Continuous and Cyclic Deep Reactive Ion etching of Borosilicate Glass by Using SF 6 and SF 6 /Ar Inductively Coupled Plasmas

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1 Journal of the Korean Physical Society, Vol. 47, November 2005, pp. S422 S428 Continuous and Cyclic Deep Reactive Ion etching of Borosilicate Glass by Using SF 6 and SF 6 /Ar Inductively Coupled Plasmas J. H. Park, N.-E. Lee and Jaichan Lee Department of Materials Science and Engineering and Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon J. S. Park and H. D. Park Korea Electronics Technology Institute, Seongnam In this study, deep reactive ion etching (DRIE) of borosilicate glass with a Ni hard mask was carried out in SF 6 and SF 6/Ar inductively coupled plasmas under continuous and cyclic etching modes. Continuous etching in SF 6 plasma resulted in glass channel profiles with an under-cut below the Ni hard mask and micro-trenching at the bottom of the etched channel. Continuous etching in SF 6/Ar plasma, on the other hand, removed the under-cut and micro-trenching but reduced the etch rate significantly. In order to enhance the glass etch rate in SF 6 plasma as well as to solve the problems such as under-cut and micro-trenching in SF 6 plasma, a cyclic etching process using SF 6/Ar (A-step) and SF 6 (B-step) plasmas was investigated by varying the period time and duty ratio. As a result, it was observed that the average etch rate during the cyclic etching process was slightly increased compared to that of the continuous SF 6 plasma etching and the profile angle was improved to >89 without under-cut and micro-trenching. PACS numbers: j, Hp, Kf, Pq Keywords: Borosilicate glass, Deep reactive ion etching, Micro-fluidic channel, Inductively coupled plasma, Ni hard mask, Cyclic etching process I. INTRODUCTION One of the most important technologies in the field of µ-tas (micro-total analysis system) development is the micromachining technology of glass, Si and polymer for microfluidic application. µ-tas [1] has been intensively investigated application in the areas of environmental monitoring [2], biomedical diagnosis [3] and other chemical analysis [4]. Recently, as an application of plasma etching technologies in microfabrication with µ-tas, DRIE of glass [5] or silica plates [6] has been developed and applied to the fabrication of microfluidic devices for biochemical [5] and environmental [6] applications. DRIE (deep reactive ion etching) is expected to be very useful for fabrication of microfluidic devices utilizing glass or quartz materials for dimensional precision and high aspect-ratio structures. In particular, DRIE is attractive because of the trend towards micrometer-scale and nanometer-scale structure fabrication by using glass or quartz. Although wet chemical etching is well developed for the fabrication of glass microfluidic channels [7 9], it has some inherent limitations due to the under-cut and the isotropic profile of the etched channel, in particular for nelee@skku.edu -S422- micro- and submicro-patterns. Dry-etching techniques, on the other hand, can generate an anisotropic and precise micro-scale profile due to the directional nature of the ion bombardment, affecting surface chemical reaction as well as physical sputtering [10 12]. In general, the controlling mechanisms of glass DRIE are chemical reaction of the neutrals with the elements and physical effects such as the ion bombardment that enhances removal of the reaction by-products [13 15]. Reactive ion etching studies of glass have been reported in previous experiments using various chemistries such as C 3 F 8 [6], CHF 3 [9], CF 4 /CHF 3 [13,18], SF 6 /Ar [14, 19], SF 6 [15, 16], CF 4 /O 2 [17, 20] and CF 4 /Ar [17, 20]. Dry etching of the glasses was carried out typically in capacitively coupled plasmas (CCP) [9, 17, 18], CCP/microwave plasma [20], and inductively coupled plasmas (ICP) [6, 14 17, 19]. Most of the etching work studied in CCP and ICP showed a low etch rate with a range of several tens to hundreds of nm/min and a low etch selectivity to the etch mask because the etched surface was protected by non-volatile compounds such as AlF 3, NaF, and KF formed during etching [6, 9, 13, 17 20]. Recently, etch rates as high as 0.6 µm/min [15, 16] and 1.2 µm/min [14,19] in SF 6 inductively coupled plasmas have been reported. The major challenges in glass plasma etching are the low etch rate, low etch selectivity of the glass to the etch mask, and fabrication of

2 Continuous and Cyclic Deep Reactive Ion etching of J. H. Park et al. a high-aspect-ratio structure. In this study, the DRIE characteristics of borosilicate glass in high-density inductively coupled plasma (ICP) were investigated by varying the various process parameters. A comparative study of continuous etching by using SF6 and SF6 /Ar plasmas and a cyclic etching process by using alternating SF6 /Ar plasma (A-step) and SF6 plasma (B-step) was carried out. The concept of cyclic etching process here is little different from the DRIE of Si by using the Bosch process that combines a repetitive side-wall passivation and etching step [21]. The use of a cyclic etching process combining the advantages of two single-step etching (i.e. continuous etching) processes was effective in removing the under-cut and microtrenching in the SF6 plasma and in improving the slower etch rate in the SF6 /Ar plasma. II. EXPERIMENTS DRIE of borosilicate glass was carried out by using a commercial 8-inch inductively-coupled-plasma (ICP) reactor (TCP 9100, Lam Research Corp.) equipped with a turbo molecular pump (2000 l/sec) backed by a dry pump. Plasma generation was controlled by MHz top-electrode power. The bottom-electrode power was supplied by a 4-MHz r.f. generator to induce the negative dc self-bias voltage (Vdc ). The glass used in this experiment was 700-µm-thick borosilicate glass (Borofloat 33 with the composition of SiO2 82 %, B2 O3 5 %, Al2 O3 7 %, Na2 O 6 %) with 1 1 cm2 square shape. After cleaning of the sample by using trichloroethylene, acetone and ethanol, adhesion and seed layers of Cr ( = 50 nm) and Cu ( = 100 nm), respectively, were sputter-deposited on the glass in order to improve the adhesion between glass and Cu seed layer and electrical conductivity for Ni electroplating. Then, a negative photoresist (SU , Microchem Inc.) with a thickness of = 15 µm was spin-coated on the Cr/Cu layer and patterns with line and spacing of µm were formed by UV (ultraviolet) photolithography. The Ni deposition was carried out by dc electroplating. After Ni electroplating, SU-8 photoresist was removed. Selective wet etching of the Cr adhesion and Cu seed layers followed. The fabricated Ni hard mask thickness of 9 µm typically showed a profile angle of 88. The samples were attached to an 8-inch Si wafer placed on the wafer chuck. The back-surface temperature of the wafer chuck during etching was held at 18 C by circulating cooling water by using a chiller. Throughout the experiments, the chamber pressure was fixed at 18 mtorr by an automatic pressure controller using a throttle valve, and the top-electrode power at 1100 W. In order to understand the etching mechanism, optical emission spectroscopy (OES) and X-ray photoelectron spectroscopy (XPS) are used to investigate the species in the plasma and the chemical binding states and composi- -S423- Fig. 1. Etch rate of glass as a function of (a) SF6 flow rate, (b) dc self-bias voltage, Vdc ; (c) SEM micrograph of glass channel etched in SF6 plasma for 15 min at Vdc of 700 V and SF6 flow rate of 200 sccm; and (d) SEM micrograph of glass channel etched in SF6 plasma for 15 min at Vdc of 500 V and SF6 flow rate of 200 sccm. tion of the etched glass surface, respectively. The optical diagnostic system for OES is equipped with a monochromator (Jobin Yvon SPEX 270M) having 1200 grooves per mm and focal length of 270 nm. The optical signal is detected by means of a photomultiplier tube (PMT) that covers a spectral range of nm. A Mg (Kα) source for X-ray photoelectron spectrometer (AES-XPS ESCA 2000) providing monochromatic X-rays at ev was used for XPS analysis. The angle of the incident X-rays was 54.7 relative to the detector axis. The take-off angle (the angle between the sample surface and the detector axis) was kept at 90 during the measurements. Etch rate and profiles were measured by scanning electron microscope (SEM, HITACHI S-3500H). III. RESULTS AND DISCUSSION First, continuous DRIE of borosilicate glass was carried out in the SF6 plasmas. The etch rate was measured as a function of SF6 flow rate and Vdc. The etch results for the channels with a width of 15 µm are shown in Figure 1(a)-(c). As shown in Figure 1(a), with increasing SF6 flow rate from 100 to 300 sccm at a fixed Vdc of 500 V, the etch rate increased from = 530 to 800 nm/min. A gradual increase in the glass etch rate with increasing SF6 flow rate was attributed to the increase in the F-radical density, which is indicated by an increase of the optical emission intensity at nm (not shown here) from the F radicals with increased SF6 flow rate. In this experiment, the selectivity was infinitely high because the Ni hard mask was covered by the non-volatile reaction by-products formed during the etching process, resulting in deposition on the Ni hard mask rather than in erosion.

3 -S424- In Figure 1(b), the etch rate was enhanced by increasing Vdc from 400 to 500 V, but it dramatically decreased at a self-bias voltage of > 500 V. A maximum etch rate of =750 nm/min was obtained at the Vdc of 500 V. In general, the etch rate is expected to increase with Vdc, due to the increased ion energy [13,22]. In the present experimental condition, however, the reason for the decreased etch rate is presumably the re-deposition of the Ni atoms physically sputtered from the Ni hard mask due to a very high ion bombardment energy under a high -Vdc range larger than 500 V. The SEM image in Figure 1(c), as an example, indicates the formation of a grass-like morphology possibly due to the re-deposition of the sputtered Ni atoms at the high Vdc value of 700 V and, as a result, a reduction in the etch rate. A SEM image of the glass channel etched for 15 min at a Vdc of 500 V and a SF6 flow rate of 200 sccm is shown in Figure 1(d). Figure 1(d) shows an undercut below the Ni hard mask due to chemical etching and micro-trenching at the bottom of the etched channel. The micro-trenching has been explained by various mechanisms such as Knudsen transport of neutrals, ion shadowing, neutral shadowing, and differential charging of insulating microstructure [14,23]. When the ions are injected into the deep channel that is being etched, scattering can be affected by the sloped Ni mask edge profile in this work [14,23]. The impact of high-energy particles at grazing angles on the sidewall, followed by specular reflection, may lead to a higher etch rate at the base of the channel sidewalls and, as a result, micro-trenching. In order to get rid of the under-cut and micro-trench formation, Ar was added to SF6 gas for the enhancement of the ion-enhanced removal of the etch by-products. The measured etch rates of the channels with width of 15 µm are shown in Figure 2(a). The Vdc and the etch time for the etch-rate measurement were 500 V and 15 min, respectively. The etch rate in Figure 2(a) was initially increased to = 580 nm/min, decreased at the Ar/(SF6 +Ar) flow ratio of 0.3, and then decreased sharply. The tendency of the etch rate variation with the Ar/(SF6 +Ar) flow ratio is similar to the variation tendency of the F radical emission intensity (see Figure 3(a)) as well as of the F concentration on the etched surface (see Figure 3(b)). This observation indicates that the etch rate measured in the SF6 /Ar plasmas is limited by the neutral flux, Γneutral, with the Ar gas added under conditions in which the ion flux is enough to remove the etch by-products. With increasing bias power, i.e. Vdc, as seen in Figure 2(b), the etch rate of the channels initially increased to a maximum etch rate of =560 nm/min, as the Vdc value increased from 400 to 500 V, and then decreased with a further increase of the Vdc value. Similarly to the case of the SF6 plasma etching (Figure 1(b)), the redeposition of the Ni atoms physically sputtered from the Ni hard mask under a high Vdc range larger than 500 V presumably contributed to the decrease in the etch rate. For the SF6 /Ar plasma etching, profile improvement Journal of the Korean Physical Society, Vol. 47, November 2005 Fig. 2. Etch rate as a function of (a) Ar/(SF6 +Ar) flow ratio, (b) dc self-bias voltage, Vdc ; (c) SEM micrograph of channel etched in SF6 /Ar plasma at Vdc of 500 V and 40 % Ar flow ratio. with no under-cut and micro-trenching was observed as compared to the SF6 plasma etching. Figure 2(c) shows the etched channel with a depth of =15 µm at a Vdc of 500 V and 40 % Ar flow ratio. As seen from the SEM micrograph in Figure 2(c), the addition of the Ar gas to the SF6 chemistry was effective for sputtering of the residues by high-flux Ar ion bombardment, leading to the improved profile as well as to the removal of the under-cut and bottom micro-trenching. The most probable reason for the profile with no under-cut below the Ni hard mask is that the sidewall passivation occurred more effectively at the initial stage of the etching process in the SF6 /Ar plasma by the re-deposition of the non-volatile reaction products due to an increased ionenhanced sputter etching of the fluorides as compared to the case of the SF6 plasma. The effective removal of the

4 Continuous and Cyclic Deep Reactive Ion etching of J. H. Park et al. -S425- Fig. 3. (a) Optical emission intensities from F + (491.5 nm), F radicals (685.5 nm), Ar + ( nm), and Ar radicals (696.5 nm) as a function of Ar/(SF 6+Ar) flow rate ratio, and (b) relative atomic compositions on etched glass surface as a function of Ar/(SF 6+Ar) flow rate ratio. micro-trenching at the trench bottom is also presumably attributed to the enhancement of the bottom etching due to the increased ion flux in the SF 6 /Ar plasmas as compared to that in the SF 6 plasmas. In order to understand the etching mechanism of the glass in the SF 6 /Ar inductively coupled plasma (ICP), OES and XPS measurements were carried out for the plasma and etched surface, respectively. The OES measurement was performed at the top power of 1100 W without substrate biasing. The Ar-flow-ratio dependence of the two F emissions at nm and nm, the Ar emission at nm, and the Ar + emission at nm in the SF 6 /Ar plasma is shown in Figure 3(a). The emission intensities of the Ar radicals and ions increased and those of the F radicals decreased with increasing Ar/(SF 6 +Ar) flow ratio. At an Ar flow ratio of 40 %, the emission intensity of the F radicals started to decrease more rapidly. It is therefore evident from the OES data in Figure 3(a) that, as the Ar gas flow ratio increased, the ion-to-neutral flux ratio, Γ ion /Γ neutral, was significantly increased due to a decrease in the radical flux, Γ neutral, as well as a significant increase in the ion flux, Γ ion. Relative atomic compositions of the glass surfaces etched in the SF 6 /Ar plasmas were obtained from the XPS spectra (not shown here) and plotted in Figure 3(b). A quantitative analysis of relative atomic concentrations on the unetched and etched glass surfaces was performed by utilizing the surface peak areas and atomic sensitivity factors [23,24]. The other elements are mainly oxygen and contaminated carbon. The tendency of the decrease in the F concentration with increasing Ar flow ratio is similar to that of the F emission intensity in Figure 3(a). As the Ar flow ratio increased, i.e. the SF 6 flow decreased, the relative atomic compositions of Al, Si, F, Na, and B decreased due to the reduction of the F-radical densities in the plasma. This result suggests that the reaction of the F radicals with the elements in the glass is limited by the F-radical density in the plasma. The combined results for the continuous etching by using SF 6 /Ar chemistry from the etch rate, OES, and XPS measurements imply that the reaction of F radicals reacting with the elements in glass, as well as the ion-assisted removal of SiF x and other fluorides on the surface, is important in the etching mechanism of glass when using SF 6 /Ar plasma chemistry. Therefore, it can be concluded that the fluorination of Al, Na, B, and Si leading to the formation of the various fluorides and ionassisted removal of the fluorides play competitive roles in the etching of the borosilicate glass. The results indicate that the etching process utilizing Ar addition helps to remove non-volatile by-products. To improve the profile problems of the under-cut and micro-trenching in the SF 6 plasma and to increase the slower etch rate in the SF 6 /Ar plasma compared to that in the SF 6 plasma, we also investigated a cyclic etching process using SF 6 /Ar (A-step) and SF 6 (B-step) plasmaetching steps. For the SF 6 /Ar-plasma etch step, SF 6 (120 sccm)/ar(80 sccm) and V dc of 500 V were used. For the SF 6 -plasma etch step, SF 6 flow of 200 sccm and V dc of 500 V were used. The cyclic process was performed by varying the process parameters of period (A-step + B-step) and duty ratio (A-step: B-step). The glass etch rates during the cyclic etching are shown in Figure 4(a) and (b) as a function of period and duty ratio, respectively. The etch rate was obtained by averaging the measured values from the several channels. In Figure 4(a), the average etch rates for the duty ratios of 1 : 1 and 2 : 3 were obtained for the various period times. For the duty ratio of 2 : 3, the average etch rate increased from = 420 to 600 nm/min as the period time increased from 1 to 5 min and then decreased with a further increase in the period time. For the duty ratio of 1 : 1, the average etch rate was not significantly changed with the period time. In Figure 4(b), the average etch rate for each period time is indicated as a function of the duty ratio. As seen from the data in Figure 4(b), the average etch rate increased on decreasing the duty ratio i.e. decreasing the etch time for A-step etching because of the faster etch rate in the SF 6 etching step. The average profile angles of the channels etched by the cyclic etching process as a function of duty ratio and period time were measured from the several etched channels, and the results are shown in Figure 5. The average-

5 -S426- Journal of the Korean Physical Society, Vol. 47, November 2005 Fig. 4. Average etch rate as a function of (a) period time (min), and (b) duty ratio (A-step : B-step). Fig. 5. Average profile angle as a function of (a) period time (min), and (b) duty ratio (A-step : B-step). profile-angle data in Figure 5(a), obtained by varying the period time for the duty ratios of 1 : 1 and 2 : 3, the trend in the profile angle variation was not clearly observed, similarly to that in the etch rate variation in Figure 4(a). As shown in Figure 5(b), on the other hand, the profile angles of glass channels were increased by decreasing the duty ratio i.e. by decreasing the etch time for A-step etching. Figure 6(a) shows a cross-sectional image of a glass channel etched for 60 min at a duty ratio of 1 : 1 and a period time of 5 min. The image shows a high-aspectratio channel structure with a profile angle of 89, a channel width of =12 µm, and a depth of =27 µm. A top-view image (combined from the several shots) of a microfluidic mixing channel etched at an optimized condition with a period of 5 min and a duty ratio of 1 : 1 is shown in Figure 6(b). As seen from the data in Figures 4-6, the cyclic etching process was effective in the improvement of the profile problems of the SF6 plasma process and the slower etch rate of the SF6 /Ar plasma process. The decrease in the duty ratio (A-step : B-step) i.e. decrease in the etch time for A-step etching (SF6 /Ar etching step) at a given period, the etch rates and the profile angles were improved, even though the variation of the period time did not give a clear tendency in the etch rate and profile angles. By using the cyclic etching process, a microfluidic mixing channel could be successfully fabricated. IV. CONCLUSION We investigated SF6 and SF6 /Ar inductively coupled plasma etching processes for microfluidic channel fabrication on borofloat 33 glass under continuous and cyclic

6 Continuous and Cyclic Deep Reactive Ion etching of J. H. Park et al. -S427- ing steps. It was observed that etch rates in SF6 and SF6 /Ar chemistries were dependent on the period time and duty ratio. An etch rate of =600 nm/min in the SF6 and SF6 /Ar cyclic etching process was achieved at a self-bias voltage of 500 V. The physical etching process in the SF6 /Ar plasma etching step helped to remove the non-volatile products due to the Ar ion bombardment effect and, as a result, a vertical etch profile angle of = 89 without under-cut and bottom micro-trenching was obtained. It was shown in this work that it is possible to fabricate a high-aspect-ratio microfluidic channel by using a cyclic etching process in SF6 /Ar and SF6 inductively coupled plasmas. ACKNOWLEDGMENTS This work was supported in part through the Electro0580 Program by the Ministry of Commerce, Industry and Energy and the Ministry of Information and Communication and in part through the Center of Excellency Program by the Korea Science and Engineering Foundation and the Ministry of Science and Technology (Grant No. R ). Fig. 6. (a) Cross-sectional SEM image of etched glass channel, and (b) top-view SEM image of microfluidic mixing channel etched for 60 min at duty ratio of 1:1 and period time of 5 min. etching conditions. It was observed that the etch rates in the SF6 and SF6 /Ar chemistries were dependent on the dc self-bias voltage and the Ar/(SF6 +Ar) ratio. Selectivity was infinitely high because Ni hard mask was deposited by the non-volatile products produced by the reaction of F with the elements in the glass. A high etch rate of =750 nm/min was achieved in the SF6 plasma at the self-bias voltage of 500 V, but profile problems such as under-cut and micro-trenching were observed. The addition of Ar to the SF6 chemistry was effective in removing the profile problems, but reduced the etch rate. The OES measurement results for the investigation of Ar/(Ar+SF6 ) flow-ratio effects suggest that the increase in the emission intensity of F radicals is closely related to the etch rate of glass, and the XPS measurement results on chemical binding states and surface composition suggest that the chemical reaction of F radicals with Si, Al, B, and Na in the glass plays an important role in determining the glass etch rate, together with an ion-enhanced removal mechanism of the fluoride by-products. To solve the problems associated with the continuous etch processes, we investigated cyclic plasma etching processes by using alternating SF6 and SF6 /Ar plasma etch- REFERENCES [1] D. R. Reyes, D. Lossifidis, P. A. Auroux and A. Manz, Anal. Chem. 74, 2623 (2002); P. A. Auroux, D. Lossifidis, D. R. Reyes and A. Manz, Anal. Chem. 74, 2367 (2002); Torsten Vilkner, D. Janaek and A. Manz, Anal. Chem. 76, 3373 (2004). [2] D. Diamond, M. Sequeira, M. Bowden and E. Minogue, Talanta 56, 355 (2002). [3] K. Sato and A. Hibara, Anal. Sci. 19, 157 (2003). [4] A. Guttman and J. Khandurina, J. Chromatography A 943, 159 (2002). [5] T. Ichiki, Y. Sugiyama, R. Taura, T. Koidesawa and Y. Horiike, Thin Solid Films 435, 62 (2003). [6] L. Ceriotti, K. Weible, N. F. de Rooij and E. Verpoorte, Microelecronic Eng , 865 (2003). [7] T. Corman, P. Enoksson and G. Stemme, J. Micromech. Microeng. 8, 84 (1998). [8] M. Kohler, Etching in Microsystem Technology (Wiley VCH Weinheim, New York, 1999), p [9] I. Rodriguez, P. Spicar-Mihalic, C. L. Kuyper, G. S. Fiorini and D. T. Chiu, Analytica Chimica Acta 496, 205 (2003). [10] H.-Y. Song, C.-W. Kim, S.-G. Park, B.-H. O, S.-G. Lee and E.-H. Lee, J. Korean Phys. Soc. 45, 748 (2004). [11] H. G. Lee, C. Y. Jeong, H. S. Moon, S. H. Kim, J. Ahn and S. Sohn, J. Korean Phys. Soc. 33, 91 (1998). [12] J.-W. Yeo, D.-P. Kim and C.-I. Kim, J. Korean Phys. Soc. 44, 1092 (2004). [13] P. W. Leech, Vacuum 55, 191 (1999). [14] Y. Sugiyama, T. Ichiki and Y. Horiilke, Dry Process Symposium (Tokyo, 2001), p. 63.

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