272-1278, Dec. 1991. [ l I] S . E. Hansen, SUPREM-III User’s Manual, Stanford University, Stanford, CA, 1985. [ 121 J. F. Ziegler, J. P. Biersck, and U. Littmark, fhe Sropping and Range of Ions in Solids. New York: Pergamon, 1985, 187 pp. [13] K. A. Pickar, “Ion implantation in silicon,” in R. Wolfe, Ed., Applied Solid State Science, vol. 5. New York: Academic Press, 1975.

Shu Qin graduated from Beijing Polytechnic University in electrical engineering in 1976 and received the M.S. degree in electrical engineering from Microelectronics Institute, Qinghua (Tsinghua) University, China, in 1982. He received the Ph.D. degree in electrical engineering from Northeastem University, Boston, MA, in 1992. From 1976 to 1979, he worked as a Professor Assistant in CAD Center of Qinghua University on VLSI design and processing. From 1982 to 1986, he worked as a Lecturer in the Beijing Institute of Posts and Telecommunications. From 1986 to 1987, he worked as a Research Scientist at ECE Dept., Lehigh University, Bethlehem, PA, on VLSI CAD. His current research interests are semiconductor processing, VLSI design, and device simulation.

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Nicol E. McCruer (S’83-M’83) received the B.S. degree in physics and the Ph.D. degree in electrical engineering in 1977 and 1983, respectively, from Michigan State Univkrsity, East Lansing. Starting in 1983, he worked for Speny Semiconductor Operations in the MOS process development and silicon research groups. At Sperry, he developed ion implantation and diffusion processes for 0.8- and 1.25-pm CMOS technologies, and investigated rapid thermal oxidation of silicon \ for MOS gate dielectrics. He is currently an assistant professor at Northeastern University, Boston, MA, with research projects in the areas of vacuum microelectronics and semiconductor fabrication

Chung Chan (S’78-M’81-SM388) was born in Canton, China, on October 23, 1956. He received the B.S. degree in electrical engineering from North Dakota State University, Fargo, in 1978, and the M.S. and Ph.D. degrees in electrical and computer engineering from the University of Iowa, Iowa City, in 1980 and 1981, respectively. From 1981 to 1984 he served as a Research Scientist at the Phaedrus Tandem Mirror at the University of Wisconsin in Madison. In September of 1984 he joined the faculty of Northeastern Uni-

versity, Boston, MA, where he currently holds the rank of Professor. He has published papers on topics related to novel plasma device, space plasma simulations, nonlinear plasma phenomena, plasma processing, microstructure fabrication, and novel plasma diagnostic techniques.

Keith Warner has been working in the field of semiconductor and thin-film device fabrication for over 10 years. He has been affiliated with Northeastern University, Boston, MA, since 1987, where he has studied processing of field emission devices, plasma source ion implantation, and contacts to high-T, superconductors. Previously, he worked in research groups at Unisys Corporation and Brown University, in the areas of laser-assisted metal deposition; MMIC packaging; interlevel dielectrics for VLSl circuits; process development of submicrometer MOS device: and fabrication of MOS and HEMT devices for quantized Hall effect studies.