EMN04, October 2004, Paris, France OPTIMIZED ULTRA-DRIE FOR THE MEMS ROTARY ENGINE POWER SYSTEM

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1 EMN04, October 2004, Paris, France OPTIMIZED ULTRA-DRIE FOR THE MEMS ROTARY ENGINE POWER SYSTEM Fabian C. Martinez, Ning Chen, Matthew Wasilik, Prof. Albert P. Pisano Berkeley Sensor and Actuator Center, University of California at Berkeley ABSTRACT Results from the development of an ultra deep reactive ion etching (DRIE) process are presented here. The research is the primary fabrication effort for the MEMS Rotary Engine Power System (MEMS-REPS), a program to develop an autonomous power generation system with a power density greater than that of conventional batteries. The assembly of the MEMS-REPS is a multi-wafer stack of DRIE processed Si components. The Wankel-type rotary engine design lends itself to MEMS fabrication techniques because of the planar geometry. Previous research has determined the importance of sealing at the apex of the Wankel-type rotary internal combustion engine [1]. These apex seals ensure a high compression ratio necessary for efficient internal combustion and have historically been the critical component in terms of development of the rotary engine; it has been demonstrated to be the same for this application. To minimize manual assembly, in-plane cantilever apex seals have been implemented into the design of the rotor [2]. The cantilevers comprise the critical dimension of the rotor fabrication with a 500 length, a seal width of 40 and a tip radius of ~20. Maintaining the resolution of these features through hundreds of microns of silicon is a challenge for ultra DRIE, which requires high integrity masking and optimized process parameters. Developments in ultra-drie have led to the fabrication of Silicon components through depths of up to 900 [3]. The implementation of 40 guard trenches to define all etch rotor features has alleviated aspect ratio dependent etching (ARDE) issues. Whereas a previous mask design required a backside DRIE processing step, the guard trench results presented here simplify the rotor profile fabrication process to a single DRIE step. This critical process step has a designed manufacturing tolerance of less than 2 for through-wafer depths of 500 and 900 [4]. Typical sidewalls resulting from DRIE are characterized by horizontal scallops resulting from the time multiplexed nature of the plasma, and vertical striations which are caused by imperfections of the masking material and limitations due to ion kinetics in deep trenches. DRIE parameters are varied throughout the process step to produce straight trench profiles and to maximize the depth of striation-free sidewalls. Masking is accomplished via patterned layers of silicon dioxide and thick (~10 ) photoresist. Imperfections of the masking layers are translated into vertical striations which are magnified as the depth of the etch is increased. These striations inevitably deteriorate the resolution of critical features and must be minimized. The work presented here is the demonstration of an intricate set of formal design of experiments for ultra DRIE. 1. MOTIVATION The goal of the MEMS Rotary Engine Power System project at the University of California at Berkeley is to develop a complete packaged system capable of producing portable electric power from a liquid hydrocarbon fuel. At the core of the system is a 1.18 mm 3 Wankel-type rotary engine; an engine design that lends itself very well to microfabrication techniques because of its planar design. When compared to the piston engine, another benefit of the rotary engine is the lack of a connecting rod assembly which leads to a smaller number of components. Historically though, the rotary engine has presented a challenge for development due to sealing of the combustion chamber. Solutions have included the Wankel-grid : complex assemblies comprised of more than 20 components. Less complicated solutions have also been developed such as by UC Berkeley s Mini- REPS team. The 12.9 mm REPS engine displayed substantial performance improvement when an apex sealing system was implemented [1]. The design of the system contained only 9 components requiring assembly. In-plane cantilever apex seal 40 guard trench Figure 1: Layout of MEMS-REPS housings and rotors defined by 40 guard trenches. Note integrated apex seals. In general, assembly of MEMS devices is a difficult procedure which tends to require specialized tools and fixtures for handling and mating components. The feasibility of the task is greatly increased by reducing the number of components requiring assembly. Therefore, the MEMS-REPS integrated in-plane cantilever apex sealing system, as shown in figure 1, is a unique solution

2 to rotary engine sealing which eliminates the need for excess components [2]. The integrated seal is manufactured as an attached feature to the rotor via an ultra-deep reactive ion etching (ultra-drie). Within the DRIE process exists the challenge of fabricating 25:1 aspect ratio structures through 500 silicon wafers; all the while maintaining 2 tolerances to meet performance requirements. 2. BACKGROUND A Surface Technology System Inductively Coupled Plasma (STS ICP) Etcher is employed to accomplish the ultra-drie process. The system uses the Bosch process of alternating passivation and etch cycles; passivating with C 4 F 8, and etching with SF 6 [5]. Previous research efforts within the MEMS-REPS project have enabled ultra-drie of depths up to 900 allowing for through-wafer etching of rotary engine components [3]. While these efforts were successful at etching through the entire wafer thickness, there were issues caused by aspect-ratio dependent etching (ARDE, phenomenon of narrow trenches etching with differing profiles and slower rates when compared to wide trenches [6]) and sidewall striations as shown in figure 2. The first step towards improvement involved attenuating the ARDE issue; this was accomplished by implementing 250 guard trenches and a 2-sided etch. Results from these modifications displayed an overall improvement in feature resolution but not enough to meet the desired tolerances. >500 um Zero width 2.1. Aspect-ratio dependent etching The minimum width trenches of the previous engine component layout (rotors and housings) were the 40 trenches which define the integrated in-plane cantilever apex seals. To avoid ARDE completely, the mask layout was revised to include 40 guard trenches to define every component feature. Therefore, the DRIE could be optimized for one trench width and the aspect ratio would be maintained at a reasonable 22.5:1 and 12.5:1 for 900 and 500 wafers respectively. By eliminating the etch rate variation due to ARDE, the DRIE process was once again simplified to a 1-sided process. This is critical to the fabrication of the engine components because of the error that would arise during backside processing for the 2-sided etch Sidewall striations Masking materials must be robust enough to survive etch times of more than four hours. Though the G-line SPR- 220 photoresist can survive for a through-wafer etch, the patterned edges of the polymer layer may lose integrity and begin to erode nonuniformily causing imperfections in the mask. Small (> 1 ) defects caused by this mask erosion translate into vertical striations often larger than 2 for 500 through-wafer etches. These striations may not start at the top edge of the etched sidewall, but they can generally be traced back vertically to their defect of origin. Mask erosion is also observed as chipping when large (~5 ) pieces of photoresist are removed. These striations begin at the top edge of the sidewall. To avoid mask erosion altogether, a patterned 2.5 layer of deposited low-temperature oxide (LTO) is used in addition to the photoresist. Along with providing the robustness necessary for masking, the LTO also eliminates any concern for selectivity; typically, oxide selectivity is 100:1 in the STS etcher. Unfortunately, sidewall striations may not be completely avoided by solving the masking issues. The dynamic effects of DRIE cause the properties of the etch process to change as the process time elapses. A typical observation is a decrease in etch rate. As selectivity is not so much of an issue, the concern becomes the onset of striations due to DRIE conditions. To control these conditions and increase the striation-free depth of the process, a formal design of experiment has been deemed necessary. Figure 2: Previous results showed (a) re-entrant profile due to ARDE, and (b) early onset of striations.

3 3. DESIGN OF EXPERIMENTS VIA THE TAGUCHI METHOD There are many parameters to consider when developing DRIE recipes: etch cycle time, etch to passivation cycle time ratio, chamber pressure, platen power, and coil power are only a select few. Other factors such as gas flow rates also play an important role in the quality of the etch, but to maintain a reasonable number of runs, these values were not varied. A full factorial design of experiments (DOE) with 4 factors and 3 levels would take 81 runs. By implementing an orthogonal design, the number of runs is reduced to nine. This saves an appreciable amount of time for DRIE experiments that may take up to 5 hours per run. The orthogonal DOE will not be as detailed as the full factorial DOE, but the knowledge gained from such an experiment may benefit the process equally. Variable Level 1 Level 2 Level 3 A Etch B Etch Bias C APC [%] D Power Table 1: The 4 DRIE parameters and their 3 levels used to complete the DOE There are many orthogonal tables which can be applied to accomplish a DOE via the Taguchi method [7]. For the DRIE experiment, the L 9 (3 4 ) table was used to test 4 parameters, each with 3 levels. The parameter values are chosen such that they cover a permissible range; the values for the DRIE experiment are shown in table 1. The orthogonal table is then completed by substituting the chosen parameter values; the resulting table is displayed in table 2. A total of 9 runs were necessary to complete the experiment. 4. SAMPLE PREPARATION AND PROCESSING A batch of 9 single-side polished 500 P-type Silicon wafers were prepared by first depositing a 2.5 layer of LTO, followed by spinning and patterning of a 9 layer of SPR220 photoresist. After patterning the LTO, the wafers were processed with the STS ICP Etcher. Etch Etch Power Power APC [%] Table 2: DRIE parameters for the nine orthogonal runs necessary to complete the DOE 4.1. Initial DRIE recipe for 1 st 100 Previous research efforts achieved greater than 100 of striation-free etching. The resulting profiles were within the required tolerance of less than 2. This depth was achieved after 45 minutes of etching; an etch rate of 2.5 /minute. The corresponding parameters for this initial recipe are presented in table 3 and an example of the resulting profile is shown in figure 3. Each of the 9 wafers was etched with this recipe for 30 minutes. Gas Flow [sccm] Pressure [mt] APC [%] ETCH 12 SF ~32 72 PASS 7.2 C4F ~ Table 3: DRIE parameters for the 1 st 100 Figure 3: The recipe developed for the 1 st 100 of striationfree etching 4.2. DOE recipes for 2 nd Following the initial etch of approximately 100, the batch of wafers was then processed with the nine Taguchi Method recipes.

4 5. SAMPLE ANALYSIS To analyze the characteristics of the DRIE, the wafers were cleaved such that the etched sidewalls and trench profiles could be observed with a scanning electron microscope (SEM). From these images, four output variables were extracted: etch rate, striation-free depth, sidewall profile angle, and selectivity. The data was then tabulated such that the effect of each parameter could be compared. To do so, the trends and the ranges of the output variables were calculated. Plotted in graphs 1 and 2 are the results for striation-free depth measurements. On graph 1, point A1 represents the average striation-free depth for the three experiments which had an etch cycle time of time of 6 seconds, A2 represents those which had a time of 8 seconds, and A3 represents those which had a time of 10 seconds. The remaining points represent the variables etch bias, APC, and coil power. Based on the data, etch cycle time has the most predominant effect on striation-free depth. It should be noted that prior research has shown that APC has a significant effect on etch smoothness and grass reduction. The results from this DOE were limited to a range of APC that did not produce this insight. 6. RESULTS The analysis showed that an increase in etch cycle time correlated with an increase in etch rate, increase in striation-free depth, and a more negative profile (widening of trench as depth increases, due to isotropy of SF 6 etching). By maintaining the passivation cycle time as constant, the increase in etch cycle time may also be observed as an increase in etch:passivation ratio. This is useful when a constant cycle time is desired. An increase in platen power resulted in an increase in rate and directionality of the etch process. The coil power had relatively small effects on the outcome of the etch results and was therefore maintained at 600 W for the final fine tuned recipes. Fundamentals show that a decrease in pressure leads to an increase in mean free path. Less collisions between ions allows for their paths to remain vertical which increases the anisotropy of the etch process. To maintain sequentially lower pressures, the APC was decreased A1 A2 Etch Graph 2: The range of the output variables shows the more dominant correlation of etch cycle time to striation-free depth The fine tuned recipe for the 2 nd 100 is presented in table 4. With the exception of a ~30 striated band located ~50 deep, this recipe produced a striation-free etch at depth of 190 (see figure 4). Striation Free Depth [um] A3 B1 Gas B2 Flow [sccm] Pressure [mt] ETCH 9.5 SF PASS 6 C4F Table 4: Parameters of the fine tuned recipe for the second 100 of etching B3 C1 C2 C3 D1 Range of Striation Free Depth [um] Etch Power APC D2 D3 Graph 1: Average striation-free depth of resulting from the 4 parameter and their 3 levels Power

5 43 49 Figure 4: Fine tuned recipe for the second 100 of etching displayed striation-free etching at depths approaching 200 A 3-step ultra-drie recipe was then developed and showed significant promise towards striation-free through wafer etching. The recipe resulted in striation-free sidewalls at a depth of 400 ; only 100 short of a through-wafer etch. Figure 5 shows that while the process resulted in a striation-free etch at large depths, progress will need to be made towards the tailoring of the sidewall profile Figure 5: Fine tuned recipe approaching through a wafer etch displayed striation-free etching at depths up to CONCLUSIONS When employing a DRIE process, researchers must often tune an existing recipe to obtain desired results. A single recipe can have a wide range of results which relate to the characteristics of the mask layout (i.e. feature size, etch depth, and exposed area). Often the DRIE step is not most critical to a device s overall process; therefore, only a limited amount of time may be spent on recipe development. This makes existing guidelines important for recipe tuning. For the purposes of the MEMS-REPS project, maximizing striation-free depth was regarded as the primary goal; this was to be accomplished by sequentially etching with three recipes. The results from the DOE showed that the striation-free depth could be increased by increasing the etch cycle time, and prior experience showed that lower pressure would also benefit the smoothness of the etch process. By taking these guides into consideration, it was possible to increase the striation-free depth by a factor of 4. To finally fulfill the requirements for the MEMS REPS fabrication process, further improvements will need to be made to maintain nearly perfectly vertical sidewalls for through-wafer etching, but with improved guidelines, it is possible to continue progressing towards this goal. 8. ACKNOWLEDGEMENTS This work is supported by DARPA through Grant #NBCHC For guidance and insight, the authors would like to thank the members of the MEMS REPS team, especially, Dr. David Walther. Finally, this work would not have been possible without the previous advancements made by Dr. Kelvin Fu and Dr. Aaron Knobloch. 9. REFERENCES [1] K. Fu, A.J. Knobloch, F. C. Martinez, D.C. Walther, A. C. Fernandez-Pello, A. P. Pisano, D. Liepmann, K. Miyasaka, and K. Maruta,, Design and Experimental Results of Small-Scale Rotary Engines, Proceedings of the 2001 ASME IMECE, [2] F.C. Martinez, A. J. Knobloch, and A. P. Pisano,, Apex Seal Design for the MEMS Rotary Engine Power System, Proceedings of the 2003 ASME IMECE, [3] A. J. Knobloch, M. Wasilik, C. F. Fernandez-Pello, and A. P. Pisano,, Micro, Internal-Combustion Engine Fabrication with 900 um Deep Features Via DRIE, Proceedings of the 2003 ASME IMECE, [4] J. Heppner, D. C. Walther, D. Liepmann, and A. P. Pisano,, Leakage Flow Analysis for a MEMS Rotary Engine, Proceedings of the 2003 ASME IMECE, [5] R. Bosch GmbH, Pat. 4,855,017 and 4,784,720 (USA) and C1, Germany, [6] A. A. Ayon, R. Braff, C. C. Lin, H. H. Sawin, and M. A. Schmidt, Characterization of a Multiplexed Inductively Coupled Plasma Etcher, Journal of The Electrochemical Society, 146, No. 1, pp , [7] A. Bendell, J. Disney, W.A. Pridmore (Eds.), Taguchi Methods: Applications in World Industry, IFS Publications, Bedford, 1989.