Selective Silicon Etching

Isotropic silicon etching Selective silicon etching Silicon etching Surface micromachining... 

In the selection of etch mask for deep glass etching, thick SU-8 is a choice, but SU-8 cannot be used in a HF bath (48%) because SU-8 does not adhere well to Si02 [123]. However, with a polycrystalline amorphous Si seed layer SU-8 adheres very well. For instance, with a 1.5-pm-thick polished poly silicon, a 50-pm-thick SU-8 can be deposited as the etch mask, leading to a maximum etch depth of 320 pm. Usually photoresist (2 pm thick) is only useful for shallow etch, less than 50 pm [123]. 

Fig. 44. Etch rate of polysilicon, oxide, and photoresist as a function of hydrogen addition to a CF4 discharge. Etch rate stops on non-oxide materials due to polymer build up at high % H - Oxide continues to etch due to polymer removal by the available oxygen. This yields high selectivity of etching oxide over silicon. After [220].

Selectivity requirements can be analyzed by comparing the etching rates of polysilicon and photoresist used as a mask. To complete the poly silicon etch before eroding the resist, the reqnired etching selectivity should satisfy the following criterion ... 

Anisotropy/sidewall angle Mask selectivity Substrate selectivity Etched surface quality Throughput A rough order of magnitude estimate of etch rates is as follows 1(X) nm/min for simple research reactors, 1 pm/min for production tools, and 10-50 pm/min for silicon DRIE reactors. Exact values depend on particular reactor design, RF power source, and other hardware factors. Silicon etch rate is much faster than oxide or polymer etch rate in the same reactor. More extensive passivation translates to reduced etch rate but also reduced undercut and more vertical sidewalls. 

Jansen HV, de Boer MJ, Unnikrishnan S, Louwerse MC, Elwenspoek MC (2009) Black silicon method X a review on high speed and selective plasma etching of silicon with profile control an in-depth comparison between Bosch and cryostat DRIE processes as a roadmap to next generation equipment. J Micromech Microeng 19 033001... 

Figure 9. Formation of micropillars from a solid precursor (schematic). (1) A175 nm thick thermal silicon dioxide film is grown on a silicon wafer. (2) A photoresist is applied by spin coating. (3) The oxide film Is selectively exposed, and then (4) selectively plasma etched with CHFs. (5) The photoresist is stripped, and (6) the silicon water is plasma etched with CI2/BCI3. (7) The residual oxide film is removed with a HF etch, and the silicon miaopillars as-formed are ready for use. Courtesy of Dr. A. Perez, Cornell University, Ithaca, NY.

Ozone is a god target reagent for microreactor applications since it is toxic, difficult to handle and very reactive. A silicon-etched 16-channel (600 (tm x 300 pm x 22.7 mm) microreactor covered with Plexiglas was used for oxidation of 1-decene into nonanal with quantitative conversion and selectivity [20]. This reaction proceeds in fact through the formation of the very reactive intermediate ozonide, which formally results from [3 + 2] addition of O3 to the C=C bond. A consecutive reduction step with P(OEt)3-EtOAc is required to yield the aldehyde. The reaction time is as short as 0.32 s. From the published data, a daily production of ca. 1600 g of nonanal per day may be obtained, which is well suited for preparation in fine chemistry. 

Porous silicon template could be carefully removed via selective chemical etching in tetramethyl ammonium hydroxide (TMAH) solution at 40-90 °C. Such dissolution process is crucial and not so easy to control. The etch rate is not the same for different Si templates. It depends on the Si dopant type and level as well as on the pore depth and diameter. The removal of template could be done using KOH solution, but the etching behavior is completely different from the case of TMAH. Ethylene diamine pyrocatechol is also reported as a chemical etch solution at 115 °C to successfully... 

Rittersma ZM, Splinter A, Bodecker A, Benecke W (2000) A novel surface-micromachined capacitive porous silicon humidity sensor. Sens Actuators B 68 210-217 Sim J-H, Cho C-S, Kim J-S, Lee J-H, Lee J-H (1998) Eight beam piezoresistive accelerometer fabricated by using a selective porous silicon etching method. Sens Actuators A 66 273-278 Steiner P, Lang W (1995) Micromachining applications of porous silicon. Thin Solid Films 255 52-58... 

Asoh H, Arai F, Ono S (2007a) Site-selective chemical etching of silicon using patterned silver catalyst. Electrochem Commun 9(4) 535-539... 

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