Si etching in high-density SF 6 plasmas for microfabrication: surface roughness formation

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1 Microelectronic Engineering (2004) Si etching in high-density SF 6 plasmas for microfabrication: surface roughness formation E. Gogolides *, C. Boukouras, G. Kokkoris, O. Brani, A. Tserepi, V. Constantoudis Institute of Microelectronics, National Center for Scientific Research NCSR Demokritos, Aghia Paraskevi, Attiki 15310, Greece Available online 14 March 2004 Abstract Silicon etching and Si surface-roughness formation in high density SF 6 plasmas was studied. Etching rates and surface roughness were measured and correlated with ion flux and neutral F atom flux measured in situ. Etching rates are an increasing function of F atom flux, while surface roughness is not a monotonic function of F atom flux, or the etching rate. In fact, it is shown that one can achieve high etching rates and small surface roughness, a result of great practical importance to MEMS fabrication. Surface roughness increases with time, while scaling analysis of the AFM data shows that in most cases the Si surfaces develop periodic mound structures with a high roughness exponent (0.8) and a small correlation length (80 nm). Ó 2004 Elsevier B.V. All rights reserved. Keywords: Si etching; Surface roughness; Scaling analysis; Plasma etching induced roughness; Roughness simulation 1. Introduction For application of Si etching in fields such as microoptics, microelectronics, and microsensors, the quality of processed surfaces is often of crucial importance to the optimal operation of the fabricated device. For example in Si-based MEMS, it has been found that the fracture strength of the plasma-etched microfabricated structures is related to the etching process and the quality of the surface produced [1]. Therefore, studies of the dependence * Corresponding author. address: evgog@imel.demokritos.gr (E. Gogolides). of the surface roughness on processing conditions and of the mechanisms responsible for its formation are of great interest recently [2,3]. SF 6 plasmas are often used for Si etching and their mixtures or alternating fluxes with fluorocarbon gases add anisotropy as in the case of the Bosch process. However, to understand roughness formation one should first start with the simplest possible system. For this reason, in this work, the surface roughness of Si surfaces etched in pure SF 6 in a high-density plasma (ICP) reactor is studied as a function of the processing conditions. Simultaneously, etching rates under various plasma conditions are determined, the plasma gas phase is characterized and correlations to surface roughness are sought /$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi: /j.mee

2 E. Gogolides et al. / Microelectronic Engineering (2004) Experimental Si surfaces are etched under various plasma conditions (plasma source power, gas pressure, etching time) in the MET Alcatel plasma etching system. Etching rates are measured with stylus profilometry on the steps created on etched pads. The plasma-induced surface roughness is measured by atomic force microscopy (AFM) using a Topometrix TMX 2000 instrument in the contact mode. The roughness scaling characteristics were determined by statistical analysis (height height correlation function) on the surface images of AFM using a home-made scaling analysis software. The details are described below in Section 3.2. In order to correlate the surface roughness with plasma parameters, the plasma gas phase is characterized by means of several methods: (a) Optical emission spectroscopy using an Acton Research SP-500 monochromator with an SBIG CCD detector ST-6i. (b) Optical emission actinometry using the nm F emission line, and the nm Ar emission line (Ar is the actinometer gas used at around 5% admixture in SF 6 ). In actinometry an inert gas such as Ar is admixed at a known concentration with the processing gas. This actinometer, as well as the species of interest (such as F atoms), emits in the plasma, and the ratio of emission intensities is usually (but not always) proportional to the ratio of atom densities. Thus, knowing the actinometer concentration gives the relative magnitude of F atom density. For calculating absolute F densities the ratio of excitation constants for Ar and F was taken equal to 2 according to [4,5]. Thus ½FŠ ¼2½ArŠI F =I Ar, where [ ] denotes atom density, and I denotes intensity. (c) Ion flux measurement using an ion flux probe by Scientific Instruments for determining ion fluxes (J þ ). 3. Results 3.1. Etching rate and plasma gas phase analysis Figs. 1(a) and (b) show the F atom density and the etching rate as a function of source power. The Fig. 1. (a) F atom density measured with actinometry versus source power of the ICP reactor, for various pressure levels. (b) Etching rate versus source power. Bias voltage is constant at )55 V. Electrode material is anodized aluminum. similarity of the two plots is evident, as well as the strong increase of the responses with power. It seems as if our Alcatel MET reactor has two regimes of operation, one characterized by low etching rates and one by high etching rates. Figs. 2(a) and (b) show the etching rate versus the F atom flux and the ion flux, respectively. A linear increase with F atom flux is observed. On the contrary, when plotted versus ion flux, the etch rate seems to lie on several parallel lines, one having a zero intercept, and the others having non-zero intercepts with the ion flux axis. Thus, the etching rate shows a clear, unique and linear correlation only with F atom flux. Such a behavior is consistent with the isotropic nature of the etching. We have recently proposed a

3 314 E. Gogolides et al. / Microelectronic Engineering (2004) Fig. 2. (a) Etching rate versus F atom flux, for various pressures. (b) Etching rate versus ion flux measured in situ. Notice the good correlation of etching rate and F atom flux. Bias voltage is constant at )55 V. Electrode material is anodized aluminum. model for Si and SiO 2 etching with an ion-enhanced etching term proportional to the ion flux and the surface coverage of F species, and a thermal or neutral etching term proportional to the F atom flux [6]. The neutral etching term is activated by ion bombardment but is not directly proportional to the ion flux. It seems that this second term can be dominant at high F fluxes Analysis of the surface roughness AFM was used to measure the surface roughness. Experiments at constant etching time (60 s) Fig. 3. Two examples of AFM images of etched Si surfaces with different morphologies quantified with the height height correlation functions shown in Fig. 4. The colors correspond to different heights shown in nanometers. Etching conditions are as follows: (a) ICP power ¼ 900 W, bias voltage ¼ )55 V, SF 6 pressure ¼ 5 Pa, etching time ¼ 60 s, Electrode material is anodized aluminum. (b) ICP power ¼ 1800 W, bias voltage ¼ )55 V, SF 6 pressure ¼ 9 Pa, etching time ¼ 60 s. Electrode material is aluminum. were conducted varying the plasma parameters. In addition experiments at constant plasma parameters were carried out varying the etching time from 10 to 120 s. High surface roughness prevented measurements at longer times with our AFM. Analysis of the AFM images followed. One of the most well-founded methods for surface roughness analysis is based on the examination of the correlations between the heights of surface points through the height height correlation function Gðx; yþ:

4 E. Gogolides et al. / Microelectronic Engineering (2004) Gðx; yþ ¼h½hðx i þ x; y j þ yþ hðx i ; y j ÞŠ 2 i 1=2 ; ð1þ Fig. 4. (a) and (b). The height height correlation functions GðrÞ of the surfaces of Fig. 3 (a) and (b), respectively, and the roughness parameters (r, a, n, k) extracted from their form. Note the oscillating behavior for r > n revealing the regular mound structure of the surfaces. where h i is the average over all surface points (x i ; y j ) [7]. Usually, one examines the one-dimensional version of Gðx; yþ, which can be either the circular average of Gðx; yþ or the average of the one-dimensional height height correlation functions GðrÞ calculated along the fast scanning direction of AFM. In Fig. 4, two examples of circularly averaged GðrÞ are shown, corresponding to the etched Si surfaces of Fig. 3. One can easily deduce that the form of GðrÞ is, in fact, determined by a small number of parameters: the RMS value r, describing the vertical development of roughness, the roughness exponent a, the correlation length n and the mean wavelength k of periodic mound surface structures, which capture spatial aspects of roughness. In particular, the roughness exponent a is related to the fractal dimension D as D ¼ 2 a, and when a < 1 ðd > 2Þ the surface can be characterized as self-affine (invariant under anisotropic scaling). Actually, the higher the value of a, the less important the high-frequency fluctuations are for surface roughness. Thus, the surface in Fig. 3(b), where low frequency roughness components are more evident, is characterized by a higher a value (a ¼ 0:9) than that of the surface in Fig. 3(a) (where a ¼ 0:8). For large r, the pffiffi GðrÞ can stabilize or oscillate regularly about the 2 r value. Regular oscillations reveal periodic mound structures on surface (mounded surface) and are usually associated with a near to 1. This is the case of both surfaces of Fig. 3, with the surface of Fig. 3(b) exhibiting a more profound regular structure with mean distance between mounds 500 nm. In Fig. 3(a), the regularity is less intense (slighter oscillation of GðrÞ) and the mean mound distance 350 nm. The transition from the power law behavior to the stabilization or oscillation is determined by the correlation length n, which in fact quantifies the density of the surface fluctuations. The denser surface of Fig. 3(a) has a lower value of n (65 nm) than the more gross-grained surface of Fig. 3(b) (n 100 nm). Mostly, in mound-like surfaces the correlation length n, and the mean mound distance behave in a similar way. In conclusion, the surface roughness is quantified by the roughness parameters r, a, n and k, which can be reliably calculated through the analysis of the height height correlation function GðrÞ of the surface. Analysis of the etched surfaces following the above methodology reveals in most cases moundlike surfaces (in agreement with [3]) with a roughness exponent larger than 0.8, a correlation length around 80 nm and a mean mound distance between 300 and 500 nm. No clear trend of the roughness exponent with etching time and plasma conditions has been observed. On the contrary, the surface roughness r (RMS value) was found to increase linearly with etching time (in agreement with [2,3,8]). The correlation length n and the mean mound distance k seem to increase slightly with etching time.

5 316 E. Gogolides et al. / Microelectronic Engineering (2004) Correlation of the surface roughness with the plasma parameters In Fig. 5(a), the variation of surface roughness r (RMS value) as a function of plasma F atom density is shown for 5 and 9 Pa. Roughness increases with F atom flux up to a maximum point beyond which it decreases. This is clearly seen for a 5 Pa pressure, while for 9 Pa roughness seems to increase and level off. A similar behavior is shown in Fig. 5(b), which shows the roughness versus the ion flux. Again surface roughness goes through a maximum versus ion flux at 5 Pa. We assume that at 9 Pa the roughness versus F atom density (or ion flux) curves have not reached the fall-off region yet and only seem to level off up to the F atom density and ion flux maximum values obtained at 9 Pa. One can justify our assumption by comparing in Figs. 1(a) and (b) the F atom density and the etching rate versus power at various pressures: at 5 Pa, both quantities have first a fast and then a slow increase with power; while at 9 Pa only a fast increase is observed, which may at higher powers (not supported by our generator) slow down. We believe that at that point roughness would decrease at 9 Pa as well. An explanation of the above phenomena will be attempted in the following section. While roughness seems to have a bell-like shape versus F atom density or ion flux, etching rate increases monotonically with F atom density or ion flux as shown in Figs. 2(a) and (b). Therefore, conditions exist under which etching occurs with high rates leading to a relatively low surface roughness. Petri et al. [8] have proposed a mathematical expression showing that the surface roughness scales with the inverse of the square root of the neutral F atom to ion flux, i.e., surface roughness decreased with increasing F flux for constant ion flux. In our experiments, we found no monotonic dependence on this ratio, but in most case the opposite behavior, namely that surface roughness increases rather than decreases with the neutral to ion flux. Their experiments are at a very low pressure (below 1 mtorr) while our experiments are above 10 mtorr. Only our lowest pressure data approximate their behavior. Fig. 5. (a) RMS (root mean square) roughness as a function of fluorine atom flux in a 5 Pa (diamonds) and 9 Pa (squares) SF 6 ICP plasma. (b) RMS roughness versus ion flux for 5 and 9 Pa. Notice the non-monotonic behavior of roughness, and that it is reduced at high fluxes where etching rates are maximized. Etching time was 60 s. Electrode material is anodized aluminum. Note that at 9 Pa RMS roughness is less compared to Fig. 4(b), since anodized aluminum has smaller sputtering yield compared to aluminum, and hence causes less micro-masking (a source of roughness). 4. Discussion There is a lot of speculation in the literature about the intricate behavior of surface roughness formation in Silicon. Drotar et al. [3] proposed that the etching species do not stick on the surface on their first approach, but are reemitted and stick only on the second and higher reemission. As a result etching species stick much less on the hills of

6 E. Gogolides et al. / Microelectronic Engineering (2004) the surface than on the valleys, the latter receiving the re-emitted flux from the hills, and consequently being etched faster than the hills leading to surface roughness growth with time. This mechanism is unnatural in our opinion for neutrals, but might be true for ions: Indeed ions hitting a sidewall surface at grazing angles can get reflected and end at the bottom of a trench. Thus, the bottoms would etch faster. This is a possible explanation for an ionenhanced dominated etching. Another possible explanation is the sputtering of electrode or ICP-dome-wall material or the deposition of sulfur-bearing polymers on the surface of silicon. A first evidence for that is supplied by the difference between the RMS roughness shown in Figs. 4(b) and 5 at 9 Pa. In Fig. 4(b), the RMS roughness is higher because micro-masking is induced from sputtering of aluminum (the electrode material), which has a higher sputtering yield compared to anodized aluminum used as an electrode in Fig. 5. This local micro-masking would also increase the surface roughness with time. In a first attempt to simulate the above phenomena, we assumed a masking material covering the Si surface at a fraction varying from f ¼ 0:20 to f ¼ 0:90. For a specific masking fraction ff g the etch rate was set to zero randomly for a fraction ff g of the surface sites at each time step. The surface topography evolution was followed in time using a level-set algorithm [9]. The calculated surface topography (texture) and the micro-masking-induced surface roughness as a function of ff g are shown in Figs. 6(a) and (b). The simulations show that roughness is increasing linearly with time (up to a point) as our experiments also verify. They also indicate a belllike shape of the roughness variation with mask coverage surface fraction (Fig. 6(b)). This behavior, compared to that of r in Fig. 5, indicates that indeed surface roughening may be attributed to micro-masking. Notice that in Fig. 6(b) ff g decreases from left to right. We plotted the simulation results in this way so as to reflect the fact that increasing F atom densities and/or ion fluxes lead to reduction of the micro-masking surface fraction coverage ff g. Fig. 6(b) can also explain the observed difference in r behavior between 5 and 9 Pa: the higher pressure favors higher surface coverage, therefore r would Fig. 6. (a) Evolution of surface topography (height) with time for isotropic etching assuming that micro-masking randomly blocks etching for 50% of the surface sites. Different sites are randomly blocked at each time step, as the profile evolution algorithm follows the topography evolution [9]. Notice the development of roughness due to micro-masking and its increase with time. (b) RMS surface roughness (in arbitrary units) versus the fraction of the surface that is micro-masked. Notice the similarity with Fig. 5, in that surface roughness goes through a maximum, suggesting micro-masking as a possible source of roughness. peak at higher F atom or ion densities where mask surface coverage values decrease. 5. Conclusions Si etching in an SF 6 ICP has been studied for surface roughness formation. The plasma gas phase was analyzed and correlated with etch rate

7 318 E. Gogolides et al. / Microelectronic Engineering (2004) and surface roughness. It has been shown that while etch rate can be increased by increasing source power and hence F atom and ion fluxes on the wafer, the surface roughness goes through a maximum and is decreased at high etching rates. Surface roughness does not correlate well with neutral or ion flux or their ratio, and first simulations indicate that surface micro-masking may account for induction of surface roughness. References [1] K.S. Chen, A. Ayon, S.M. Spearing, J. Am. Ceram. Soc. 83 (6) (2000) [2] P. Brault, P. Dumas, F. Salvan, J. Phys.: Condens. Matter 10 (1998) L27 L38. [3] J. Drotar, Y.-P. Zhao, T.-M. Lu, G.-C. Wang, Phys. Rev. B 64 (4) (2000) [4] Y. Kawai, K. Sasaki, K. Kadota, Jpn. J. Appl. Phys. 36 (Pt 2 9A/B) (1997) L1261 L1264. [5] M.J. Schabel, V.M. Donnelly, A. Kornblit, W.W. Tai, J. Vac. Sci. Technol. A 20 (2) (2002) [6] E. Gogolides, P. Vauvert, G. Kokkoris, G. Turban, A. Boudouvis, J. Appl. Phys. 88 (10) (2000) [7] B.Y. Zhao, G.-C. Wang, T.-M. Lu, in: Experimental Methods in the Physical Sciences, vol. 37, Academic Press, [8] R. Petri, P. Brault, O. Vatel, D. Henry, E. Andre, P. Dumas, F. Salvan, J. Appl. Phys. 75 (1994) [9] G. Kokkoris, A. Tserepi, A.G. Boudouvis, E. Gogolides, J. Vac. Sci. Technol. A 22 (4) (2004), in press.