Surface contamination control during plasma etching
`
`H. Miyatake, K. Kawai, N. Fujiwara, M. Yoneda,
`K. Nishioka+ and H. Abe
`
`LSI Laboratory, Mitsubishi Electric Corporation,
`4-1 Mizuhara,
`Itami, Hyogo 664,
`Japan
`+Kita-Itami Works, Mitsubishi Electric Corporation,
`4—1 Mizuhara,
`Itami, Hyogo 664, Japan
`
`ABSTRACT
`
`in
`is developed by employing NF3 gas
`Reactive ion etching (RIE)
`order
`to avoid the fluorocarbon contamination on
`the Si
`surface ex—
`
`posed to the plasma.
`A high SiOZ etch rate is achieved with magneti-
`cally enhanced RIE
`because of efficient Species generation. An aniso—
`tropic etching profile of $102 is obtained due to the low pressure and
`low temperature operation. The
`reaction layers
`on Si
`surfaces
`are
`investigated by x-ray photoelectron spectrOSCOpy and cross-sectional
`transmission electron microscopy.
`It
`is
`found
`that
`the NF3 plasma
`etching is more effective to maintain a clean surface than the CHF3
`plasma etching.
`In addition the photoresist which is used as
`a mask
`during via-hole etching is easily removed without
`any residues by 02
`plasma ashing because the fluorocarbon contamination is avoided.
`
`l.INTRODUCTION
`
`surface during plasma etching is
`a Si
`of
`Contamination control
`a high quality device. Fluorocarbon plas-
`required for
`fabrication of
`mas
`have been' studied extensively and
`have
`been used
`for
`etching
`polysilicon and silicon oxide. However, device degradation is caused
`by contamination originating from fluorocarbon deposition during dry
`etching.l The polymer deposition on
`the Silicon surface
`in an NF3
`discharge is minimal compared to etching in a
`fluorocarbon plaSma. The
`NF3 gas plasma
`is often used
`for
`surface cleaning treatment after
`conventional
`reactive ion etching (RIE)
`to remove
`the
`fluorocarbon
`film deposited from the reactive gas plasma-2—4
`In this work,
`an RIE process was developed by employing NF3 gas. A
`high Si02 etch rate was achieved and an anisotropic etching profile
`was
`formed in the NF3 plasma with magnetically enhanced RIE (MERIE)-
`The
`composition of
`reaction layers
`on Si
`surfaces
`exposed
`to
`the
`plasma was
`investigated by x-ray photoelectron Spectroscopy (XPS). The
`surface quality of
`the Si substrate was also characterized by cross-
`Sectional
`transmission electron microscopy (TEM)
`in more detail.
`
`2.EXPERIMENTAL
`
`The MERIE System was used in this study. The wafers were clamped to
`the rf powered electrode. Helium back Side cooling was used to main-
`
`0-8794-0724-0/92/8400
`
`IP Bridge Exhibit 2223
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`SP/E Vol. 7593 Dry Etch Techno/ong7997j/ 47
`
`

`

`tain a constant wafer
`temperature. Clean (100) Si wafers were etched
`with 700 W of
`rf
`power applied using NF3
`0r CHFS
`for
`60
`sec. The
`chamber pressure was
`50 mTorr
`and the electrode temperature was 5°C.
`The other process parameters such as gas
`flow (40 Sccm)
`and magnetic
`field strength (90 C) were held constant
`in this experiment.
`The Surface of
`the wafers was analyzed using XPS. The XPS Spectra
`were excited with Mg Km x-ray at
`10
`kV. For
`TEM observation (Oll)
`cross—sectional Specimens were prepared. The observation of
`surface
`profile imaging was carried out with a high-resolution electron micro—
`scope operated at 200 kV.
`thermal-
`SiOZ etch rates were measured on samples that consisted of
`ly grown SiO2
`layer
`on
`a silicon substrate with a photoresist mask.
`The Si02 profiles
`etched by NF3
`gas were observed using scanning
`electron microscopy (SEM). The chamber pressure was varied from 20 to
`200 mTorr
`and the electrode temperature was varied from -50 to 20°C.
`Finally,
`the surface residues of
`the samples after via-hole etching at
`low pressure (20 mTorr) and low temperature (-50°C) were investigated
`by
`SEM. The
`samples were observed after
`the
`resist
`removal
`by
`02
`plasma ashing.
`
`3.RESULTS AND DISCUSSION
`
`sample etched by NF3 and CHF3 gas are shown
`the Si
`XPS Spectra of
`in Fig.1. The intensities of
`the Si
`2S and 2p peaks are Strong for
`the
`sample etched in NF3. For
`the sample etched in CHF3 they are weak. For
`the NF3 etched sample peaks due to O Auger, O ls, F Auger and F is are
`observed,
`and the C ls peak is very small. For
`the CHF3 etched sample
`the peaks due
`to oxygen appear
`to be smaller, while the peaks due
`to
`fluorine and the C is peak is much larger. This results can be Simply
`explained by the formation of
`a
`fluorocarbon layer
`on
`the Si
`surface
`after etching in CHF3. On
`the other hand
`there is
`an oxide
`layer
`which contains F atoms on the surface exposed in NF3.
`In Fig.2 cross—sectional TEM images of NF3 and CHF3 plasma-exposed
`Si Surface are Shown.
`It Shows
`(200)
`lattice planes parallel
`to the Si
`surface. The
`(200) plane spacing is 0.27 nm. For
`the NF3 etched sample
`there are no defects in evidence and the Si surface is smooth within a
`few monolayers
`[Fig.2(a)]. Extensive defects are found in the Surface
`layer after etching in CHF3
`[Fig.2 (b)]. Small amorphouslike regions
`and a high density of planer defects are observed. They are heavily
`decorated by
`impurities, possibly H,
`C or F.5 Figure 2(a)
`Shows
`the
`a
`presence of
`2
`nm thick amorphous
`film (indicated by arrows) which
`was
`found by XPS
`to contain mostly oxygen and
`thus
`represents
`the
`native oxide on the surface eXposed in NF3. AS Shown
`in Fig 2(b)
`a
`3
`nm thick amorphous
`film (alSo indicated by
`arrows)
`on
`the
`surface
`etched by CHF3
`is observed. This
`film was
`identified to be
`a
`fluoro—
`carbon film by XPS.
`function of
`a
`as
`the etch rates of Si02 in NF3
`Figure 3 Shows
`change very
`pressure.
`It
`is noted that
`the SiOZ etch rate does not
`much with pressure. A high Si02 etch rate is achieved with the MERIE
`because of efficient Species generation. Figure 4
`shows
`the etch rates
`of SiO2
`as
`a
`function of
`temperature. The Si02 etch rate slightly
`
`48 / SPIE Vol, 7 593 Dry Etch Technology/1997)
`
`

`

`keeps nearly constant
`
`it
`but
`temperature
`lowering
`increases with
`to 20°C.
`within the temperature range from —50
`02
`by
`removal
`resist
`SEM micrographs
`of
`the
`samples after
`the
`the NF3 pressure
`ashing are shown in Fig.5. For
`the sample etched at
`of
`200 mTorr
`and
`the
`temperature of
`5 °C,
`the
`sidewall profile is
`slightly bowed with a positive taper
`[Fig.5(a)]. As
`the pressure is
`decreased to 20 mTorr,
`the bowed
`feature of
`the sidewall disappears
`and the profile exhibits a slightly positive slope [Fig.5(b)]. For
`the
`sample etched at
`the temperature of
`-50°C and the NF3 pressure of
`20
`mTorr,
`the straight
`sidewall
`is produced [Fig.5(c)]. An anisotropic
`etching profile is
`formed due
`to
`low pressure and
`low temperature
`operation. Etching at higher pressures tends towards Chemical process-
`es where ion energies are lower and the density of reactive Species is
`higher. Etching at
`lower pressures emphasizes physical processes and
`etching at
`lower
`temperatures further enhances them. Figure 5(d)
`shows
`the cross-sectional view of
`the sample etched in CHF3 at
`the tempera-
`ture of
`-50°C and the preSSure of
`20 mTorr. The
`tapered sidewall
`is
`formed. The
`tapered etching profile is attained by
`the
`simultaneous
`progress of etching and deposition. The deposition of
`a
`fluorocarbon
`film on the sidewall
`induces the tapered profile formation at
`the low
`temperature.6 During SiOZ etching in NF3
`the
`polymer
`film is
`not
`deposited
`on
`the
`sidewall
`for
`lack
`of deposition gas
`such
`as
`hydrocarbon.
`For
`the
`sample
`etched
`in NF3
`on
`the
`same
`etching
`condition as CHF3,
`therefore,
`the straight sidewall
`is obtained.
`the
`Figure 6
`shows
`the profile of
`the via-hole etched in NF3 at
`temperature of
`-50°C and the pressure of
`20 mTorr. The
`samples were
`treated in HF Solution before the plasma etching in NF3. Undercutting
`by the HF treatment
`is observed at
`the interface between the photore—
`sist
`and the SiO2 film. The surface residues of
`the samples etched in
`NF3 and CHF3 after the resist
`removal by 02 ashing are shown in Fig.7.
`For
`the sample etched in CHF3
`the residual
`films still
`remain around
`the via-holes
`[Fig.7(b)].
`The
`fluorocarbon film which contains Al
`atoms Sputtered during overetching was
`redeposited on the sidewall and
`the surface of
`the photoresist. On
`the sample etched in NF3
`there are
`no
`residues
`[Fig.7(a)],
`because
`the
`fluorocarbon contamination
`is
`avoided.
`
`4.CONCLUSION
`
`surface modification in Si Substrate induced during plasma
`The
`etching was
`studied by XPS
`and
`cross—sectional
`TEM.
`For
`pure NF3
`etching gas,
`a native oxide film 2
`nm thick grown on
`the Si
`surface
`was observed and there were no extended surface defects.
`It was
`found
`
`the NF3 plasma etching was effective to maintain a clean surface
`that
`as compared to the sample exposed to the CHF3 plasma.
`In addition,
`the
`photoresist used during via—hole etching was easily removed without
`any residues by 02 ashing because the fluorocarbon contamination was
`avoided. A clean RIE process was developed by employing the NF3 gas. A
`high Si02 etch rate was achieved and an aniSOtropic etching profile
`was obtained in the NF3 plasma with the MERIE.
`
`SP/E Vol. 7593 Dry Etch Technology {7991) / 4.9
`
`

`

`5.REFERENCES
`
`1. Y. H. Lee, G. S. Oehrlein and C. Ranson, ”RIE-Induced Damage and
`Contamination in Silicon,” Radiation Effects
`and Defects
`in Solids,
`vol 111&112, pp.221-232, 1989.
`2. H. Cerva, E. G. Mohr
`and H. Oppolzer, ”Transmission Electron
`Microscope Study of Lattice Damage
`and Polymer Coating Formed after
`Reactive Ion Etching of 8102,"
`J. Vac. Sci. Technol., vol B5,
`no 2,
`pp.590-593, 1987.
`
`”Remoyal of RIE Induced
`3. T. Akimoto, K. Kasama and M. Sakamoto,
`Damage Layer Using NF3/02 Chemical Dry Etching,” Proc. of 10th Sympo-
`sium on Dry Process, pp 92-97, 1988.
`4. T. Ogawa, K. Kawai, H.
`Ito, M. Yoneda and K. Nishioka, ”Si Sur-
`face Cleaning Using NF3 after Glow Plasma
`and Deep UV Irradiation,”
`Proc. of 11th Symposium on Dry Process, pp.94—99, 1989.
`5. S.
`J.
`Jeng and G. S. Oehrlein, ”Microstructural Studies of Reac-
`tive Ion Etched Silicon,” Appl. Phys. Lett., vol 50,
`no 26,
`pp 1912-
`1914, 1987.
`
`6. T. Ohiwa, K. Horioka, T. Arikado,
`I. Hasegawa
`and H. Okano,
`”SiOZ Tapered etching employing Magnetron Discharge,” Proc.
`of 12th
`Symposium on Dry Process, pp.105-109, 1990.
`
`INTENSWY(au)
`
`1000
`
`800
`
`600
`
`400
`
`200
`
`O
`
`BINDING ENERGY (eV)
`
`Fig.1. XPS Spectra of NF3 and CHF3 etched Si samples.
`
`50 / SP/E Vol. 1593 Dry Etch Techno/ogyl199 I)
`
`

`

`
`
`(b)
`
`Fig 2. TEM cross sections of Si surface etched in (a) NF3 and (b) CHF3
`plasmas.
`
`SPIE Vol. 7593 Dry Etch Technology {199 7)/ 57
`
`

`

`
`
`200
`
`10
`
`100
`
`PRESSURE(mTon)
`
`400
`
`\O\
`
`O\
`
`
`
`ETCHRATE(nm/min)
`
`72‘
`E\
`Ecv
`LU
`I—
`
`in a
`Fig.3. Si02 etch rates
`NF3 plasma
`as
`a
`function of
`chamber
`pressure.
`The
`elec-
`trode temperature was 5°C.
`
`200
`
`0
`
`in a
`SiOZ etch rates
`Fig.4.
`NF3 plasma
`as
`a
`function of
`electrode
`temperature.
`The
`chamber pressure was 20 mTorr.
`
`-5O
`
`0
`
`50
`
`TEMPERATURE (°c)
`
`EI
`
`O'
`
`—U
`
`J
`
`52 / SPIE Vol. 1593 Dry Etch Technology( 1997 j
`
`

`

`
`
`(:1)
`
`samples were
`SEM cross sections of 8102 sidewall profiles. The
`Fig.5.
`etched at
`(a)
`the NF3 pressure of 200 mTorr
`and the temperature of
`5
`°C,
`(b)
`the NF3 pressure of
`20 mTorr
`and the temperature of 5°C,
`(c)
`the NF3 pressure of 20 mTorr and the temperature of ~50°C, and (d)
`the
`CHF3 pressure of 20 mTorr and the temperature of
`-50°C.
`
`SPIE Vol. 7593 Dry Etch Technology (1997} / 53
`
`

`

`Fig.6.
`
`SEM cross section of via—hole etched in NF3 plasma.
`
`
`
`
`
`(b)
`
`SEM micrographs of surface residues after the resist
`Fig.7.
`02 plasma ashing. The
`samples were etched in
`(a) NF3
`and
`plasma.
`
`removal by
`(b) CHF3
`
`54 / SPIE Vol. 7593 Dry Etch Technology( 1991 )
`
`

`

`DRY ETCH TECHNOLOGY
`
`Volume 1593
`
`SESSION 2
`
`Dry Etch Requirements
`for Advanced Photoresist
`
`Chair
`G. Ken Herb
`
`AT&T Bell Laboratories
`
`