Micromachining Silicon Structures on Thin Membranes Using Plasma Etching

Zhong Ren, Mark E McNie, Colin C Welch, Mike Cooke

Oxford Instruments Plasma Technology, Yatton, BS49 4AP, United Kingdome-mail: zhong.ren@oxinst.com

Keywords: inductively coupled plasma (ICP) etch; silicon membrane; cantilever; Bosch process

electro-mechanical systems (MEMS); deep silicon etch (DSiE)

Micromachining detailed silicon structures on thin membranes is regarded as one of the primary MEMS fabrication techniques, but also one of the more difficult processes. There is achemical loading effect, as the silicon clears from the membrane layer whilstfrom etching structures on a membrane decreases significantly as the membrane is approached. exothermic etch reaction can even cause the resist pattern and etch passivation associated heat is focused in a small area. There is a need for very high etch rates to remove bulk material to expose the membrane, but also a need for very precise profile control in fabricating features. The bulk etch can use an anisotropic wet etch that is selective to the <111> planes, such as KOH or TMAH. The wet chemistry is only suitable for low aspect ratio (AR) features andresults in a characteristic slope at a 54.74º for <100> silicon wafers that limitspacking density [1]. A Bosch (or gas-chopping) deep silicon etching (DSiE) processdeposition and etch steps can realize a vertical profile at high aspect-ratio 30µm/min, and can etch arbitrary shapes as defined by the mask pattern [2, 3]

This paper reports a technique which addresses these problems, and gives example results on structures, using a DSiE tool (Oxford Instruments PlasmaPro 100 Estrelas) in the Bosch process (chemistry). Most DSiE process tools are optimised for high rate anisotropic etchability to perform precision, lower rate processes. Such fast switching significant minimum power to maintain the plasma during the transition between esteps. This tool can operate successfully over more than an order of magnitude plasma (ICP) source power, from <500W to >5kW, opening the possibility of both high rate and precision, low rate processes in the same chamber. The minimum time of a single process step is 100removing polymer which enables a full cycle of deposition, break through and etch to Results given here used cycle times up to 2.5 s. Whilst an electrostatic chuckwafer surface in contact and to conductive layers on the membrane, where membranes are prouclamping plane the gap is large and so heat removal efficiency via Helium back side cooling is dramaticallyreduced. Thus in order to be able to etch structures on membranes without overheating and maintaining profile control, a low power process capability is also required.

Three devices have been realized using this combination of high ate and precision proceresonator high-Q cavity with a vertical profile (90º) and uniform scallop sizeperiodic deep grating with 50:1 high AR (Figures 3 and 4); and a large lengthphotoresist (PR) membrane where avoiding mask overheating and not damaging the polymer underneath the cantilever was key (Figures 5 and 6). Moreover, high quality also achieved on these etched structures.

[1] H. Seidel, L. Csepregi, A. Heuberger, H. Baumgartel, J. Electrochem. Soc. 137 (11) (1990) 3614[2] F. Laermer, A. Schilp, German Patent DE-4241045, 1994 [3] Z.Ren, M.E.McNie, Microelec. Eng. 141 (2015) 261-266

The authors would like to thank Dr. S Vollmeke (CiS Forschungsinstitut fur for providing cantilever samples for etching at Oxford Instruments and permission to use the SEM images

Using Plasma Etching

, Mike Cooke

tton, BS49 4AP, United Kingdom

antilever; Bosch process; micro-

regarded as one of the primary MEMS here is a very sharp transition in whilst the lateral heat dissipation

tching structures on a membrane decreases significantly as the membrane is approached. The and etch passivation to degrade when the ery high etch rates to remove bulk material

to expose the membrane, but also a need for very precise profile control in fabricating micro- or nano-scale n anisotropic wet etch that is selective to the <111> crystallographic

The wet chemistry is only suitable for low aspect ratio (AR) features and limits geometric freedom and

deep silicon etching (DSiE) process with alternating (AR) with etch rates of up to ] with high packing density.

which addresses these problems, and gives example results on three typical in the Bosch process (SF6-C4F8

process tools are optimised for high rate anisotropic etch but may be limited in their switching etch processes often need a

on between etching and deposition order of magnitude of inductively coupled

5kW, opening the possibility of both high rate and precision, The minimum time of a single process step is 100-300 ms for

cycle of deposition, break through and etch to be as low as 1 s. electrostatic chuck (ESC) clamps over the whole membrane, where membranes are proud of the

back side cooling is dramatically reduced. Thus in order to be able to etch structures on membranes without overheating and maintaining

have been realized using this combination of high ate and precision processing: a micro-vertical profile (90º) and uniform scallop size (Figures 1 and 2); a fine

ength-width ratio cantilever on a overheating and not damaging the polymer film

igh quality and smooth sidewalls were

Baumgartel, J. Electrochem. Soc. 137 (11) (1990) 3614–3626

Forschungsinstitut fur Mikrosensoik GmbH, Erfurt) and permission to use the SEM images.

Figure 1. Schematic section of a micro-resonator on a Si3N4 membrane: back cavity by KOH etching and the top pattern defined in a 60µm thick silicon membrane using the Bosch process

Figure 3. Schematic section of gratings in a SOI wafer: the back cavity defined by a high rate Bosch etch and fine period optical gratings realised in the device layer using a precision Bosch etch.

Figure 5. Challenges on Si membrane etch: (a) grass and striations in corners; (b) optimized etch gives striation-free and smooth sidewall; (c) PR mask burnt out during Bosch etch; (d) optimized etch avoids overheating on PR mask.

Figure 2. Micro-resonator structure: (a) side view with vertical profile and a 60µm etch depth; (b) sidewall roughness (scallop size) of 78nm

Figure 4. High aspect-ratio (50:1) fine period (200nm) gratings etched by a Bosch process: (a) Etch depth 5µm; (b) minimal bow at top of trenches

Figure 6. Si cantilever: (a) Illustration of cantilever fabrication in Si membrane by means wet-etching and Bosch process; (b) 5 mm length × 200 µm width × 50 µm thickness cantilever; (c) and (d) Enlarged view of the cantilever after polymer removal.