D. Hilgart, Laserage Technology Corporation, Waukegan, IL (private communication). 6. D. Hickman, Questech Services Corporation, Garland, TX (private ...
Silicon Microfabrication: Laser Ablation vs. Inductively Coupled Plasma (ICP) Etch Megan M. Owens, Joseph W. Soucy, Thomas F. Marinis Electronics Packaging and Prototyping Division, The Charles Stark Draper Laboratory, Inc., Cambridge, MA 02139, U.S.A. ABSTRACT Mechanical prototypes of a silicon interposer device, having high-aspect ratio straight walls, were required for experiments to qualify various packaging assembly options. The ideal solution would yield expendable, quickly made, and relatively inexpensive replica parts. This need led to the investigation of micromachining of silicon via laser ablation. We sought to optimize the results by varying several parameters, the most influential of which was shown to be laser wavelength. Silicon wafers of thickness 500 µm were machined using laser wavelengths of 10,600 nm, 1064 nm, and 355 nm. As was predicted according to the theoretical silicon absorption profile, the best results were obtained using 355 nm wavelength light from an ultraviolet laser source. Several post-machining cleaning routines, to remove melted silicon debris, were evaluated as well. Pictorial results of both laser processing and cleaning are shown and discussed. ICP was concurrently investigated as a viable alternative to laser machining. These cuts were markedly cleaner and straighter than those induced by laser; additionally, cut features were more well defined. Pictorial results are compared and contrasted with those of laser ablation. Cost and time are traded for quality when choosing between laser ablation and ICP etch. Therefore, process choice should be application-dependent. For wafers with moderately complex cutout features, laser micromachining is significantly lower in cost and manufacturing time. However, the laser process does produce residual thermal stress surrounding the cut region, machined edges are relatively rough, and fine features are more difficult to produce. Laser ablation is an attractive manufacturing option for quick-turnaround prototyping as well as high-volume production; ICP is better suited to producing high-definition parts without potential for thermal stress damage. INTRODUCTION Many micro-electromechanical systems (MEMS) devices perform best when isolated from stresses in their operating environment; for example, vibration and temperature changes are often factors to be avoided. To this end, an interposer may be used as a physical bridge between the MEMS device and its external packaging, dampening the impact of temperature changes as well as unexpected mechanical forces. Silicon is a highly desirable interposer material, as MEMS devices are typically silicon-based structures; both the interposer and the device would share a similar coefficient of thermal expansion (CTE). This match would reduce concerns regarding potential thermal variation impacts to the system.
We sought to fabricate just such an interposer. The piece was designed of single crystal silicon with a cutout width-to-depth aspect ratio of 4:1, the cut width being 125 µm and the depth 500 µm. The goal was production of several expendable prototypes for assembly process experimentation and a few final pieces of the highest quality for the deliverable product. Two material removal options were considered: laser micromachining and ICP etching. While acceptable results may be obtained using both processes, important tradeoffs do exist. In general, process choice should be guided by the individual application. Both methods have the advantage of being dry processes, as opposed to the more traditional wet chemical etch. Laser micromaching is a more rapid and less costly process than ICP for wafers of moderate cutout complexity, based on vendor quotes; however, ICP is capable of producing finer features. While laser micromachining yields highly reproducible features regardless of location on the wafer, ICP results tend to vary by radial distance from the wafer center. Both fabrication methods require material mounting for machining and then removal, as well as post-process part cleaning. PROCESSES OVERVIEW The ICP process, patented by R. Bosch GmbH, involves isotropic etching via chemical cycling [1]. The silicon is first exposed to only SF6, removing target material, and is then exposed to only C4F8, passivating the exposed surface area. The subsequent SF6 step removes more material from the bottom of an exposed area than from the sides. The following passivation step protects all surfaces equally, and so the cycle continues [2]. Laser micromachining removes material by ablation, a result of the energy transfer from laser radiation to target material. The transfer occurs in two stages: absorption and conversion. Absorption excites electrons to the conduction band; conversion occurs as the electrons return to the valence band, emitting thermal energy. This process can produce melting, vaporization, ionization, and material ejection [3]. Material removal only occurs if the laser energy imparted exceeds both the bandgap energy, Eg, and the bond dissociation energy, D0. These energy levels correlate to specific wavelengths:
λ=
hc , E
(1)
where E is the energy, h is Plank’s constant, 6.6261 x 10-34 Js, and c is the speed of light, approximately 3 x 108 m/s. The optimal laser wavelength depends on these two factors as well as the material’s absorption spectrum. The absorption coefficient defines the achievable efficiency of energy transfer for a material. The maximum wavelengths for the bandgap energy and bond dissociation energy criterion are in Table I. Table I. Energy levels of Si and corresponding laser parameter implications. Energy (J) MAX λ (nm) Bandgap Energy 1.79 x 10-19 1106 Bond Dissociation Energy 3.25 x 10-19 612
These metrics indicate that the ideal wavelength is less than or equal to 612 nm. Considering the absorption profile of silicon, as shown in Figure 1, the ideal wavelength is expected to be 285 nm, the wavelength at which silicon is most absorptive.
Si Absorption Coefficient vs. Wavelength 1.00E+07
Peak: λ = 2.85E+02 α = 2.39E+06
Absorption Coefficient, α
1.00E+06
1.00E+05
1.00E+04
1.00E+03
1.00E+02
1.00E+01
Ultraviolet 1.00E+00 150
350
Infrared
Visible Light 550
750
950
1150
1350
Wavelength, λ (nm)
Figure 1. Absorption spectrum of Si [4]. FABRICATION TRIALS AND RESULTS Laser Micromachining Laser micromachining of silicon was attempted at three wavelengths: 10,600 nm, 1064 nm, and 355 nm. In the 1064 nm case, the wafer was spun with photoresist and soft-baked prior to processing. This layer served to shield the silicon from machining debris; the photoresist was removed in post-fabrication cleaning. A CO2 laser, operating at 10,600 nm, was unable to produce an acceptable part due to cracking, while two Nd:YAG lasers were able to yield whole parts. Qualitatively, the Nd:YAG laser operating at the fundamental wavelength of 1064 nm gave less desirable results than did the frequency-tripled laser at 355 nm. The uncleaned pieces of material processed by the CO2 laser showed pulse indentations along the sidewalls. The surface of the piece was speckled with silicon debris ejected during the machining process. Regions of thermal damage along the top surface clearly followed the laser pathline. (Figure 2) These trials led to the conclusion that an operational wavelength of 10,600 nm is unacceptable for machining silicon. [5]
108 µm 93 X
Figure 2. Cleaved part machined with CO2 laser at wavelength 10,600 nm. Interposer prototypes were successfully produced using an Electro Scientific Industries (ESI) Model 44 Nd:YAG laser operating at wavelength 1064 nm [6]. Although the machining did yield whole parts, the uncleaned surfaces were heavily sputtered with ejected debris and microcracking was evident. 37.6 µm 266 X
130 µm 136 X
Figure 3. Uncleaned interposer surfaces post laser machining at 1064 nm. Parts machined at wavelength 355 nm using an ESI Model 4440 Nd:YAG laser were of acceptable prototype quality. Uncleaned pieces showed straight sidewalls, clean lines, and top surfaces relatively clear of particulate. Some thermal damage along the cutting edge was evident, but was not as dramatic as that produced at longer wavelengths. [7] The presence of post-processing surface debris was common in all laser trial pieces. Several chemical cleaning techniques were evaluated (Table II); the goal was to maximize debris removal while maintaining the intended part shape and dimensions. Of these cleaning variations, best results were accomplished using treatment #9. Results of cleaning interposers fabricated using the 1064 nm and 355 nm wavelength sources are shown below (Figure 4 and Figure 5). The part machined at 1064 nm was still somewhat littered with silicon debris, especially along the top surface edges. The interposer made at 355 nm, however, showed a clean surface. The structure remained intact. No remnants of edge roughness remained. Though the sidewalls were not perfectly smooth, this process yielded an acceptable prototype for the application of interest.
Table II. Cleaning treatments. A = most clean; E = most dirty. No. 1 2 3 4 5 6 7 8 9
Treatment Acetone Mechanical Scrub Acetone U/S 30 s Acetone U/S 30 s, HF Etch 10 s Acetone U/S 30 s, HF Etch 10 s, TMAH Etch 23 min Acetone U/S 2 min, PRS U/S 30 min PRS U/S 30 min PRS U/S 30 min, 1:1 30% H2O2 : H2SO4 2 min PRS U/S 30 min, 1:1 30% H2O2 : H2SO4 5 min 1:1 30% H2O2 : H2SO4 10 min, PRS U/S 5 min
179 µm 56 X
88 µm 113 X
Result D C E B C C B C B
60 µm 167 X
Figure 4. Cleaned (treatment no. 9) interposer surfaces post laser machining at 1064 nm. 114 µm 88 X
56.8 µm 176 X
37.9 µm 264 X
Figure 5. Cleaned (treatment no. 9) interposer surfaces post laser machining at 355 nm. ICP Etching Parts were also micromachined via ICP etch. Silicon wafers were coated with photoresist, exposed, and developed. Wax-mounted on a handle wafer, the wafer was alternately etched and passivated until the parts were completely machined. The interposers were then separated from the carrier wafer and cleaned to remove the photoresist. This process, plus post-fabrication cleaning, yielded cleaner surfaces and more well defined features than did laser machining. Overall, ICP etching produced a part more dimensionally true to the original design (Figure 6).
189 µm 53 X
Figure 6. Interposer fabricated via ICP. RECOMMENDATIONS/ CONCLUSIONS This investigation validates the possibility of silicon wafer laser machining for fabrication of interposers used in packaging MEMS devices. To achieve best results, silicon should be machined in the ultraviolet spectrum. For many applications, laser micromachining may be a viable, even attractive, alternative to conventional silicon fabrication processes that might expend more time, effort, and money overall. Laser micromachining is economically well suited for prototyping and small batch size applications, as well as for volume manufacturing of parts that do not require the finest features.
ACKNOWLEDGEMENTS The authors appreciate valuable contributions from: J. Haley for internal laser trials; D. Hilgart at Laserage, D. Hickman at Questech, and K. Fahey and S. Upham at ESI for part fabrication; D. Gay for SEM photography; and M. Singleton for investigation of cleaning techniques.
REFERENCES 1. Robert Bosch GmbH, Pat. 4,855,017 and 4,784,720 (USA) and 4241045C1 (Germany) (1994). 2. A. A. Ayon, R. Braff, C. C. Lin, H. H. Sawin, and M. A. Schmidt, J. Electrochem. Soc., 146 (1) 339 (1999). 3. A. Ludwig, W. Pfleging and E. Quandt in Conference Proceedings: 6th International Conference on New Actuators, 1998, p. 367. 4. E. D. Palik, ed., Handbook of Optical Constants of Solids, (Academic Press, Orlando, 1985) pp. 529 – 535. 5. D. Hilgart, Laserage Technology Corporation, Waukegan, IL (private communication). 6. D. Hickman, Questech Services Corporation, Garland, TX (private communication). 7. K. Fahey and S. Upham, ESI, Portland, OR (private communication).
