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New plasma source with an UHF (500 MHz) antenna
ELSEVIER
Thin Solid Filrns 281-282 (1996) 169-171
~::
New plasma source with an UHF (500 MHz) anter-' Y. Nakagawa a,*, S. Samukawa b, H. Ueyama c, T. Tsukada u, K. Shinohara
C
~ ANELVA
corp., Yotsuya 5·8·1, Fuchu, Tokyo 183. Japan h Microelectronic.• Research Lab.• NEe, MiYllkigaoka 34, Tsukuba, lbaraki 305, Japan ': Nihon Koshuha Co. LId., Nakayama-elsa II /9, Midori·kll, Yokohama, Kunagawa 226, Japan
Absi'ract
A high density, low temperature, and uniform plasma was achieved using an UHF plasma source. The UHF power was coupled wilh a spokewlse antenna assembly. The density of the CF4/02 plasma was larger than 4 X 10 111 em -) over a plasma diameter of 300 mm and the electron temperature was around 2.5 eV. The properties ofthe UHF plasma source are applicable to the future ULSI processing. Keywords: Fluorine; Optical spectroscopy; Plasma processing and dcposition
for large diameter (12 inches or more) wafer processing
1. Introduction
r3,4]. Several types of low pressure. high density plasma sources such as Helicon wave sources, inductively coupled plasma sources and ECR plasma sources have been developed asthe dry-etching apparatus for future ULSI processing. High rate, precious etch process of aluminum film was realized using the Helicon wave source [PMT, MORI™] r 1] . However. for the Si0 2 etch processes using fluorocarbon gases, these sources have some problems and the most serious one isthe low selectivity between SiD2 and poly-Si etch rates. The etch rate selectivity isdue tothe properties ofthe surface polymer layer, which is formed from the CFx molecules in the plasma. Intheconventional high density plasma sources, the fluorocarbon gasmolecules are highly dissociated by the high energy electron impact and itcauses the lack of the CFx molecules in the plasma. Thus, the most important subject to apply high density plasma sources to the SiD:! etching processes is tokeep the electron temperature low even inthe high density plasma. We have already proposed a new plasma source with a spokewise antenna assembly which couples the UHF power tothe plasma [2,3]. The excitation frequency of a plasma source has an important role for the electron acceleration inthe plasma and the lower electron temperature is expected under the condition of the higher excitation frequency [4]. Furthermore, the wavelength of the UHF (60 emfor 500MHz) issuitable todesign the spokewise antenna
In this report, we show some results of the UHF plasma diagnostics, and discuss the plasma properties incomparison with the Helicon wave plasma source.
2. Experimental apparatus
Fig. I shows the schematic diagram of the UHF discharge system. The system contains the UHF antenna assembly, the discharge chamber, the wafer stage, and the vacuum pumping system. The UHF power was supplied to the antenna assembly and radiated into the discharge chamber through the quartz plate. Spokewise Antenna Assembly
I
• Corrctponuing author. Tel.: +8J ·423·34·0240; Fax: +81-423·60· 22';'':! 0040·6090/S'6;$15,OO e t996 Elsevier Science SA All rights reserved PI/ 50040·6090 (96) 08603-8
Optical Fiber to the Spectrometer
++
Langmuir Probe
WaJerStage
PuQ\plng System
Fig. I. Schcmatic diagram of the UHF discharge system.
Y. Nakagawa etal. /tu« Solid Films 281-282 (1996) 169 -171
170
A cylindrical Langmuir probe was positioned at 75 mm from the quartz plate to measure the plasma density and the electron temperature. The optical emission spectrum was analyzed with a spectrometer toobserve the states of the gas dissociation in the plasma. To compare the performances of the plasma sources, the Helicon wave plasma source [PMT, MORI™] was installed inplace of the UHF plasma source.
3. UHF antenna assembly Fig. 2 shows a schematic illustration ofthe antenna assembly, which has twelve sets of spokes and stub plates. The length of each spoke was 120 mm, that was about 1/4 of the wave length of 500MHz [2,3 J. The resonance frequency of each spoke was individually tuned by adjusting the space between the spoke and the stub plate. The UHF power was supplied toa couple of the spokes.
4. Plasma production mechanism The design of the spokewise antenna assembly is based on the design of an interdigital filter, which was composed of the parallel cylindrical rods (spokes) arranged between parallel grounded plates [5]. The rods are resonantly coupled by the stray capacitance between the adjacent rods and the inductance of the rods itself. Thus, this filter operates as a band pass filter if the resonance frequency of the every rod was tuned to the same frequency. We applied this structure todesign ourspokewise antenna. Therods are arranged like the spokes of a wheel and one of the grounded plates was replaced with the plasma. The electromagnetic wave propagates the spokes consecutively with the phase difference of90 deg, and the UHF current flows on every spoke. The current on the spoke couples both inductively and capacitively to the plasma (eM coupling) and the induction current in the plasma accelerates the electron to sustain the plasma. By using the spokewise antenna, the induction current flows near the every spoke and forms a uniform plasma. Cylindrical Antenna Housing
5. Results and discussion
5. J. Probe measurement The uniformity ofthe plasma density was measured by the Langmuir probe. Fig. 3 shows the radial distribution of the ion saturation current density of the UHF Ar plasma. The non-uniformity was ± 2.3% and ± 12% within a diameter of 200 mm and 300 mm, respectively. We consider that the plasma uniformity is suitable for the 12 inches wafer processing. Furthermore, the uniformity should be improved by the optimization of the antenna assembly and / 01' the installation of a bucket magnetic field. Fig. 4shows the source power dependenciesofthe electron density and the electron temperature of the UHF CF4 / 0 2 plasma. The plasma density increased linearly with the UHF power and was about 4 X10'0 em-3 at the UHF power of 1000 W. The electron temperature was lower than 3 eVwith the UHF power of larger than 500Wand decreased slightly with the power. Fig. 5 shows the dependencies of the electron density in the UHF plasma source and in the Helicon wave plasma source (HWS) on the discharge pressure. The distance between the probe and the wafer stage was almost the same for each plasma source. The gas flow rate was the same as the case ofFig. 4. The density of the UHF plasma was increased with the pressure below 10 mTorr and then saturated. On the other hand, the density of the Helicon plasma was decreased with Center Axis
±2.3%( 1J 200mm)
±12% ( ¢ 300mm)
20 15 10 5 ProbePosition (em)
0
Fig. 3. Radial profile of theion saturation current oftheUHF plasma.
------.8 CF4/02 -1.0/20 seem
lOmTorr \
\. \ \
\
\\
\
\
\.....----T-----:& I
UHF Power Feeder Fig. 2. Schemalic illusrration of the UHF spokcwise antenna assembly.
o
..... ' ,. , ...,'" " 0 250 500 750 1000 UHF Power (W) Fig, 4. Dependence of theUHF plasma characteristic 011 the power.
o
Y. Nakagawa et at./Thill Solid Films 281-282 (1996) /69-171
4)(10
10
UHF 1000W
----------.
CF4/02
''0-.
0
5
...... --'0
oQ... ...
10
15
20
~,I
HWS, Source region
........ ....A
-.
HWS, Diffusion ;;&i~
-.r-e-.a.-.... ~ o
UHF plasma
o
5
10
...~'st"'''te
15 20
2S
C4FS 10mTorr
2000W
25 30
10 ....- . - - - - - - - . . , CF4/02 \ ,
C2
HWS
Source region
F
Pressure (mTorr ) Fig. 5. Pressure dependence of the plasma density in the UHF plasma (e) and inthe diffusion region of the HWS (0).
~
C4FS 10mTorr
F
HWS, 20DOW, Diffusion region
o
171
W
Pressure (rnTorr ) Fig. 6. Pressure dependence ofthc electron temperature in the UHF plasma (e) and inthe source region (.6.) and the dliffusion region (0) oftheHWS.
the pressure. At the gas pressure of 10 mTorr, that is the typical operating pressure for the: dry etching process, the density of the UHF plasma was twice as much as that of the HWS. Hence, wecanestimate that the efficiency of the UHF plasma source is 4 times larger than that of the HWS. In the HWS, a super-high density plasma (about 10 12 em- 3) was l--'foduced inthelimited area and the long diffusion length was needed for the uniform processing of the large area. Therefore, the plasma density neat the wafer was suppressed with increasing the gas pressure. In the case of the UHF plasma source, the plasma production area isequivalent to the area of the spokewise antenna assembly and the uniformity is expected to be good if the wafer stage is set near the plasma production area. Fig. 6 shows the pressure dependencies of the electron temperature in theUHF plasma in comparison with those in the diffusion region and in the source region of the HWS. The electron temperatures in the UHF plasma and in the diffusion region of the HWS were the same. On the other hand, the temperature in the source region of the (fWS was higher than that intheUHF plasma source. This indicates the formation of high energy electrons in the HWS.
5.2. Optical emission spectra Fig. 7 shows the optical emission spectrum of the C4Fs (octatluorocyclobutane) plasma in the UHF source and in the source region of the HWS. For the application ofthe high
300
400
500
600
700
800
Wavelength (nm)
Fig, 7. Optical emission spectra of the UHF plasma source and the source region of the HWS.
density plasma sources, C4Fs isoften employed asthe etching gas for the silicon dioxide films. These spectra show the difference ofthe gas dissociation states between the sources. The most remarkable difference isthe relative intensity ofthe CFz molecule. Inthe UHF plasma source, the gas dissociation was limited to the lower state even in the plasma production region, which was corresponded to the low electron temperature in the UHF plasma. In the source region of the HWS, themain part of thegaswas decomposed to F atoms. which was corresponded to the higher electron temperature as shown in Fig. 6. These results suggest that the UHF plasma promises appropriate etching characteristics for Si0 2• because the, electron temperature is low and thus the concentration ofthe CF2 molecules is high. 6. Summary
A high density, led ,-;ressure and large diameter uniform plasma was obtained using the newly developed UHF (500 MHz) plasma source with the spokewise antenna assembly. The electron temperature was low in the plasma production region. The power efficiency of the plasma production was larger than the Helicon wave source. These features of the UHF plasma source arevery attractive forthe application of the large diameter wafer processing, especially for the silicon dioxide etching process. ){{eferenCI~S
[]] T.Tsukada, H. Nogami, Y. Nakagawa and E.Wani, Jpn. J. Appl. Phys.. 33 (1994) 4433.
[21 S.Samukawa, Y. Nakagawa, T.Tsukada, H. Ueyama and K. Shinohara, Appl. Phys, Lett.• 67 ( 1995) 1414. [3] S.Samukawa, Y. Nakagawa, T.Tsukada, H.Ueyama and K. Shinohara, Digest of Papers oJrfre 8thInt. Microi'rocess COIJ/., The Japan Society 'Of Applied Physics, Sendai, 1995, p. ISO. [4] S.Samukawa, Y. Nakagawa, T.Tsukada, H. Ueyama and K. Shinohara, lpn. J. Appl. Phys; 34 (1995) 6805. (5) RJ. Wenzel. IEEE. Trans. Microwav(! Theory and Tech; MIT·13 (1965) 559.
Thin Solid Filrns 281-282 (1996) 169-171
~::
New plasma source with an UHF (500 MHz) anter-' Y. Nakagawa a,*, S. Samukawa b, H. Ueyama c, T. Tsukada u, K. Shinohara
C
~ ANELVA
corp., Yotsuya 5·8·1, Fuchu, Tokyo 183. Japan h Microelectronic.• Research Lab.• NEe, MiYllkigaoka 34, Tsukuba, lbaraki 305, Japan ': Nihon Koshuha Co. LId., Nakayama-elsa II /9, Midori·kll, Yokohama, Kunagawa 226, Japan
Absi'ract
A high density, low temperature, and uniform plasma was achieved using an UHF plasma source. The UHF power was coupled wilh a spokewlse antenna assembly. The density of the CF4/02 plasma was larger than 4 X 10 111 em -) over a plasma diameter of 300 mm and the electron temperature was around 2.5 eV. The properties ofthe UHF plasma source are applicable to the future ULSI processing. Keywords: Fluorine; Optical spectroscopy; Plasma processing and dcposition
for large diameter (12 inches or more) wafer processing
1. Introduction
r3,4]. Several types of low pressure. high density plasma sources such as Helicon wave sources, inductively coupled plasma sources and ECR plasma sources have been developed asthe dry-etching apparatus for future ULSI processing. High rate, precious etch process of aluminum film was realized using the Helicon wave source [PMT, MORI™] r 1] . However. for the Si0 2 etch processes using fluorocarbon gases, these sources have some problems and the most serious one isthe low selectivity between SiD2 and poly-Si etch rates. The etch rate selectivity isdue tothe properties ofthe surface polymer layer, which is formed from the CFx molecules in the plasma. Intheconventional high density plasma sources, the fluorocarbon gasmolecules are highly dissociated by the high energy electron impact and itcauses the lack of the CFx molecules in the plasma. Thus, the most important subject to apply high density plasma sources to the SiD:! etching processes is tokeep the electron temperature low even inthe high density plasma. We have already proposed a new plasma source with a spokewise antenna assembly which couples the UHF power tothe plasma [2,3]. The excitation frequency of a plasma source has an important role for the electron acceleration inthe plasma and the lower electron temperature is expected under the condition of the higher excitation frequency [4]. Furthermore, the wavelength of the UHF (60 emfor 500MHz) issuitable todesign the spokewise antenna
In this report, we show some results of the UHF plasma diagnostics, and discuss the plasma properties incomparison with the Helicon wave plasma source.
2. Experimental apparatus
Fig. I shows the schematic diagram of the UHF discharge system. The system contains the UHF antenna assembly, the discharge chamber, the wafer stage, and the vacuum pumping system. The UHF power was supplied to the antenna assembly and radiated into the discharge chamber through the quartz plate. Spokewise Antenna Assembly
I
• Corrctponuing author. Tel.: +8J ·423·34·0240; Fax: +81-423·60· 22';'':! 0040·6090/S'6;$15,OO e t996 Elsevier Science SA All rights reserved PI/ 50040·6090 (96) 08603-8
Optical Fiber to the Spectrometer
++
Langmuir Probe
WaJerStage
PuQ\plng System
Fig. I. Schcmatic diagram of the UHF discharge system.
Y. Nakagawa etal. /tu« Solid Films 281-282 (1996) 169 -171
170
A cylindrical Langmuir probe was positioned at 75 mm from the quartz plate to measure the plasma density and the electron temperature. The optical emission spectrum was analyzed with a spectrometer toobserve the states of the gas dissociation in the plasma. To compare the performances of the plasma sources, the Helicon wave plasma source [PMT, MORI™] was installed inplace of the UHF plasma source.
3. UHF antenna assembly Fig. 2 shows a schematic illustration ofthe antenna assembly, which has twelve sets of spokes and stub plates. The length of each spoke was 120 mm, that was about 1/4 of the wave length of 500MHz [2,3 J. The resonance frequency of each spoke was individually tuned by adjusting the space between the spoke and the stub plate. The UHF power was supplied toa couple of the spokes.
4. Plasma production mechanism The design of the spokewise antenna assembly is based on the design of an interdigital filter, which was composed of the parallel cylindrical rods (spokes) arranged between parallel grounded plates [5]. The rods are resonantly coupled by the stray capacitance between the adjacent rods and the inductance of the rods itself. Thus, this filter operates as a band pass filter if the resonance frequency of the every rod was tuned to the same frequency. We applied this structure todesign ourspokewise antenna. Therods are arranged like the spokes of a wheel and one of the grounded plates was replaced with the plasma. The electromagnetic wave propagates the spokes consecutively with the phase difference of90 deg, and the UHF current flows on every spoke. The current on the spoke couples both inductively and capacitively to the plasma (eM coupling) and the induction current in the plasma accelerates the electron to sustain the plasma. By using the spokewise antenna, the induction current flows near the every spoke and forms a uniform plasma. Cylindrical Antenna Housing
5. Results and discussion
5. J. Probe measurement The uniformity ofthe plasma density was measured by the Langmuir probe. Fig. 3 shows the radial distribution of the ion saturation current density of the UHF Ar plasma. The non-uniformity was ± 2.3% and ± 12% within a diameter of 200 mm and 300 mm, respectively. We consider that the plasma uniformity is suitable for the 12 inches wafer processing. Furthermore, the uniformity should be improved by the optimization of the antenna assembly and / 01' the installation of a bucket magnetic field. Fig. 4shows the source power dependenciesofthe electron density and the electron temperature of the UHF CF4 / 0 2 plasma. The plasma density increased linearly with the UHF power and was about 4 X10'0 em-3 at the UHF power of 1000 W. The electron temperature was lower than 3 eVwith the UHF power of larger than 500Wand decreased slightly with the power. Fig. 5 shows the dependencies of the electron density in the UHF plasma source and in the Helicon wave plasma source (HWS) on the discharge pressure. The distance between the probe and the wafer stage was almost the same for each plasma source. The gas flow rate was the same as the case ofFig. 4. The density of the UHF plasma was increased with the pressure below 10 mTorr and then saturated. On the other hand, the density of the Helicon plasma was decreased with Center Axis
±2.3%( 1J 200mm)
±12% ( ¢ 300mm)
20 15 10 5 ProbePosition (em)
0
Fig. 3. Radial profile of theion saturation current oftheUHF plasma.
------.8 CF4/02 -1.0/20 seem
lOmTorr \
\. \ \
\
\\
\
\
\.....----T-----:& I
UHF Power Feeder Fig. 2. Schemalic illusrration of the UHF spokcwise antenna assembly.
o
..... ' ,. , ...,'" " 0 250 500 750 1000 UHF Power (W) Fig, 4. Dependence of theUHF plasma characteristic 011 the power.
o
Y. Nakagawa et at./Thill Solid Films 281-282 (1996) /69-171
4)(10
10
UHF 1000W
----------.
CF4/02
''0-.
0
5
...... --'0
oQ... ...
10
15
20
~,I
HWS, Source region
........ ....A
-.
HWS, Diffusion ;;&i~
-.r-e-.a.-.... ~ o
UHF plasma
o
5
10
...~'st"'''te
15 20
2S
C4FS 10mTorr
2000W
25 30
10 ....- . - - - - - - - . . , CF4/02 \ ,
C2
HWS
Source region
F
Pressure (mTorr ) Fig. 5. Pressure dependence of the plasma density in the UHF plasma (e) and inthe diffusion region of the HWS (0).
~
C4FS 10mTorr
F
HWS, 20DOW, Diffusion region
o
171
W
Pressure (rnTorr ) Fig. 6. Pressure dependence ofthc electron temperature in the UHF plasma (e) and inthe source region (.6.) and the dliffusion region (0) oftheHWS.
the pressure. At the gas pressure of 10 mTorr, that is the typical operating pressure for the: dry etching process, the density of the UHF plasma was twice as much as that of the HWS. Hence, wecanestimate that the efficiency of the UHF plasma source is 4 times larger than that of the HWS. In the HWS, a super-high density plasma (about 10 12 em- 3) was l--'foduced inthelimited area and the long diffusion length was needed for the uniform processing of the large area. Therefore, the plasma density neat the wafer was suppressed with increasing the gas pressure. In the case of the UHF plasma source, the plasma production area isequivalent to the area of the spokewise antenna assembly and the uniformity is expected to be good if the wafer stage is set near the plasma production area. Fig. 6 shows the pressure dependencies of the electron temperature in theUHF plasma in comparison with those in the diffusion region and in the source region of the HWS. The electron temperatures in the UHF plasma and in the diffusion region of the HWS were the same. On the other hand, the temperature in the source region of the (fWS was higher than that intheUHF plasma source. This indicates the formation of high energy electrons in the HWS.
5.2. Optical emission spectra Fig. 7 shows the optical emission spectrum of the C4Fs (octatluorocyclobutane) plasma in the UHF source and in the source region of the HWS. For the application ofthe high
300
400
500
600
700
800
Wavelength (nm)
Fig, 7. Optical emission spectra of the UHF plasma source and the source region of the HWS.
density plasma sources, C4Fs isoften employed asthe etching gas for the silicon dioxide films. These spectra show the difference ofthe gas dissociation states between the sources. The most remarkable difference isthe relative intensity ofthe CFz molecule. Inthe UHF plasma source, the gas dissociation was limited to the lower state even in the plasma production region, which was corresponded to the low electron temperature in the UHF plasma. In the source region of the HWS, themain part of thegaswas decomposed to F atoms. which was corresponded to the higher electron temperature as shown in Fig. 6. These results suggest that the UHF plasma promises appropriate etching characteristics for Si0 2• because the, electron temperature is low and thus the concentration ofthe CF2 molecules is high. 6. Summary
A high density, led ,-;ressure and large diameter uniform plasma was obtained using the newly developed UHF (500 MHz) plasma source with the spokewise antenna assembly. The electron temperature was low in the plasma production region. The power efficiency of the plasma production was larger than the Helicon wave source. These features of the UHF plasma source arevery attractive forthe application of the large diameter wafer processing, especially for the silicon dioxide etching process. ){{eferenCI~S
[]] T.Tsukada, H. Nogami, Y. Nakagawa and E.Wani, Jpn. J. Appl. Phys.. 33 (1994) 4433.
[21 S.Samukawa, Y. Nakagawa, T.Tsukada, H. Ueyama and K. Shinohara, Appl. Phys, Lett.• 67 ( 1995) 1414. [3] S.Samukawa, Y. Nakagawa, T.Tsukada, H.Ueyama and K. Shinohara, Digest of Papers oJrfre 8thInt. Microi'rocess COIJ/., The Japan Society 'Of Applied Physics, Sendai, 1995, p. ISO. [4] S.Samukawa, Y. Nakagawa, T.Tsukada, H. Ueyama and K. Shinohara, lpn. J. Appl. Phys; 34 (1995) 6805. (5) RJ. Wenzel. IEEE. Trans. Microwav(! Theory and Tech; MIT·13 (1965) 559.