Charging effects in plasma immersion ion implantation for ...

Charging effects in plasma immersion ion implantation for microelectronics Shu Qin, James D. Bernstein, Zhuofan Zhao,a) Wei Liu, and Chung Chan Plasma Science and Microelectronics Laboratory, Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115

Jiqun Shao and Stuart Denholm Eaton Corporation, Semiconductor Equipment Division, Beverly, Massachusetts 01915

~Received 27 April 1995; accepted 16 June 1995! The charging effects of plasma immersion ion implantation ~PIII! doping experiments have been investigated using a dynamic sheath model and PDP1 plasma simulation code. When the target has a dielectric film, charge accumulation during PIII can have a profound impact on doping results. Under certain process conditions, it can significantly reduce implant energy and dose and thereby alter the implant profile. In addition, it may degrade device reliability, especially for ultralarge-scale integrated circuit devices. In order to minimize charging effects, shorter pulse widths along with moderate values of plasma density and pulse potential should be used. © 1995 American Vacuum Society.

I. INTRODUCTION Plasma immersion ion implantation ~PIII! is a promising method for semiconductor doping1–5 and hydrogenation of thin-film transistors ~TFTs! for flat panel displays.6 PIII doping has advantages over conventional ion implantation for such applications because of its higher dose rate at low energy, independent of implant area. The high dose rate requires special consideration when processing devices with insulator films, such as field or gate oxides for conventional devices and silicon-on-insulator ~SOI! structures for TFTs. During the PIII high voltage pulse, charge will accumulate on the dielectric surface. Although the charge is neutralized by electrons between pulses, the charge accumulation during the pulse can have a profound effect on doping results and device characteristics. This becomes especially serious when processing at high dose rates. Emmert7 used a dynamic sheath model to study the charging at dielectric surfaces during plasma source ion implantation for material processing. En et al.8 used the SPICE circuit simulator to study thin oxide charging on metal-oxide-semiconductor ~MOS! device structures during PIII. In this article, the charging effects of hydrogen PIII are investigated with a dynamic sheath model9–15 and PDP1 simulation code.16,17 An analysis of the effects on implant energy, dose, impurity profile, and device reliability as functions of PIII process conditions and device structure is presented. Hydrogen PIII has been chosen to study the charging effects because of its many applications to semiconductor doping processing, including its use as a diluting gas for safety considerations and for passivation of semiconductor grain boundary and interface defects. Most dopant gases in microelectronics contain hydrogen ~e.g., B2H6 , PH3 , AsH3 , etc.!. When these gases are used for plasma, hydrogen often accounts for significant fractions of the species compositions due to its high ionization rate. Hydrogen plasma produces the largest ion current for a given implant voltage due to its a!

Current address: Varian Associates, Gloucester, Massachusetts 01930.

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J. Vac. Sci. Technol. B 13(5), Sep/Oct 1995

light mass. Consequently, hydrogen PIII can have serious charging effects. II. DYNAMIC SHEATH MODEL OF PIII The collisionless dynamic sheath model9–15 for PIII can be expressed by considering the Child–Langmuir law j5

4 e 9 0

A

2q V 3/2 s , M s2

~1!

where j is the ion current density crossing the sheath edge, e0 is the free-space permittivity, q is the ion charge, M is the ion mass, V s is the absolute value of the pulse potential, and s is the sheath thickness. The ions implanted into the target is due to the uncovering of ions at the sheath edge during sheath expansion and the ambipolar diffusion of ions toward the sheath boundary at the Bohm acoustic speed u b 5(qT e /M ) 1/2 . The implant current density resulting from these two factors is j5qn i

S

D

ds 1u b , dt

~2!

where n i is the ion density and T e is electron temperature in units of electron volts. There is still some controversy as to whether u b should be included in the dynamic sheath model.9–12 We believe that the dynamic sheath model minus the u b term is a good approximation for most material applications and some semiconductor applications when heavier ions are implanted, ion density is relatively low, and the pulse potential is relatively high. However, for our hydrogen PIII experiments where the ion density range is from 131010 to 131011/cm3 and the pulse potential is less than 20 kV, the u b term cannot be neglected because the sheath expansion speed is not always higher than the ion acoustic speed. This assumption is consistent with the results obtained from PDP1 simulation described later, and is also confirmed by the experimental results of secondary ion mass spectroscopy ~SIMS! dose analysis. The relations between PIII process conditions and the dynamic sheath model need to be investigated further.

0734-211X/95/13(5)/1994/5/$6.00

©1995 American Vacuum Society

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Qin et al.: Charging effects in plasma immersion ion implantation

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The sheath expansion, s(t), can be obtained by solving Eqs. ~1! and ~2!. After the dynamic sheath is established, the total ion current density during the high-voltage pulse can be derived directly from the Child–Langmuir law with the addition of a displacement current term,14,15 so that j i~ t ! 5

4 e 9 0

A

S

D

s 20 2q 3/2 1 Vs 1 , M s~ t !2 s~ t !4

~3!

where s 0 is the initial ion-matrix sheath thickness. The ion dose per pulse can be obtained by integrating the ion current density over the pulse width, d5

1 q

E

tp

0

~4!

j i ~ t ! dt.

When a wafer has an insulator layer such as SiO2 or a quartz substrate, charge will accumulate on the insulator surface during the high-voltage pulse. The charge, in turn, builds up an opposing electric field that partially cancels the exterior field. Thus, the implanted ions may not be monoenergetic even when a perfect pulse is applied to a collisionless plasma. The implanted ion dose may be decreased because of the reduced effective sheath potential. The accumulated charge on the dielectric film during the pulse is Q~ t !5

E

t

0

~5!

j i ~ t ! dt,

and V s in Eqs. ~1! and ~3! is no longer a constant, and becomes V s ~ t ! 5V 0 2

1 C0

E

t

0

j i ~ t ! dt,

~6!

where V s (t) is the effective sheath potential, V 0 is the initial pulse potential, and C 0 is the capacitance per unit area of the insulator layer. Using Eq. ~2!, the effective sheath potential is V s ~ t ! 5V 0 2

qn i @ s ~ t ! 2s 0 1u b t # , C0

~7!

and the electric field in the dielectric film caused by the charge accumulation is E~ t !5

Q~ t ! , e 0e r

FIG. 1. Comparison of the effective sheath potential vs pulse width for different ion densities when V5210 kV and t ox50.5 mm.

will cease when this occurs. Figure 2 shows effective sheath potential versus SiO2 thickness for different ion densities at the end of a 10 ms pulse for a 210 kV applied pulse potential. The reduction of the effective sheath potential for dielectric films thinner than 0.1 mm can be neglected under these process conditions. For example, the effective sheath potential only decreases by approximately 6 V in 10 ms for a SiO2 layer with thickness t ox50.1 mm in a hydrogen plasma with ion density n i 5131011/cm3. This is negligible compared to a 210 kV applied potential. However, the effects of charge accumulation can be significant for thicker dielectric films. As shown in Fig. 2, because oxide capacitance is inversely proportional to dielectric thickness, the effective sheath potential decreases with increasing oxide thickness. Ions experience a significant decrease in implant energy as charge accumulates during the time of the pulse. B. Implanted dose and impurity profile

For thicker dielectric films, the decrease in the effective sheath potential due to the charging effect will reduce the

~8!

where e r is the relative permittivity of the dielectric material. III. RESULTS AND DISCUSSION A. Effective sheath potential

Figure 1 shows the effective sheath potential versus time as affected by charge accumulation during a 10 ms pulse when the SiO2 layer has thickness t ox50.5 mm such as in the case of a TFT substrate. A 210 kV pulse potential is applied to a target immersed in hydrogen plasmas with ion densities from 13109 to 131012/cm3. The magnitude of the effective sheath potential decreases over the pulse duration from 10 to approximately 6.6 and 1 kV for ion densities of 131010/cm3 and 2.531010/cm3, respectively. The effective sheath potential drops to zero after 3 and 0.3 ms for ion densities of 131011/cm3 and 131012/cm3, respectively. Ion implantation JVST B - Microelectronics and Nanometer Structures

FIG. 2. Comparison of the effective sheath potential vs SiO2 thickness for different ion densities at the end of a 10 ms pulse. V5210 kV.

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FIG. 3. Implant dose vs pulse width for a conducting substrate and a substrate with a 0.5-mm-thick SiO2 film when V5210 kV and n i 5131011/cm3.

implant dose to a value that is significantly lower than what would be expected from Eq. ~4!. Figure 3 compares the implant dose into a conducting substrate and a substrate with a 0.5 mm SiO2 film as a function of pulse width. Here, the applied pulse potential is 210 kV and ion density is 131011/cm3. The implant dose saturates in 3 ms for a substrate with a 0.5-mm-thick SiO2 film. This corresponds to the n i 5131011/cm3 curve in Fig. 1 at time t53 ms when the effective sheath potential drops to zero. Figure 4 predicts the dose reduction as a function of pulse width for the different densities when a 210 kV pulse is applied to a substrate with a 0.5 mm SiO2 film. The dose reduction is defined as a ratio of the implant dose with charging effect and the dose without charging effect. The implant dose is significantly reduced when the ion density is high. Figure 5 predicts dose reduction versus SiO2 thickness for different ion densities for a 210 kV applied pulse potential and 10 ms pulse width. For the case of a 0.5 mm SiO2 TFT substrate, the implant dose decreases by approximately 8% and 73% for ion densities of 131010 and 131011/cm3, respectively. For a substrate with a

FIG. 4. Comparison of the dose reduction vs pulse width for different ion densities when V5210 kV and t ox50.5 mm. J. Vac. Sci. Technol. B, Vol. 13, No. 5, Sep/Oct 1995

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FIG. 5. Prediction of the dose reduction vs SiO2 thickness for different ion densities when V5210 kV and t p 510 ms.

100-nm-thick SiO2 film, the dose reduction is less than 0.01%. Dose reduction is negligible for oxide films thinner than 10 mm with current process conditions. The implanted impurity profile is altered by the wider ion energy distribution and reduced implant dose caused by the charging effect. Figure 6 shows implanted hydrogen profiles in a silicon substrate for V 0 5210 kV, n i 5531010/cm3, t p 510 ms, and pulse repetition rate5500 Hz. Implant time is 10 min. Lindhard–Scharff–Schiott ~LSS! theory18 is used to calculate the impurity profile, so that the hydrogen profile N(x) is a symmetrical Gaussian function, N~ x !5

d

A2 ps p

S

exp 2

~ x2R p ! 2

2 s 2p

D

,

~9!

where d is the implant dose, R p is the projected range, and s p is the projected straggle. The hydrogen profile can be calculated by Eq. ~9! with constant R p and s p , which are determined by the implant ion energy when there is no charging effect. The charge ac-

FIG. 6. Comparison of the impurity profiles for PIII doping of a conducting substrate and a substrate with a 0.5 mm SiO2 film. V5210 kV, n i 5531010/cm3, t p 510 ms, pulse repetition rate5500 Hz, and implant time510 min. The solid line represents the profile for the substrate with SiO2 film.

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FIG. 7. Electric field for different ion densities in a 100-nm-thick SiO2 film during a 10 ms pulse. V 0 5210 kV.

cumulation on a substrate with a dielectric film during PIII doping will cause a wider energy distribution and reduce the implant dose. For this case, the hydrogen profile can be calculated by the superposition of the profiles of ions with different energies and doses. Thus N~ x !5

(i

di

A2 ps pi

S

exp 2

~ x2R pi ! 2

2 s 2pi

D

,

~10!

where i is the index for the energy of ions and R p and s p are no longer constants. The implant profiles as calculated by Eqs. ~9! and ~10! are shown in Fig. 6. The profile for a SOI structure, that is, a silicon film with a 0.5 mm SiO2 substrate, as indicated by the solid line, is wider and shallower than for a conducting substrate. The total dose is reduced by 49% from 2.5231017 to 1.2931017/cm2. C. Electric fields in dielectric films and device reliability

Although a thinner dielectric film will have a smaller effect on implant energy and dose, the electric field across the film caused by the accumulated charge could damage the film. This is important for MOS devices with thin gate oxides. Figures 7 and 8 show the electric fields in 100-nm and 0.5-mm-thick SiO2 dielectric films during a 10 ms pulse for different ion densities when the pulse potential is 210 kV. The thicker dielectric film experiences a smaller electric field for the same PIII conditions, resulting from reduced dose due to the charging effect, especially at higher ion densities. Figure 9 shows the induced electric fields versus pulse potential for the different ion densities on a 100-nm-thick SiO2 film at the end of a 10 ms pulse. Although the electric fields produced in our current PIII experiments ~n i >1010 –1011/cm3! are less than the breakdown field of SiO2 ~63108 V/m!, as shown in Figs. 7, 8, and 9, gate oxide damage has been found experimentally. This damage was manifested through increased leakage current through the gate oxide, and could degrade the MOS transistor’s performance. Nonplanar and nonuniform device structures and locally concentrated electric fields, such as those that may exist between the gate and JVST B - Microelectronics and Nanometer Structures

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FIG. 8. Electric field for different ion densities in a 0.5-mm-thick SiO2 film during a 10 ms pulse. V 0 5210 kV.

the corner of the source or drain regions, may be responsible for this damage. Hydrogen PIII experiments have been performed on MOS transistors with 100-nm-thick gate oxide layers to monitor gate oxide damage. A comparison of the gate currents before, after PIII, and after a 350 °C annealing is shown in Table I. With an ion density of 731010/cm3 and the magnitude of the pulse potential greater than 15 kV, the gate current increased dramatically. This potential corresponds to an electric field of approximately 63107 V/m shown in Fig. 9. The damage was permanent and could not be repaired with a high temperature anneal. When the magnitude of the pulse potential was less than 15 kV, although the gate current was increased after the PIII process, the gate leakage current could be recovered with a 350 °C anneal. IV. PDP1 SIMULATION The PDP1 plasma simulation code16,17 was used to simulate charging effects during the hydrogen PIII process. The simulated results include the effective sheath potential, implant dose, accumulated charge density, and induced electric field in a dielectric substrate.

FIG. 9. Electric field as a function of pulse potentials and ion densities in a 100-nm-thick SiO2 film at the end of a 10 ms pulse.

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TABLE I. Comparison of gate current of MOS transistors before and after hydrogen PIII processing. @Device dimension: W/L550 mm/50 mm, gate oxide thickness51000 Å, PIII process conditions: pressure58 mTorr, hydrogen plasma density5731010/cm3, pulse width510 ms, pulse repetition rate5200 Hz; annealing conditions: ambient is forming gas ~0.5% H2 in N2!, temperature5350 °C, time530 min.# I G ~pA! at V G 510 V Pulse voltage ~2kV! 35 25 15 10 5

As fabricated

Hydrogen PIII

Annealing

0.14 0.08 0.15 0.19 0.16

2600 71.53 4.54 2.84 7.0

41.64 1.46 0.22 0.19 0.20

PDP1 is a one-dimensional planar bounded plasma simulator, described in detail by Birdsall.16 The particle-in-cell method is implemented in PDP1 to solve for the particle and field parameters self-consistently. The code also uses a Monte Carlo scheme to model charged and neutral particle collisions. The PIII simulation parameters are as follows: gas: hydrogen, plasma density n i 51010/cm3, plasma length50.3 m, target area5100 cm2 ~planar!, pulse voltage V 0 5210 kV, pulse width t p 510 ms, T e 52.0 eV, T i 50.2 eV, number of particles52250. time step51310211 s, The pressure is set to 0.1 mTorr to maintain the collisionless condition. The external capacitance C is set to the values calculated from the thickness of the SiO2 dielectric films. Figure 10 shows the simulated ion and electron densities of hydrogen PIII at 1.4 ms. The left-hand side ~x50! represents the target to which a 210 kV pulse is applied. The

FIG. 10. Ion and electron densities for a hydrogen PIII simulated by PDP1 code. V5210 kV, n i 5131010/cm3, and time51.4 ms.

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sheath has expanded to ;5 cm at this moment. In Fig. 10, a presheath can be seen between x>5 and x>8 cm from ion and electron spatial profiles. The appearance of the presheath supports our assumption that the ion acoustic speed term u b should be included in the dynamic sheath model for our PIII conditions. Figures 1 and 2 compare the PDP1 simulated effective sheath potential to the dynamic sheath model. A similar comparison of the PDP1 simulated dose reduction to the dynamic sheath model is shown in Fig. 4. PDP1 simulated electric fields in 100-nm and 0.5-mm-thick SiO2 films are shown in Figs. 7 and 8, respectively. Good agreement between the PDP1 simulations and our model is demonstrated. V. CONCLUSION Charge accumulation on an insulator surface during PIII processing can reduce implant energy and dose. It can change the implant profile and damage thin gate oxide films. Calculations based on the dynamic sheath model are consistent with PDP1 computer simulations, but the dynamic sheath model is simpler and more efficient. A shorter pulse width along with moderate values of plasma density and pulse potential will reduce positive charge accumulation. ACKNOWLEDGMENTS The authors wish to thank Ryne Allen, Joe Genevich, and Keith Warner for their technical assistance. This work was supported in part by the Advanced Research Projects Agency ~ARPA! and the Air Force Office of Scientific Research ~AFOSR!. 1

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