Selective SiO2-to-Si3N4 etching in inductively coupled ... - IREAP

Selective SiO2-to-Si3N4 etching in inductively coupled fluorocarbon plasmas: Angular dependence of SiO2 and Si3N4 etching rates Marc Schaepkens and Gottlieb S. Oehrleina) Department of Physics, University at Albany, State University of New York, Albany, New York 12222

Christer Hedlund, Lars B. Jonsson, and Hans-Olof Blom

Solid State Electronics, A˚ngstro¨m Laboratory, Box 534, S-751 21, Uppsala, Sweden

~Received 21 July 1998; accepted 21 August 1998! In the fabrication of microstructures in SiO2 , etch selectivity of SiO2 to masking, etch stop, and underlayer materials need to be maintained at corners and inclined surfaces. The angular dependence of the SiO2-to-Si3N4 etch selectivity mechanism in a high density fluorocarbon plasma has been studied using V-groove structures. The SiO2 etch rate on 54.7° inclined surfaces is lower than on flat surfaces, while the SiO2 etch yield ~atoms/ion! is a factor of 1.33 higher. The results are consistent with a chemical sputtering mechanism. The Si3N4 etch yield is greater by a factor of 2.8 for 54.7° inclined surfaces than for flat surfaces. This large enhancement is explained by a fluorocarbon surface passivation mechanism that controls Si3N4 etching. The fluorocarbon deposition is decreased at 54.7° whereas the fluorocarbon etching rate is increased at 54.7°. This produces a thinner steady-state fluorocarbon film on the inclined Si3N4 surface, and results in a large enhancement of the Si3N4 etch yield. © 1998 American Vacuum Society. @S0734-2101~98!05706-6#

I. INTRODUCTION In the fabrication of integrated circuits, SiO2 etching needs to take place in a highly selective fashion with respect to masking, etch stop, and underlayer materials. The SiO2 etch selectivity mechanism in fluorocarbon plasma etching of blanket films has been studied extensively.1–3 In a companion article4 we discussed the influence of thin steady-state fluorocarbon films on Si, Si3N4 , and SiO2 etching. The etch rate/yield of the substrate material was found to be inversely proportional to the thickness of this fluorocarbon film. The fluorocarbon film thickness reflects the ability of the etched substrate to consume fluorocarbon film precursors. The requirements for achieving SiO2-to-Si3N4 selectivity were found to be analogous to achieving SiO2-to-Si selectivity, i.e., a relatively thick fluorocarbon film needs to suppress the Si3N4 etch rate, while the SiO2 surface stays relatively clean so that fast chemical sputtering can occur. When forming actual microstructures using fluorocarbon plasma etch processes it is important that SiO2 etch selectivity to Si and Si3N4 is maintained at corners and inclined surfaces.5,6 The latter requirement has been proven to be an extremely challenging task, and a reduced SiO2 selectivity is commonly observed. For example, Fig. 1 shows a micrograph of the cross section of an unsuccessfully etched selfaligned contact ~SAC! test structure using an inductively coupled fluorocarbon plasma. The frame of solid lines that is placed over the micrograph indicates the SAC structure before etching. It clearly shows that the SiO2 etch process is significantly slowed down on the flat parts of the Si3N4 etch stop layer, e.g., on top of the poly-Si gate. At the position where the SiO2 etch process encounters a curved/inclined a!

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Si3N4 etch stop layer, the Si3N4 is etched fast with respect to the flat parts. After etching through the curved Si3N4 etch stop layer, the SiO2 spacer is exposed to the plasma and will ultimately be removed. The origin of the etch rate enhancement, and thus selectivity loss, at curved and inclined surfaces is often attributed to the angular dependence of the physical sputtering yield, which shows a maximum around 60° angle of incidence.7 In a fluorocarbon plasma environment, where ion bombardment energies are typically in the order of 100 eV, etch processes can be controlled by processes different from physical sputtering. The ion penetration depth is typically smaller or of the same order as the thickness of steady-state fluorocarbon films present on Si3N4 surfaces. This fluorocarbon film can prevent direct ion impact which is required for physical sputtering. The suggestion of enhanced physical sputtering thus seems unsatisfactory. This article summarizes results of a study aimed at developing more detailed insights in the angular dependence of selective SiO2-to-Si3N4 fluorocarbon etch processes. Etching of 54.7° V-groove samples covered with SiO2 and Si3N4 was examined and compared to etch rates measured for flat SiO2 and Si3N4 samples. The surface modifications due to fluorocarbon plasma exposure have been investigated on both Vgroove and flat Si3N4 samples by postplasma x-ray photoemission spectroscopy. II. EXPERIMENT The inductively coupled plasma source setup is similar to that described in our companion article.4 A planar coil, supplied with 1400 W inductive power at 13.56 MHz, generates the plasma through a 19-mm-thick quartz coupling window. The plasma is fed with 20 sccm of hexafluoropropene (C3F6)

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©1998 American Vacuum Society

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wet etching of silicon substrates,8 and subsequently coated with either 380 nm thermal oxide or a 100 nm thermal SiO2/200 nm LPCVD Si3N4 stack. The angle of the surface in the V-groove samples is 54.7° with respect to the normal of the average wafer surface. The V-groove dimensions are at least an order of magnitude smaller than the plasma sheath, so that ion bombardment remains normal to the macroscopic wafer surface. After etching, the residual SiO2 or Si3N4 film thicknesses on both types of samples were measured using scanning electron microscopy ~SEM! to determine thin film etch rates. The surface chemistry of etched flat and V-groove Si3N4 samples was examined using postplasma in situ x-ray photoelectron spectroscopy ~XPS!, prior to SEM analysis. FIG. 1. Micrograph of selfaligned contact structure etched at a condition with only marginal selectivity of the SiO2 etch process to the curved Si3N4 etch stop layer.

or 40 sccm trifluoromethane (CHF3) and the operating pressure is 6.5 mTorr. A wafer is clamped electrostatically to a chuck that is cooled to 10 °C. To the backside of the wafer He at a pressure of 5 Torr is applied for good thermal contact. The distance between the chuck and the induction coil is 8.5 cm. The chuck is biased using a variable power supply at 3.4 MHz. The selfbias voltage ranged from 15 V ~0 W bias power! to 2175 V ~200 W bias power!. Two types of samples were employed for this work. The first type of samples consisted of either 330-nm-thick thermal SiO2 or 200 nm low-pressure chemical vapor deposition ~LPCVD! Si3N4 on top of 100 nm thermal SiO2 on ~100! single crystal silicon. The second type of samples are Vgroove structures, see Fig. 2, prepared by highly anisotropic

FIG. 2. SEM of an oxide covered 54.7° V-groove sample: ~a! That was exposed to the plasma without a bias applied to the substrate, such that fluorocarbon film has been deposited, and ~b! that was partially etched at 265 V selfbias voltage. The V grooves are wet etched into a c-Si wafer and subsequently coated with a thin film material. This specific V groove is not fully developed, resulting in a flat area between the grooves. J. Vac. Sci. Technol. A, Vol. 16, No. 6, Nov/Dec 1998

III. BLANKET SiO2-TO-Si3N4 SELECTIVITY MECHANISM The mechanism underlying blanket SiO2-to-Si3N4 selectivity has been discussed in detail in a companion article.4 Since the results obtained in this study on etching 54.7° Vgroove structures will be directly compared to blanket etching results, it is appropriate to briefly review the blanket selectivity mechanism. In the absence of high energy ion bombardment, highdensity fluorocarbon plasmas result in fluorocarbon deposition. When a radio frequency ~rf! bias is applied to the substrate, ion bombardment in combination with the presence of fluorine atoms at the fluorocarbon surface will cause the thick fluorocarbon film to be etched. Once the fluorocarbon film is thin enough that some of the ion bombardment energy and fluorine atom flux will be consumed by etching of the substrate material instead of the fluorocarbon film, the fluorocarbon etch rate is reduced. Steady-state conditions are reached, once the fluorocarbon etch and fluorocarbon deposition rates balance. The steady-state condition for SiO2 etching, however, differs from Si3N4 . As a result of the presence of oxygen in the SiO2 substrate, most of the deposited fluorocarbon material is being consumed by net substrate etch reactions.9 Si3N4 , on the other hand, has a reduced ability to aid in the fluorocarbon removal from its surface during etching. At highly selective SiO2-to-Si3N4 conditions, e.g., in high-density plasmas fed with highly polymerizing, fluorine-deficient fluorocarbon gases with H2 and CO as possible additives,10–13 Si3N4 is typically covered with a thicker steady-state fluorocarbon film than SiO2 . Figure 3 shows, for example, that for such condition the intensity of the C 1s spectrum is much higher on a processed Si3N4 than on SiO2 . The Si3N4 etch rate is suppressed by a relatively thick fluorocarbon film, while the SiO2 surface stays relatively clean so that chemical sputtering is possible. IV. RESULTS When etching inclined surfaces, the balance between fluorocarbon deposition, fluorocarbon etching, and fluorocarbon consumption due to substrate etching, is possibly different from blanket etching conditions. In this section, we present

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TABLE I. Fluorocarbon deposition rates and sticking yields at 0 W bias, and fluorocarbon/SiO2 /Si3N4 etch rates and etch yields at 2100 V selfbias measured in discharges of C3F6 and CHF3 ~1400 W inductive power, 6 mTorr operating pressure! for both 0° and 54.7°. The yields were calculated assuming an atomic density of 2.2 g/cm3 for oxide, 3.2 g/cm3 for nitride and for fluorocarbon material the atomic density of Teflon is used, i.e., 2.2 g/cm3. The ratio of the 54.7° yield to 0° yield is also included.

Process

FIG. 3. Carbon 1s XPS spectra obtained from blanket oxide and nitride samples etched at a selective oxide to nitride etching condition.

results on the angular dependence of the fluorocarbon balance and the corresponding SiO2 and Si3N4 etch processes. A. Fluorocarbon deposition and etching

Figure 2~a! shows a micrograph of a SiO2 covered Vgroove sample that was exposed to a C3F6 plasma without a bias applied to the substrate. These process conditions resulted in the deposition of a fluorocarbon film on top of the oxide. The deposited fluorocarbon film has a uniform thickness along the inclined surface, indicating that the fluorocarbon deposition is to a significant extent controlled by lineof-sight processes. An isotropic deposition process would suffer from geometric shadowing at positions deeper in the V groove, resulting in a fluorocarbon film with varying thickness along the inclined surface. Similar data were obtained in a CHF3 plasma. Table I shows the fluorocarbon deposition rates measured in CHF3 and C3F6 on blanket and angled surfaces. The deposition rates on angled surfaces are lower than on the flat surfaces. Since the ion flux to the angled surface is lower by a factor of cos(54.7°), it is useful to also plot the data in terms of number of atoms deposited per incident ion, i.e., fluorocarbon sticking yield. The deposition per incident ion, see Table I, is increased on the inclined surface. In Sec. III we mentioned that next to the fluorocarbon deposition, fluorocarbon etching is an important factor in the formation of the steady-state fluorocarbon film.2,4 In order to evaluate if the same mechanism that explains the fluorocarbon film formation on blanket surfaces holds for the angled surfaces, the fluorocarbon etch rate was measured at 2100 V selfbias voltage, using a V-groove and flat sample on which fluorocarbon material was deposited at 0 W RF bias power. The measured rates and also the number of atoms removed per incident ion, i.e., fluorocarbon etch yield, are also included in Table I for discharges of CHF3 or C3F6 at 1400 W inductive power and 6 mTorr operating pressure. In both JVST A - Vacuum, Surfaces, and Films

0° rate 54.7° rate 0° yield 54.7° yield Yield ~nm/min! ~nm/min! ~atoms/ion! ~atoms/ion! ratio

CFx deposition ~C3F6 , 0 W bias! CFx deposition ~CHF3 , 0 W bias! CFx etching ~C3F6, 2100 V! CFx etching ~CHF3, 2100 V! SiO2 etching ~C3F6, 2100 V! SiO2 etching ~CHF3, 2100 V! Si3N4 etching ~C3F6, 2100 V! Si3N4 etching ~CHF3, 2100 V!

480

380

0.51

0.69

1.35

299

211

0.35

0.43

1.23

380

540

0.40

0.99

2.48

216

413

0.25

0.83

3.32

480

340

0.57

0.69

1.22

378

265

0.45

0.54

1.20

210

424

0.36

1.25

3.47

397.5

279

0.68

0.82

1.21

discharges, the fluorocarbon on the inclined surface etched more rapidly than on the flat surface. In the described blanket etching mechanism, the increase in fluorocarbon etch rate combined with the decrease in fluorocarbon deposition rate, as observed on inclined surfaces, implies a reduced thickness of the steady-state fluorocarbon film. This would lead to an increase in substrate etch yield on the angled surface. If this increase in substrate etch yield would be larger than the decrease in ion flux to the angled surface, then this mechanism would even lead to an increase in substrate etch rate. B. SiO2 etching

Figure 2~b! shows the cross section of a SiO2 covered V-groove sample that was partially etched in C3F6 at a selfbias voltage of 265 V. Similar to the deposited fluorocarbon film, the SiO2 films after etching also had a uniform thickness along the inclined surface. Again, this indicates that the process is to a significant extent controlled by line-of-sight processes, such as direct ion impact. Figure 4~a! shows the SiO2 etch rates in a C3F6 plasma as a function of selfbias voltage, measured normal to either the flat or angled surface. Since the ion flux to the angled surface is lower by a factor of cos(54.7°), it is useful to also plot the data in terms of number of atoms removed per incident ion, i.e., etch yield, see Fig. 4~b!. For all surface orientations the well known transition from fluorocarbon deposition at low bias voltages to thin film etching at high bias voltages is observed. Similar observations were made for SiO2 etching in CHF3 plasmas, see Table I. The SiO2 etch rates are lower on the angled surface, while the SiO2 etch yield is higher on the angled surface than on

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FIG. 4. ~a! SiO2 etch rates and ~b! etch yields as a function of selfbias voltage for both 0° and 54.7° in a C3F6 discharge ~1400 W inductive power, 6 mTorr operating pressure!.

FIG. 5. ~a! Si3N4 etch rates and ~b! etch yields as a function of selfbias voltage for both 0° and 54.7° in a C3F6 discharge ~1400 W inductive power, 6 mTorr operating pressure!.

blanket. The SiO2 yield ratio between 54.7° and 0° is found to be 1.3360.11 on average for the various selfbias voltage conditions. These results are in good agreement with results from CHF3 /O2 reactive ion beam sputtering of thermal SiO2 and tetraethylorthosilicate ~TEOS! by Barklund and Blom,14,15 and support the reactive sputtering mechanism suggested for SiO2 etching by Rueger et al.9 It is not surprising that the oxide etch results can solely be explained in terms of sputtering, while not including effects expected from the fluorocarbon deposition and etching behavior as a function of angle. Since there is only a relatively thin steady-state fluorocarbon film on 0° SiO2 surfaces,4,9 a further reduction in the fluorocarbon film at 54.7° is hardly possible. This effect, therefore, contributes only slightly to the angular dependence of the etch rate.

sults for SiO2 etching are obtained for Si3N4 . Table I shows that the Si3N4 etch rate is lower on the angled surface than on the blanket surface, while the etch yield is a factor of 1.21 higher. These results can be explained in a similar fashion as the SiO2 data. Since there is only a relatively thin steadystate fluorocarbon film on 0° Si3N4 surfaces, as we showed in our companion article,4 a further reduction in the fluorocarbon film at 54.7° will have hardly any effect on the ion limited etch process through which Si3N4 etches at these conditions.

C. Si3N4 etching

Figure 5~a! shows the Si3N4 etch rates, and Fig. 5~b! the Si3N4 etch yields in a C3F6 plasma as a function of selfbias voltage, measured normal to either the flat or angled surface. For the blanket etching condition, Si3N4 etches much slower than SiO2 , i.e., selective SiO2-to-Si3N4 etching occurs. As opposed to SiO2 etching, the Si3N4 etch rate is higher on the angled surfaces, consistent with the observed selectivity loss on curved Si3N4 etch stop layers. The Si3N4 etch yield at the 54.7° surface is much higher than at the 0° surface, see Table I. This large etch yield enhancement is similar to the enhancements observed when etching fluorocarbon. It can therefore be expected that the Si3N4 etch yield enhancement is in agreement with what we expected from the change in fluorocarbon balance, i.e., a reduction in steady-state fluorocarbon film thickness leading to an increased etch rate/yield. On the other hand, when etching Si3N4 at nonselective SiO2-to-Si3N4 etch conditions in a CHF3 plasma, similar reJ. Vac. Sci. Technol. A, Vol. 16, No. 6, Nov/Dec 1998

D. Surface analysis

In order to validate the suggested mechanism for the SiO2-to-Si3N4 selectivity loss on inclined surfaces, we examined the angular dependence of the steady-state fluorocarbon film thickness on Si3N4 surfaces as a function of angle by XPS analysis. The XPS measurements are performed on both flat and V-groove Si3N4 samples etched in a C3F6 plasma, i.e., selective SiO2-to-Si3N4 etching conditions for blanket samples. The thickness of the steady-state fluorocarbon films was calculated by comparing the C (1s) photoemission intensities I ss from processed samples to the intensity I ` measured on a similar sample, i.e., V-groove or blanket, covered with 200 nm fluorocarbon material ~deposited at 0 W RF bias power!. The latter acts as a semi-infinitely thick fluorocarbon film for XPS analysis since the probing depth is limited to the surface (,10 nm). The thickness d cfx of the steady-state fluorocarbon film measured perpendicular to the sample surface can then be calculated in the following manner: d cfx52l ln~ 12I ss /I ` ! cos~ u ! ,

~1!

where l is the C (1s) photoelectron attenuation length ~'2.3 nm from Ref. 16!, and u the angle between the normal

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FIG. 7. ~a! The nitride etch rates on both flat and angled samples plotted as a function of the ion flux after penetration of the fluorocarbon film. The fluxes are normalized to the flux on the flat sample at 265 V selfbias condition. ~b! The Si3N4 etch rates on both flat and angled samples plotted as a function of thickness of the steady-state fluorocarbon film.

FIG. 6. Thickness of steady-state fluorocarbon films on both flat and angled nitride surfaces as a function of selfbias voltage. The film thickness d cfx is measured perpendicular to the nitride surface. The thickness d cfx /cos(54.7°) is the thickness through which ions that are incident normal to the average wafer surface will have to penetrate in order to contribute to etching of the angled nitride surface.

to the surface and the photoelectron detector. Both blanket and V-groove sample were analyzed at an angle of 0° between the normal to the macroscopic wafer surface and the photoelectron detector. For the V-groove sample it needs to be taken into account, however, that the microscopic surface is inclined. On blanket samples u is therefore equal to 0°, while for the V-groove sample u is 54.7°. Figure 6 shows the thickness of the steady-state fluorocarbon films calculated from C (1s) XPS intensities, obtained from processed V-groove and flat nitride samples. The fluorocarbon film on inclined Si3N4 surfaces is found to be thinner than on flat Si3N4 for all conditions. Also plotted in Fig. 6 is the fluorocarbon film thickness on the inclined surface divided by the factor of cos(54.7°), i.e., the thickness through which ions would have to penetrate in order to contribute to etching of the substrate. It shows that ions need to penetrate through a fluorocarbon film on the angled sample that has a greater effective thickness than on the flat sample.

V. DISCUSSION In the above, a reduction in the thickness of the steadystate fluorocarbon film on inclined Si3N4 surfaces has been found to correspond to an increase in Si3N4 etch rate. In this section two different models are assessed for their ability to explain these observations. The first model relies on sputtering of Si3N4 through the steady-state fluorocarbon film, while the second model is based on the etch rate limitation by transport of atomic fluorine through the steady-state fluorocarbon film.2,4 In the sputtering model Si3N4 etching is ion driven. In order to contribute to sputtering, ions need to penetrate the fluorocarbon film to reach the Si3N4 substrate. The ion flux that contributes to etching of the Si3N4 substrate G sub has JVST A - Vacuum, Surfaces, and Films

been calculated for the various conditions assuming that the ion flux decreases exponentially in the fluorocarbon film d cfx : G sub5G inc cos~ u ! exp$ 2d cfx / @ d att cos~ u !# % ,

~2!

where G inc the ion flux incident to the macroscopic wafer surface, u the angle of incidence, and d att the attenuation depth for ion penetration ~'1 nm from Ref. 17!. Since the surface area of the angled sample is a factor of 1/cos(54.7°) larger than for the flat sample, the total ion flux is going to be lower on the inclined surface. In addition, on the angled sample the steady-state fluorocarbon film has a greater effective thickness than on the flat sample, see Fig. 6. Therefore, on the angled sample the ion flux that arrives at the fluorocarbon film surface will be attenuated to a greater extent before it reaches the Si3N4 . Figure 7~a! shows the Si3N4 etch rates as a function of the ion flux. Due to the net flux reductions on the angled surface discussed before, the Si3N4 etch rate is much higher on the angled than on the flat surface for a given ion flux at the fluorocarbon/Si3N4 interface. This effect cannot be explained by a sputtering mechanism that includes direct ion impact, unless a 54.7° to 0° etch yield ratio of 2.860.2 or larger ~dependent on selfbias! is assumed. This value is much higher than the value of 1.21 that we measured when etching nitride V-groove and flat samples in a CHF3 plasma. It is also much higher than the value of 1.26 that is found by Barklund and Blom for XeF21Ar1 chemical sputtering of Si3N4 . 15 This result strongly indicates that Si3N4 etching occurs through a mechanism different from sputtering. On the other hand, the arrival of atomic fluorine through the fluorocarbon film may be of overriding importance for substrate etching.2,4 In our companion article4 this mechanism is described in detail. We found for all conditions that the substrate etch yield decreases exponentially with the steady-state fluorocarbon film thickness. In Fig. 7~b! the Si3N4 etch yield is plotted as a function of the steady-state fluorocarbon film thickness ~measured normal to the microscopic surface!. Indeed, the Si3N4 etch yield on both inclined and flat surfaces respond in a similar fashion to the actual thickness of the fluorocarbon film, i.e., the etch yield decreases exponentially with increasing fluorocarbon film

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thickness. The thinner steady-state fluorocarbon layer and an increased fluorine diffusion appear to be dominantly responsible for the angular dependence of Si3N4 etching at selective blanket SiO2-to-Si3N4 conditions. VI. SUMMARY AND CONCLUSIONS The angular dependence of SiO2 etching has been found to be consistent with a reactive sputtering mechanism. The angular dependence of Si3N4 at nonselective SiO2-to-Si3N4 conditions is similar to SiO2 . The angular dependence of Si3N4 etching at selective conditions, on the other hand, is controlled by the angular dependence of fluorocarbon deposition and etching, which produces a thinner steady-state fluorocarbon film on the inclined Si3N4 surface. The SiO2-toSi3N4 selectivity loss mechanism at curved and inclined surfaces can therefore be explained consistently with the blanket selectivity mechanism. ACKNOWLEDGMENTS The authors would like to thank Peter Matsuo for his supporting work in XPS analysis and Neal Rueger, Marcus Doemling, Theo Standaert, Mark Chang, and Fei Wang for helpful discussions. This work was supported in part by Lam Research Corporation and Advanced Micro Devices. G. S. Oehrlein and Y. H. Lee, J. Vac. Sci. Technol. A 5, 1585 ~1987!; G. S. Oehrlein and H. L. Williams, J. Appl. Phys. 62, 662 ~1987!; G. S. Oehrlein, S. W. Robey, and M. A. Jaso, Mater. Res. Soc. Symp. Proc. 98, 229 ~1987!.

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