Abstract
In plasma etching for microelectronics fabrication, the quality of the process is in large part determined by the ability to control the ion energy distribution (IED) onto the wafer. To achieve this control, dual frequency capacitively coupled plasmas (DF-CCPs) have been developed with the goal of separately controlling the magnitude of the fluxes of ions and radicals with the high frequency (HF) and the shape of the IED with the low frequency (LF). In steady state operation, plasma properties are determined by a real time balance between electron sources and losses. As such, for a given geometry, pressure, and frequency of operation, the latitude for controlling the IED may be limited. Pulsed power is one technique being investigated to provide additional degrees of freedom to control the IED. In one configuration of a DF-CCP, the HF power is applied to the upper electrode and LF power is applied to the lower electrode which is serially connected to a blocking capacitor (BC) which generates a self dc-bias. In the steady state, the value of the dc-bias is, in fact, constant. During pulsed operation, however, there may be time modulation of the dc-bias which provides an additional means to control the IED. In this paper, IEDs to the wafer in pulsed DF-CCPs sustained in Ar/CF4/O2 are discussed with results from a two-dimensional plasma hydrodynamics model. The IED can be manipulated depending on whether the LF or HF power is pulsed. The dynamic range of the control can be tuned by the dc-bias generated on the substrate, whose time variation depends on the size of the BC during pulsed operation. It was found that high energy ions can be preferentially produced when pulsing the HF power and low energy ions are preferentially produced when pulsing the LF power. A smaller BC value which allows the bias to follow the change in charged particle fluxes produces a larger dynamic range with which to control IEDs.
This work was supported by the Semiconductor Research Corp., Department of Energy Office of Fusion Energy Sciences, and the National Science Foundation.
I. INTRODUCTION
II. DESCRIPTION OF THE MODEL
III. PLASMA PROPERTIES OF PULSE POWERED DF-CCP SUSTAINED IN Ar/CF_{4}/O_{2}
IV. CONTROL OF THE IED IN PULSE POWERED DF-CCP USING BLOCKING CAPACITANCE
V. CONCLUSIONS
Key Topics
- Electrodes
- 27.0
- Plasma sheaths
- 12.0
- Capacitors
- 9.0
- Capacitance
- 8.0
- Plasma properties
- 6.0
H01L21/3065
Figures
Operating system for this investigation. (a) Geometry of the DF-CCP chamber. The LF (10 MHz) is applied on the lower electrode, and the HF (40 MHz) is applied on the upper electrode. One of the two frequencies is operated in pulse mode with a few tens of kHz PRF. (b) Electrical schematic for the DF-CCP system. The BC is connected in series with the lower electrode.
Operating system for this investigation. (a) Geometry of the DF-CCP chamber. The LF (10 MHz) is applied on the lower electrode, and the HF (40 MHz) is applied on the upper electrode. One of the two frequencies is operated in pulse mode with a few tens of kHz PRF. (b) Electrical schematic for the DF-CCP system. The BC is connected in series with the lower electrode.
(Color online) Electron density (left) and temperature (right) when pulsing the HF power at different times during the pulsed cycle (as indicated in the lower figure). (Ar/CF4/O2 = 75/20/5, 40 mTorr, 200 sccm, LF= 250 V at 10 MHz cw, HF = 250 V at 40 MHz in pulse mode with BC = 1 μF, PRF = 50 kHz and 25% duty-cycle.) The electron density is modulated by about 30% during the pulse cycle while the electron temperature shows nearly instantaneous changes as the HF power toggles on and off, especially near the sheaths due to enhanced stochastic heating.
(Color online) Electron density (left) and temperature (right) when pulsing the HF power at different times during the pulsed cycle (as indicated in the lower figure). (Ar/CF4/O2 = 75/20/5, 40 mTorr, 200 sccm, LF= 250 V at 10 MHz cw, HF = 250 V at 40 MHz in pulse mode with BC = 1 μF, PRF = 50 kHz and 25% duty-cycle.) The electron density is modulated by about 30% during the pulse cycle while the electron temperature shows nearly instantaneous changes as the HF power toggles on and off, especially near the sheaths due to enhanced stochastic heating.
(Color online) Electron density and temperature when pulsing the LF power at different times during the pulsed cycle (as indicated in the lower figure). (Ar/CF4/O2 = 75/20/5, 40 mTorr, 200 sccm, LF = 250 V at 10 MHz in pulse mode with BC = 1 μF, PRF = 50 kHz and 25% duty-cycle, HF = 250 V at 40 MHz cw.) Pulsing the LF power produces nominal intercycle changes in electron density and temperature over the pulse period as the majority of the LF power is dissipated in ion acceleration.
(Color online) Electron density and temperature when pulsing the LF power at different times during the pulsed cycle (as indicated in the lower figure). (Ar/CF4/O2 = 75/20/5, 40 mTorr, 200 sccm, LF = 250 V at 10 MHz in pulse mode with BC = 1 μF, PRF = 50 kHz and 25% duty-cycle, HF = 250 V at 40 MHz cw.) Pulsing the LF power produces nominal intercycle changes in electron density and temperature over the pulse period as the majority of the LF power is dissipated in ion acceleration.
(Color online) Plasma potential, VP , and dc-bias, Vdc , during one pulse period when pulsing the HF power (PRF = 50 kHz, 25% duty-cycle). (a) BC = 10 nF and (b) BC = 1 μF. The sheath potential is VS = VP − Vdc . The LF power is always on and the HF power is on only during the pulse window of 25%. Due to the smaller RC time constant with the small BC, the dc-bias responds more quickly. Since the voltage amplitude of the LF power rides on the dc-bias, the maximum envelope of the plasma potential has the same shape as the dc-bias.
(Color online) Plasma potential, VP , and dc-bias, Vdc , during one pulse period when pulsing the HF power (PRF = 50 kHz, 25% duty-cycle). (a) BC = 10 nF and (b) BC = 1 μF. The sheath potential is VS = VP − Vdc . The LF power is always on and the HF power is on only during the pulse window of 25%. Due to the smaller RC time constant with the small BC, the dc-bias responds more quickly. Since the voltage amplitude of the LF power rides on the dc-bias, the maximum envelope of the plasma potential has the same shape as the dc-bias.
(Color online) Plasma potential, VP , and dc-bias, Vdc , during one period when pulsing the LF power (PRF = 50 kHz, 25% duty-cycle). (a) BC = 10 nF and (b) BC = 1 μF. The sheath potential is VS = VP − Vdc . The HF power is always on and the LF power is on only during the pulse window of 25%. The plasma potential is mainly determined throughout the pulse period by the voltage amplitude of the cw HF power. The dynamic range of dc-bias is larger with the smaller BC.
(Color online) Plasma potential, VP , and dc-bias, Vdc , during one period when pulsing the LF power (PRF = 50 kHz, 25% duty-cycle). (a) BC = 10 nF and (b) BC = 1 μF. The sheath potential is VS = VP − Vdc . The HF power is always on and the LF power is on only during the pulse window of 25%. The plasma potential is mainly determined throughout the pulse period by the voltage amplitude of the cw HF power. The dynamic range of dc-bias is larger with the smaller BC.
(Color online) Total IEDs for all ions with different sizes of the BC for the base case (40 mTorr, 250 V at 10 MHz, 250 V at 40 MHz). (a) cw operation, (b) pulsing HF power, and (c) pulsing LF power. Pulsing has a PRF of 50 kHz and 25% duty-cycle. The IED is insensitive to the size of BC with cw operation while its shape depends on the size of BC with pulsed operation.
(Color online) Total IEDs for all ions with different sizes of the BC for the base case (40 mTorr, 250 V at 10 MHz, 250 V at 40 MHz). (a) cw operation, (b) pulsing HF power, and (c) pulsing LF power. Pulsing has a PRF of 50 kHz and 25% duty-cycle. The IED is insensitive to the size of BC with cw operation while its shape depends on the size of BC with pulsed operation.
(Color online) Total IEDs for all ions for different PRFs when pulsing the HF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The IED becomes single-peaked in appearance with the smaller BC while the IED maintains a multiple-peaked shape with the larger BC. The IEDs with larger PRFs extend to the higher energies.
(Color online) Total IEDs for all ions for different PRFs when pulsing the HF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The IED becomes single-peaked in appearance with the smaller BC while the IED maintains a multiple-peaked shape with the larger BC. The IEDs with larger PRFs extend to the higher energies.
(Color online) DC-bias as a function of normalized time (which is time divided by the length of each pulse period) with different PRFs when pulsing the HF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The LF power is cw. During power-on period, the dc-bias becomes less negative with some overshoot with smaller PRFs.
(Color online) DC-bias as a function of normalized time (which is time divided by the length of each pulse period) with different PRFs when pulsing the HF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The LF power is cw. During power-on period, the dc-bias becomes less negative with some overshoot with smaller PRFs.
(Color online) Ion energy distributions for O^{+}, Ar^{+}, and CF3 ^{+} when pulsing the HF power. (a) BC = 10 nF and (b) BC = 1 μF.
(Color online) Ion energy distributions for O^{+}, Ar^{+}, and CF3 ^{+} when pulsing the HF power. (a) BC = 10 nF and (b) BC = 1 μF.
(Color online) Total IEDs for all ions for different PRFs when pulsing the LF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The IED extends to higher energies with the smaller BC.
(Color online) Total IEDs for all ions for different PRFs when pulsing the LF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The IED extends to higher energies with the smaller BC.
(Color online) DC-bias as a function of the normalized time (which is time divided by the length of each pulse period) with different PRFs when pulsing the LF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The HF power is cw. If the size of BC is small enough for the dc-bias to response to the voltage on the electrode, the temporal behavior of dc-bias is similar for different PRFs.
(Color online) DC-bias as a function of the normalized time (which is time divided by the length of each pulse period) with different PRFs when pulsing the LF power with a 25% duty-cycle. (a) BC = 10 nF and (b) BC = 1 μF. The HF power is cw. If the size of BC is small enough for the dc-bias to response to the voltage on the electrode, the temporal behavior of dc-bias is similar for different PRFs.
(Color online) IEDs for O^{+}, Ar^{+}, and CF3 ^{+} when pulsing the LF power. (a) BC = 10 nF and (b) BC = 1 μF.
(Color online) IEDs for O^{+}, Ar^{+}, and CF3 ^{+} when pulsing the LF power. (a) BC = 10 nF and (b) BC = 1 μF.
(Color online) Total IEDs for all ions for different duty-cycles when pulsing the HF power with a PRF of 50 kHz. (a) BC = 10 nF and (b) BC = 1 μF. The LF power is cw. The smaller duty-cycle tends to produce an extended energy range in the IED.
(Color online) Total IEDs for all ions for different duty-cycles when pulsing the HF power with a PRF of 50 kHz. (a) BC = 10 nF and (b) BC = 1 μF. The LF power is cw. The smaller duty-cycle tends to produce an extended energy range in the IED.
(Color online) Temporal behavior of dc-bias with different duty-cycles when pulsing the HF power with a PRF of 50 kHz. (a) BC = 10 nF and (b) BC = 1 μF. The LF power is cw. The dynamic range of the dc-bias is from 0 V to −200 V with the smaller BC while the range is only from −60 to −90 V with larger BC.
(Color online) Temporal behavior of dc-bias with different duty-cycles when pulsing the HF power with a PRF of 50 kHz. (a) BC = 10 nF and (b) BC = 1 μF. The LF power is cw. The dynamic range of the dc-bias is from 0 V to −200 V with the smaller BC while the range is only from −60 to −90 V with larger BC.
(Color online) Total IEDs for all ions for different duty-cycles when pulsing the LF power with a PRF of 50 kHz. (a) BC = 10 nF and (b) BC = 1 μF. The HF power is cw. The amplitude of the low energy peak diminishes while the amplitude of the high energy peak increases as the duty-cycle increases. The IED becomes similar to that of the cw case with further increase of the duty-cycle.
(Color online) Total IEDs for all ions for different duty-cycles when pulsing the LF power with a PRF of 50 kHz. (a) BC = 10 nF and (b) BC = 1 μF. The HF power is cw. The amplitude of the low energy peak diminishes while the amplitude of the high energy peak increases as the duty-cycle increases. The IED becomes similar to that of the cw case with further increase of the duty-cycle.
(Color online) Temporal behavior of dc-bias with different duty-cycles when pulsing the LF power with a 50 kHz PRF. (a) BC = 10 nF and (b) BC = 1 μF. The HF power is cw. The dynamic range is from −40 to +80 V with the smaller BC while the range is at most ±15 V at 25% duty-cycle with larger BC. Note that the range of oscillation the dc-bias is similar for different duty-cycles with the smaller BC while the range is shifted by duty-cycle with the larger BC.
(Color online) Temporal behavior of dc-bias with different duty-cycles when pulsing the LF power with a 50 kHz PRF. (a) BC = 10 nF and (b) BC = 1 μF. The HF power is cw. The dynamic range is from −40 to +80 V with the smaller BC while the range is at most ±15 V at 25% duty-cycle with larger BC. Note that the range of oscillation the dc-bias is similar for different duty-cycles with the smaller BC while the range is shifted by duty-cycle with the larger BC.
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