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C2Cl3F3 plasma
361
Microelectronic Engineering 23 (1994) 361-3@ Elsevier
Experimental study of anisotropy mechanisms during reactive ion etching of silicon in a SF&2QF3 plasma V.A. Yunkina, I.W. Rangelowb, J.A. Schaeferb, D. Fischef, E. VogesC and S. Sloboshanind a7nstitute of Microelectronics Technology, Russian Academy of Sciences, Chemogolovka, 142432, Moscow distr., Russia bUniversity of Kassel, FB- 18, Heinrich-Plett-Str.40, D-34 132 Kassel, Germany C,Universitit Dortmund, Lehrstuhl fiir Hochfrequenztechnik, Postfach 50 05 00, D-44221 Dortmund, Germany d3nstitute of Solid State Physics and Semiconductors, Minsk, Belorussia The experimental results of the investigations of the mechanisms of anisotropy of silicon RIE, using a SF&,Cl,F, plasma, are presented and discussed.
1.
INTRODUCTION
Reactive ion etching of deep trenches in single crystal silicon with photoresist masks and SF, / C2Cl,F, plasma which is characterized by a high anisotropy, a high etch rate and a good selectivity has been developed [l]. Using other materials as masks (SiO,, Al) has resulted in trench profiles with high undercut of the mask as for pure SF6 RIE. Similar effects during silicon etching in the plasma etch mode with mixtures of SF, with C,ClF, or CBrF, have been found 121.It has been shown that the anisotropic etching with these plasmas is caused by the probable deposition of a halocarbon polymer layer which inhibits lateral etching, and which is related to the masking photoresist which appears to release hydrocarbons, that polymerize in the presence of active species in the plasma. However, the actual inhibitor layer responsible for preventing lateral etching has not been found and identified. This paper presents experimental results of investigations of mechanisms which determine the anisotropic character of silicon etching using RIE in a SF&,Cl,F, plasma.
2. EXPERIMENTAL The etching experiments were carried out in a planar RIE system (Plasma Technology RIE 80). Process conditions resulting in highly anisotropic reactive ion etching of deep trenches in silicon using photoresist mask [l] have been used in most of the experiments. The operating pressure of the gas mixture (80% SF6 / 20% C.&F,) was in the range of 47-50 mTorr. The rf power was kept at 130 W. Four types of samples were prepared: (a) Si (100) with a patterned 1.4 pm-AZ 5214E mask; (b) Si with a 1 l.tm-Si02 mask patterned using photolithography; (c) Si with a double photolithographically patterned mask layer of AZ 5214E and SiO,; (d) Si with a mask of 0.2 pm-Al film on top of the AZ 5214E photoresist layer. The depths of etched Si-trenches were measured by a stylus (AlphaStep 200) or/and optical (UBM) profilometers. The profile and lateral undercut of the trenches were observed and evaluated by scanning electron (JSM-6400, JEOL and H-4000, Hitachi) or/and optical microscopes.
0167-9317/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved.
V.A. Yunkin et al. I Experimental
362
3.
RESULTS
AND
study of anisotropy
mechanisms
DISCUSSION
After etching the samples (a), (b). (c), and (d) under the same conditions different trench profiles were obtained. Figure 1 shows that the highest anisotropy is achieved on samples with the photoresist masks. When the top layer was not a photoresist material the etch profile was isotropic. Achieved results suggested that high anisotropy is related to sidewall passivation effects by polymer deposition. The probable mechanisms of the formation of polymeric films have been considered as follows: (i) formation of monomers in gaseous SF&$l,F, plasma and their polymerization on the silicon surface; (ii) formation of monomers in the plasma and their catalytic polymerization in the presencc of the photoresist; (iii) releasing of hydrocarbons from the photoresist surface that polymerize in the presence of active species in the plasma; (iv) generation of polymers on the photoresist surface in the presence of active species from the plasma, their removal by ion sputtering, and their redeposition.
(a>
(b)
tc>
(4
Figure 1. SEM cross-sectional micrographs of the trenches with different masks etched under the same plasma parameters: (a) AZ 5214E, (b) SiO, mask, (c) double-layer (AZ 5214E/SiO,) mask, (d) double-layer (Al/AZ 5214E) mask.
Figure 2. SEM micrographs of the trenches with SiO, masks etched with different proportion of C,Cl,F, in the SF&,Cl,F, gas mixture: (a) 20 %. (b) 27 %, (c) 32 %. The total gas flow rate was 15 seem. Self-bias voltage was -131, -139, and - 143 V: respectively. The etching time was 15, 25 and 25 min, respectively. Etching results of samples with double-layer (aluminium/photoresist) masks have refuted the second mechanism (Figure Id). Experiments using samples with SiOJmasks and plasmas with different SF&,Cl,F, mrxtures have provided no evidence to support the first proposed mechanism of the anisotropy (Figure 2.).
Figure 3. Test-structure used for investigation of the lateral extent of the photoresist effect on etching characteristics.
363
V.A. Yunkin et al. / Experimental study of anisotropy mechanisms
A
Ts 10 3
5 4 0
A_
t-
9
The lateral extent of the photoresist effect on etching characteristics was examined in order to test the mechanisms (iii) and (iv). Test-structure used in these experiments is shown schematically in Figure 3. The resist covered one edge of the substrates with an oxide masks patterned on them. The undercut of the SiO, mask and the depth of trenches after RIE were measured as a function of distance from the edge of the resist layer (Figure 4). The values of the undercut and the depth are maximal under and near the edge of the resist. With increasing distance from the photoresist edge these values decrease to their minima at about 14 pm from the edge. These values rapidly increase again in the range up to 4 mm and increase gradually as the distance increases beyond 4 mm. These results indirectly confirm the photoresist related polymer deposition effecting on the degree of anisotropy. In addition, the presence of the maximal undercut and of the depth near the edge of the resist suggest that the fourth mechanism is valid,
*
-
I.
.
. .
n-..
08001 00;
1
01 DISTANCE
10°
(mm)
Figure 4. Dependence of the undercut (M) and the depth (A) of trench on the distance from the photore,$st mask edge. The structure was etched under the same conditions as shown in Figure 1. The etching tfme was 16 min.
41
I’
1
I
I
’
I
’
I
I
284.3
688.6
I------
before etching
l
lXPS
1
I
after etching
Ihv=1253.6& L___________-_J
532.8
c12p
Fls
201.1 4
I
I
I
695 690 685
y/l
I
I,
540 535 530 BINDING
l/l
I
I
I
I
I
I;-b
295 290 285 280 ENERGY
I
I
205 200 195
(eV)
Figure 5. XPS-spectra (Fls, Ols, Cls, C12p) from the photoresist masks before (s..) and after (-)
etching.
V.A. Yunkin et al. I Experimental
364
Table 1. Composition
of photoresist
study of anisotropy
mechanisms
surface (%).
W&),, before RLE
95
after RIE
45
(CF, Hyln
0
Cl
5 45
i.e. generation of polymers on the photoresist surface in the presence of active species from the plasma and their redeposition by ion sputtering. The shadow field near the edge of the resist appears to be related to the transport of ion-sputtered species. The results of photoresist stripping by acetone after RIE have shown, that there is a thin polymer-like film on the resist surface. For identification a chemical analysis of the photoresist (before and after etching) was performed by X-ray photoelectron spectroscopy (XPS) with MgK a-radiation (E = 1253.6 cV) by monitoring the core level lines of Fls, 0 1s, Cls, and C12p (Figure 5). Table 1 summarizes the XPS results. By comparison with data from literature [3], we find that the photoresist drastically changes its composition after etching. This is evident (see Figure 5) from the reduction of the Cls-signal at a binding energy at 284.3 cV, which is typical for (CH2)“. The rather broad Cls-signal at higher binding energies up to a value of 290.2 eV is typical for different (CF,Hy),-species. In this case, higher Cls-binding energies are Identified with higher fluorine concentrations in the reacted resist material. The carbon/hydrogen-compound (95 %) decreases to 45 %. Simultaneously, a carbon/fluorine/ hydrogen-compound (45 %) is formed, and the oxygen signal increases from 5 % to 7 %. In addition, minor amounts of carbon/chlorine (3 %) are detected after etching. This supports the hypothesis that there is a strong reaction between the photoresist and the SF&$l,F, plasma. As a result, it seems possible that some of the reaction products are desorbed by ion sputtering and passivate trench sidewalls. However, by using high resolution field emission microscopy (Hitachi-4000) we haven’t seen any passivation film on the sidewalls.
7
4.
3
CONCLUSION
It has been shown that anisotropic etching of silicon trenches in the presence of a photoresist with a SF&,Cl,F, plasma is related to polymer deposition on the sidewalls to inhibit lateral etching. The experiments suggest that an ion-sputtered polymer from the modified resist surface is a main sidewall passivation compound. The mechanisms of polymer transport to the sidewalls, the polymer composition on the sidewalls and the etching mechanisms of very deep structures about 100 pm are not clear yet. The developed very deep Si-etch technology is a very promising tool for the fabrication of micromechanical and optoelectronical structures.
ACKNOWLEDGEMENTS V.A. Yunkin and S. Sloboshanin wish to thank the DAAD (Deutscher Akademischer Austauschdienst’) for financial support.
REFERENCES V.A.Yunkin, D.Fischer, E.Voges (these proceedings). J.P.McVittie and C.Gonzalez, Proc. of the Fifth Symp. on Plasma Processing, The Electrochem. Society, Vo1.85-1(1985), pp.552. XPS-Handbook, Perkin Elmer (1978), Physics Electronics Division.
Microelectronic Engineering 23 (1994) 361-3@ Elsevier
Experimental study of anisotropy mechanisms during reactive ion etching of silicon in a SF&2QF3 plasma V.A. Yunkina, I.W. Rangelowb, J.A. Schaeferb, D. Fischef, E. VogesC and S. Sloboshanind a7nstitute of Microelectronics Technology, Russian Academy of Sciences, Chemogolovka, 142432, Moscow distr., Russia bUniversity of Kassel, FB- 18, Heinrich-Plett-Str.40, D-34 132 Kassel, Germany C,Universitit Dortmund, Lehrstuhl fiir Hochfrequenztechnik, Postfach 50 05 00, D-44221 Dortmund, Germany d3nstitute of Solid State Physics and Semiconductors, Minsk, Belorussia The experimental results of the investigations of the mechanisms of anisotropy of silicon RIE, using a SF&,Cl,F, plasma, are presented and discussed.
1.
INTRODUCTION
Reactive ion etching of deep trenches in single crystal silicon with photoresist masks and SF, / C2Cl,F, plasma which is characterized by a high anisotropy, a high etch rate and a good selectivity has been developed [l]. Using other materials as masks (SiO,, Al) has resulted in trench profiles with high undercut of the mask as for pure SF6 RIE. Similar effects during silicon etching in the plasma etch mode with mixtures of SF, with C,ClF, or CBrF, have been found 121.It has been shown that the anisotropic etching with these plasmas is caused by the probable deposition of a halocarbon polymer layer which inhibits lateral etching, and which is related to the masking photoresist which appears to release hydrocarbons, that polymerize in the presence of active species in the plasma. However, the actual inhibitor layer responsible for preventing lateral etching has not been found and identified. This paper presents experimental results of investigations of mechanisms which determine the anisotropic character of silicon etching using RIE in a SF&,Cl,F, plasma.
2. EXPERIMENTAL The etching experiments were carried out in a planar RIE system (Plasma Technology RIE 80). Process conditions resulting in highly anisotropic reactive ion etching of deep trenches in silicon using photoresist mask [l] have been used in most of the experiments. The operating pressure of the gas mixture (80% SF6 / 20% C.&F,) was in the range of 47-50 mTorr. The rf power was kept at 130 W. Four types of samples were prepared: (a) Si (100) with a patterned 1.4 pm-AZ 5214E mask; (b) Si with a 1 l.tm-Si02 mask patterned using photolithography; (c) Si with a double photolithographically patterned mask layer of AZ 5214E and SiO,; (d) Si with a mask of 0.2 pm-Al film on top of the AZ 5214E photoresist layer. The depths of etched Si-trenches were measured by a stylus (AlphaStep 200) or/and optical (UBM) profilometers. The profile and lateral undercut of the trenches were observed and evaluated by scanning electron (JSM-6400, JEOL and H-4000, Hitachi) or/and optical microscopes.
0167-9317/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved.
V.A. Yunkin et al. I Experimental
362
3.
RESULTS
AND
study of anisotropy
mechanisms
DISCUSSION
After etching the samples (a), (b). (c), and (d) under the same conditions different trench profiles were obtained. Figure 1 shows that the highest anisotropy is achieved on samples with the photoresist masks. When the top layer was not a photoresist material the etch profile was isotropic. Achieved results suggested that high anisotropy is related to sidewall passivation effects by polymer deposition. The probable mechanisms of the formation of polymeric films have been considered as follows: (i) formation of monomers in gaseous SF&$l,F, plasma and their polymerization on the silicon surface; (ii) formation of monomers in the plasma and their catalytic polymerization in the presencc of the photoresist; (iii) releasing of hydrocarbons from the photoresist surface that polymerize in the presence of active species in the plasma; (iv) generation of polymers on the photoresist surface in the presence of active species from the plasma, their removal by ion sputtering, and their redeposition.
(a>
(b)
tc>
(4
Figure 1. SEM cross-sectional micrographs of the trenches with different masks etched under the same plasma parameters: (a) AZ 5214E, (b) SiO, mask, (c) double-layer (AZ 5214E/SiO,) mask, (d) double-layer (Al/AZ 5214E) mask.
Figure 2. SEM micrographs of the trenches with SiO, masks etched with different proportion of C,Cl,F, in the SF&,Cl,F, gas mixture: (a) 20 %. (b) 27 %, (c) 32 %. The total gas flow rate was 15 seem. Self-bias voltage was -131, -139, and - 143 V: respectively. The etching time was 15, 25 and 25 min, respectively. Etching results of samples with double-layer (aluminium/photoresist) masks have refuted the second mechanism (Figure Id). Experiments using samples with SiOJmasks and plasmas with different SF&,Cl,F, mrxtures have provided no evidence to support the first proposed mechanism of the anisotropy (Figure 2.).
Figure 3. Test-structure used for investigation of the lateral extent of the photoresist effect on etching characteristics.
363
V.A. Yunkin et al. / Experimental study of anisotropy mechanisms
A
Ts 10 3
5 4 0
A_
t-
9
The lateral extent of the photoresist effect on etching characteristics was examined in order to test the mechanisms (iii) and (iv). Test-structure used in these experiments is shown schematically in Figure 3. The resist covered one edge of the substrates with an oxide masks patterned on them. The undercut of the SiO, mask and the depth of trenches after RIE were measured as a function of distance from the edge of the resist layer (Figure 4). The values of the undercut and the depth are maximal under and near the edge of the resist. With increasing distance from the photoresist edge these values decrease to their minima at about 14 pm from the edge. These values rapidly increase again in the range up to 4 mm and increase gradually as the distance increases beyond 4 mm. These results indirectly confirm the photoresist related polymer deposition effecting on the degree of anisotropy. In addition, the presence of the maximal undercut and of the depth near the edge of the resist suggest that the fourth mechanism is valid,
*
-
I.
.
. .
n-..
08001 00;
1
01 DISTANCE
10°
(mm)
Figure 4. Dependence of the undercut (M) and the depth (A) of trench on the distance from the photore,$st mask edge. The structure was etched under the same conditions as shown in Figure 1. The etching tfme was 16 min.
41
I’
1
I
I
’
I
’
I
I
284.3
688.6
I------
before etching
l
lXPS
1
I
after etching
Ihv=1253.6& L___________-_J
532.8
c12p
Fls
201.1 4
I
I
I
695 690 685
y/l
I
I,
540 535 530 BINDING
l/l
I
I
I
I
I
I;-b
295 290 285 280 ENERGY
I
I
205 200 195
(eV)
Figure 5. XPS-spectra (Fls, Ols, Cls, C12p) from the photoresist masks before (s..) and after (-)
etching.
V.A. Yunkin et al. I Experimental
364
Table 1. Composition
of photoresist
study of anisotropy
mechanisms
surface (%).
W&),, before RLE
95
after RIE
45
(CF, Hyln
0
Cl
5 45
i.e. generation of polymers on the photoresist surface in the presence of active species from the plasma and their redeposition by ion sputtering. The shadow field near the edge of the resist appears to be related to the transport of ion-sputtered species. The results of photoresist stripping by acetone after RIE have shown, that there is a thin polymer-like film on the resist surface. For identification a chemical analysis of the photoresist (before and after etching) was performed by X-ray photoelectron spectroscopy (XPS) with MgK a-radiation (E = 1253.6 cV) by monitoring the core level lines of Fls, 0 1s, Cls, and C12p (Figure 5). Table 1 summarizes the XPS results. By comparison with data from literature [3], we find that the photoresist drastically changes its composition after etching. This is evident (see Figure 5) from the reduction of the Cls-signal at a binding energy at 284.3 cV, which is typical for (CH2)“. The rather broad Cls-signal at higher binding energies up to a value of 290.2 eV is typical for different (CF,Hy),-species. In this case, higher Cls-binding energies are Identified with higher fluorine concentrations in the reacted resist material. The carbon/hydrogen-compound (95 %) decreases to 45 %. Simultaneously, a carbon/fluorine/ hydrogen-compound (45 %) is formed, and the oxygen signal increases from 5 % to 7 %. In addition, minor amounts of carbon/chlorine (3 %) are detected after etching. This supports the hypothesis that there is a strong reaction between the photoresist and the SF&$l,F, plasma. As a result, it seems possible that some of the reaction products are desorbed by ion sputtering and passivate trench sidewalls. However, by using high resolution field emission microscopy (Hitachi-4000) we haven’t seen any passivation film on the sidewalls.
7
4.
3
CONCLUSION
It has been shown that anisotropic etching of silicon trenches in the presence of a photoresist with a SF&,Cl,F, plasma is related to polymer deposition on the sidewalls to inhibit lateral etching. The experiments suggest that an ion-sputtered polymer from the modified resist surface is a main sidewall passivation compound. The mechanisms of polymer transport to the sidewalls, the polymer composition on the sidewalls and the etching mechanisms of very deep structures about 100 pm are not clear yet. The developed very deep Si-etch technology is a very promising tool for the fabrication of micromechanical and optoelectronical structures.
ACKNOWLEDGEMENTS V.A. Yunkin and S. Sloboshanin wish to thank the DAAD (Deutscher Akademischer Austauschdienst’) for financial support.
REFERENCES V.A.Yunkin, D.Fischer, E.Voges (these proceedings). J.P.McVittie and C.Gonzalez, Proc. of the Fifth Symp. on Plasma Processing, The Electrochem. Society, Vo1.85-1(1985), pp.552. XPS-Handbook, Perkin Elmer (1978), Physics Electronics Division.