Deep plasma silicon etch for microfluidic applications

Surface and Coatings Technology 116–119 (1999) 461–467 www.elsevier.nl/locate/surfcoat

Deep plasma silicon etch for microfluidic applications K. Richter a, *, M. Orfert a, S. Howitz b, S. Thierbach a a Dresden University of Technology, Semiconductor and Microsystems Laboratory, D-01062 Dresden, Germany b GeSiM Company for Silicon Microsystems mbH, D-01454 Großerkmannsdorf, Germany

Abstract Various microfluidic devices like micro pumps, micro dispensers, micro pipettes, etc. are produced by the GeSiM mbH Großerkmannsdorf. Fabrication of such microsystems includes the realization of three-dimensional silicon substrates. Plasmabased silicon etch processes are key technologies for exact patterning the substrates from both surfaces. Typically, etch depths of 10 to 500 mm, aspect ratios >25 and the application of conventional mask systems are required. Etch rates between 2 and 6 mm/min, a uniformity (3s) below 5%, an excellent anisotropy and a selectivity of >50:1 for photoresists and >150:1 for SiO can be realized using an etch system produced by Surface Technology Systems Limited (STS ), UK and the 2 ASE@ process based on fluorine etch chemistry. This technique overcomes the disadvantages of conventional wet silicon etch processes, like the influence of crystal orientation on etch rate. Our intention was to investigate the influence of gas flow rates, process gas pressure, supplied power and time sequence on etch rate and edge profile. The results should be used to vary process parameters according to the requirements of application. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Microfluidic devices; Plasma etching; Silicon

1. Introduction Three-dimensional silicon structures are important components of microfluidic devices like micro-pumps, micro-dispensers and micro-pipettes. The silicon patterning process has to fulfil the following requirements: $ $ $

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etch depth 10–500 mm; aspect ratios >25; etch rates between 2 and 6 mm/min for <50% to <10% exposed area; variable etch profiles, adapted to the application; any lateral shape, for instance small pores and holes, comb structures, structures with optimized bend; uniformity 5%; application of conventional mask systems like photoresists, SiO and CVD-SiN; 2 selectivity >50:1 for these masks; low roughness of sidewalls.

These requirements also apply for the fabrication of * Corresponding author.

microfluidic devices. The GeSiM mbH Großerkmannsdorf produces various microfluid devices, like micro-pumps, micro-dispensers, micro-pipettes. For several years GeSiM used anistropic wet silicon etch processes based on KOH for patterning silicon. An important disadvantage of this method is the limitation of the shape of the edge profile, because the silicon etch rate strongly depends on crystal orientation. Therefore, cross-sections of etched patterns show a V-profile with a large mask undercut. Using the Advanced Silicon Etch system of Surface Technology Systems Limited (STS), UK, with a high density inductively coupled plasma source and biased substrate electrode and applying the ASE@ process enables patterning of silicon substrates effectively with high etch rates, good uniformity and excellent anistropy. Microstructures with etch depths of several 100 mm and any lateral shape can be realized. For example, microfluidic structures with holes, comb structures, as well as structures with optimized bend for an improved behavior during filling or leakage can be produced by plasma etching. During the fabrication of current sensors, membranes consisting of three layers (Si N –Pt–Si N ) with a crooked shape are to be etched 3 4 3 4 free to a depth of 420 mm.

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 11 2 - 7

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K. Richter et al. / Surface and Coatings Technology 116–119 (1999) 461–467

2. Experimental The Advanced Silicon Etch system used in this investigation is shown schematically in Fig. 1. This equipment possesses a high density inductively coupled plasma source (13.56 MHz, 1 kW ). The substrate electrode could be biased, the maximum supplied forward power (13.56 MHz) could be chosen between 30 and 300 W. Substrates were mechanically clamped and cooled by helium backside cooling. N , Ar, O , SF , CF , and 2 2 6 4 other higher molecular weight gases in a pressure range of 2.5 to 13 Pa can be used in the etch system. The equipment enables the carrying out of cyclic processes. The kinds of gas admitted, the flow rates, pressures and the supplied power at the coil and platen can be varied during the alternating process steps. Technological investigations were carried out using silicon wafers with etching masks of photoresist (thickness: ~2 mm) or SiO (thickness: 1.5–2 mm). The mask 2 patterns had a lateral dimension in the range of 50 mm to several millimeters.

plasma. The ion and radical species undergo polymerization reactions. A passivating layer of nCF is deposited 2 in the etched patterns, on the sidewalls and on the mask. A possible reaction scheme is CF +e−CF+ +CF · +F · +e− 4 x x nCF · nCF nCF x 2 adsorbed 2 film The deposited polymer film can be removed by ion bombardment with low energy. During the etching step the etching gas (for instance SF ) is dissociated by the plasma, liberating high 6 amounts of etching species. These species are necessary to remove the polymer film at the bottom of the pattern and to react with the silicon effectively. The etching reactions [2,3] involve removal of the passivation layer nCF

2 film

+F · + ion energyCF

x (adsorbed)

CF

x (gas)

and silicon etching Si+F · Si–nF+ion energySiF SiF x (adsorbed) x (gas)

3. Results of experimental investigations 3.1. Advanced Silicon Etch (ASE@) for fabrication of microfluidic devices 3.1.1. General The ASE@ process is based on a technique invented by La¨rmer and Schilp [1] and was introduced by STS. Sequentially alternating etching and deposition steps enable an accurate control of the anistropy. In the deposition step the precursor gas (CHF , 3 CF , C F or higher molecular gas) is dissociated by the 4 2 6

Based on the ASE@ process, silicon substrates for microfluidic devices were patterned. The etching mask was an SiO layer with a thickness of 1.5 to 2 mm. 2 Figs. 2 and 3 show some examples. 3.1.2. Influence of aspect ratio on the etch rate The etch rate is in the order 4–5 mm/min and depends strongly on the lateral dimensions of the etched patterns. Fig. 4 shows the normalized etch rate dependence on the aspect ratio during the etching process through a silicon wafer.

Fig. 1. Schematic setup of the Advanced Silicon Etch system.

K. Richter et al. / Surface and Coatings Technology 116–119 (1999) 461–467

Fig. 2. Leak of a micro-pump, patterned by ASE@.

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3.1.3. Characterization by means of optical emission spectroscopy The characterization of cyclic plasma processes with periods of some seconds requires sufficient fast measurement procedures. The applied 15 cm grid spectrograph (S&I ) is suitable for this in principle; however, a characterization of the spectral range of the near UV–IR is very expensive. For the application of the measurement technique that is used normally for continuous processes, the cycle time was enlarged by two orders of magnitude and spectra in the range 250–900 nm were registered (time of measurement: 1 min). Representative partial spectra recorded during the etching step (modified process parameters: SF : 50 sccm; 2.66 Pa; 1000 W ) are 6 shown in Fig. 5. Fig. 6 shows the intensities of isolated lines normalized to their maximum degree. These lines are assigned as follows: CF : 251.8 nm; CS : 258.1 nm; S : 283.6 nm; 2 2 2 SiF: 440.5 nm; C : 516 nm; F: 703.7 nm; S: 896.4 nm. 2 The following results should be noted. The intensity of fluorine continually rises with increasing conversion of the reaction products of the preceding polymerization. The SiF intensity increases drastically and the fluorine intensity is reduced after about 25 min processing time. Moreover, a later decrease of the SiF line is obvious.

Fig. 3. Structures of a micro-sieve. Slopes are etched by wet process; holes are etched by ASE@.

Fig. 5. Partial spectra for the etching period.

Fig. 4. Normalized etch rate dependence on aspect ratio during patterning of silicon wafers by ASE@.

The decrease of the etch rate at a growing aspect ratio (so called RIE lag) mainly results from reduced transport of etching species and reaction products in small patterns, scattering of ions due to collisions with other gas particles during crossing the sheath, as well as due to geometrical shading. The RIE lag can be reduced by low process gas pressure to reduce the number of collisions, as well as low bias voltage and high plasma density to minimize sheath thickness [4,5].

Fig. 6. Normalized intensities of isolated lines to their maximum degree.

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3.2. Influence of gas flow rates, process gas pressure and supplied power 3.2.1. SF flow rate 6 Fig. 7 shows the silicon etch rate dependence on normalized SF flow. As a reference value the flow rate 6 of the original ASE@ process was used. At first the etch rate rises with increasing SF flow 6 rate, caused by a growing amount of atomic fluorine. After going through a maximum the etch rate drops slightly. A relationship to the retention time of the particles in the reactor is assumed for this behavior. The retention time decreases with increasing flow rate. A decreasing retention time of species in the reactor results in a decreasing probability that gas molecules interact with the plasma and excited species react at the substrate surface.

3.2.2. Flow rate of precursor gas for deposition The following effects are related to CF deposition: x $

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etch rate depends on the thickness and stability of the deposited CF layer; x polymer film influences the roughness of the sidewalls; polymer residues on the bottom of the structures, which are not removed during the etching step, can cause micromasking, resulting in needle-form silicon residues.

The influence of the normalized flow rate of the precursor gas on the etch rate is shown in Fig. 8. Since the pumping power is constant, the amount of precursor gas in the reactor decreases with reduced gas flow rate. Therefore, less ion and radical species generated in the plasma are available for deposition of passivation layers, resulting in a decrease of the film thickness. During the etching step the thinner polymer films are removed faster and the effective etch rate is higher. The influence of polymer deposition on the roughness of the sidewalls and the effect of micromasking will be discussed later.

Fig. 7. Silicon etch rate dependence on normalized SF flow rate. 6

Fig. 8. Influence of deposition precursor gas flow on etch rate (reference value is the flow rate of the original ASE@ process).

3.2.3. Process gas pressure Process gas pressure is an important parameter with reference to etch rate, roughness of sidewalls, etch residues and etch uniformity. Fig. 9 shows the etch rate dependence on process gas pressure. The reference value is the process gas pressure p of the original ASE@ 0 process. Before reaching the value p (used during ASE@) the 0 etch rate rises with increasing pressure due to a growing amount of etching species. Then it reaches a maximum and essentially does not change anymore. Passivation layers that are deposited at decreased process gas pressure probably have a lower thickness. Therefore, they are removed earlier near the bottom of the structures during the etching step. The surfaces of the sidewalls are smoother; the roughness of the sidewalls decreases with longer etching action. Figs. 10 and 11 demonstrate this effect, especially near the bottom of the patterns. If the process gas pressure rises further than 1.5p 0 needle-form residues are found on the bottom of the structures, as seen in Fig. 12. Probably, an increased deposition of polymers takes place at higher process gas pressures. Simultaneously, the ion energy decreases due to a reduced mean free path of the gas particles. Therefore, the passivation layer cannot be completely removed during the etching step. Polymeric residues cause micromasking of very small areas, resulting in needle-form silicon spikes. The etch uniformity dependence on the process gas

Fig. 9. Influence of process gas pressure on etch rate.

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Fig. 13. Influence of the process gas pressure on the etch uniformity.

of the wafer is comparable to that available near the edge. Fig. 10. Cross-section through an etched structure, p=0.47p . 0

Fig. 11. Cross-section through an etched structure, p=1.5p . 0

3.2.4. Supplied power The influence of supplied coil and platen power on the etch rate is shown in Fig. 14. Reference values of P and P are those of ASE@ process. The ICP coil platen plasma source produces a plasma of high density, even at low pressure. According to Fig. 14, only a slight influence of the power supplied to the coil P on the coil etch rate is to be seen. The r.f. power supplied to the substrate electrode influences the self-bias essentially, and so determines the character of the process. Unfortunately, the bias voltage could not be measured during the experiments and so only the variation of P can be discussed. If P platen platen is low (that means low bias), the ion bombardment is insignificant. The deposition of a passivating polymer layer is dominant compared with etching. The etch rate drastically decreases at reduced P . At higher values platen of P the increasing ion bombardment changes the platen process character. At higher P the passivation layers platen are removed faster during the etching step and the resulting etch rate is higher. For a P higher than platen the original value of ASE@ the etching reactions are dominant. 3.3. Influence of time sequence The process character and the form of the edge profiles can be changed by varying the time sequence. The time sequence determines which reactions are domi-

Fig. 12. Cross-section through an etched structure at p=1.9p . 0

pressure is shown in Fig. 13. According to Fig. 13 the etch uniformity can be improved by reducing the process gas pressure. The mean free path of the gas particles grows with decreasing pressure. Therefore, at low pressure, the distribution of etching species by diffusion all over the substrate surface is more regular. This means that the amount of reactants reaching the central region

Fig. 14. Etch rate dependence on supplied power.

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nant: the deposition or the etching reactions. The results of these investigations are shown in Figs. 15–19. The thickness of the passivation layers grows with increasing deposition time. During a short etching step it is impossible to remove the thick polymer films completely. Consequently, perfect sidewall covering

Fig. 18. Changing roughness of sidewalls and negative edge profile at dominating etching step.

Fig. 15. Perfect sidewall covering achieved by short etching step and long deposition step resulting in a stop of the etching process.

Fig. 19. Smooth sidewalls and mask undercut at dominating etching step.

Fig. 16. Needle-form residues caused by micromasking effects.

( Fig. 15), an etch rate of approximately zero and micromasking effects resulting in many needle-form residues are seen (Fig. 16). At increasing etching time more polymers are removed. After a complete elimination, the etching reactions determine the etch depth, the roughness of the sidewalls and the form of the edge profiles (Figs. 17–19).

4. Conclusions

Fig. 17. High anisotropy at optimum ratio of deposition/etching.

Three-dimensional silicon structures are used to produce microfluidic devices like micro-pumps, micro-dispensers and micro-pipettes. Patterns with any lateral shape are realized in silicon with aspect ratios >15, etch rates between 2 and 6 mm/min and selectivity >50:1 for photoresist, SiO and CVD-SiN masks by the ASE@ 2 process. The ASE@ process consists of sequentially alternating etching and deposition steps to enable an accurate control of the anisotropy.

K. Richter et al. / Surface and Coatings Technology 116–119 (1999) 461–467

During the deposition step a polymer film for the sidewall passivation is deposited on all vertical and horizontal surfaces of the substrate. The thickness of the film grows with increasing flow rate of the deposition precursor gas, increasing process gas pressure, decreasing ion bombardment (determined by the power supplied to the platen) and increasing deposition time. If the thickness and stability of the passivation layer exceed a critical value (depending on the parameters of the etching step), the deposition step is the dominant process, i.e. a complete removal of the polymer film is impossible during the etching step. This results in sidewall covering with rough sidewalls, low etch rates and micromasking effects causing needle-form residues. During the etching step the polymer film is removed and the etching species react with the silicon. The etch rate increases with decreasing thickness and stability of the passivation layer, growing amount of available etching species (high P and P ), increased ion bomcoil platen bardment (high P ) and lengthened time of the etch platen step. A short retention time of the particles in the reactor

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and hindered particle transport at high aspect ratios limit the etch rate. If the etching step is dominant, high etch rates, minor roughness of the sidewalls and a smooth bottom of the patterns are characteristic for the etching process.

References [1] F. La¨rmer, A. Schilp, Method of anisotropically etching silicon, German Patent DE 4 241 045. [2] J. Bhardwaj, H. Ashraf, A. McQuarrie, Dry silicon etching for MEMS, Symposium on Microstructures and Microfabricated Systems at the Annual Meeting of the Electrochemical Society, Montreal, (1997). [3] S. Vogel, U. Schaber, K Ku¨hl, R. Schafflik, H. Pradel, F. Kozlowski, B. Hillerich, Novel microstructuring technologies in silicon, Mikrosystemtechnik-Symposium zur Productronica, (1997). [4] H.C. Jones, R. Bennet, J. Singh, Proceedings of 8th Symposium on Plasma Processing vol. 90-14, ECS, Pennington, 1990, p. 59. [5] J.K. Bhardwaj, H. Ashraf, Proceedings reprint from SPIE, vol. 2639, Micromachining and Microfabrication Process Technology, Austin, Texas (1995) 224.