Micralyne Newslyne - September 2007

Achievements & Perspectives of DRIE Technology for the Microsystems Market

Alcatel Micro Machining Systems
The use of DRIE (Deep Reactive Ion Etching) for the fabrication of mechanical devices has completely changed the research in microsensors and microactuators to develop Micro Electro Mechanical Systems (MEMS or Microsystems).

Introduction

The use of DRIE (Deep Reactive Ion Etching) for the fabrication of mechanical devices has completely changed the research in microsensors and microactuators to develop Micro Electro Mechanical Systems (or MEMS or Microsystems). This is a rapidly emerging technology combining electrical, electronic, mechanical, optical, material, chemical, and fluids engineering disciplines. Nowadays, the mature technology is widely used, the total sales in the MEMS market reached $5.4 billion last year (source: Yole Développement). For example, MEMS devices incorporated into cell phones include silicon microphones, 3D accelerometers, gyroscopes for image stabilization and GPS, microfuel cells, and also biochips for personal weather stations and health care monitoring.

1. DRIE technology

The DRIE process prevents lateral etching of the Silicon resulting in highly anisotropic etch profiles at high etch rates and with high aspect ratio compared to wet chemical etching such as Potassium Hydroxid (KOH) or Tetra Methyl Ammonium Hydroxid (TMAH).

1.1. Principle

Bosch Process with low roughness at 14 nm RMS
Fig.1: Bosch Process with low roughness at 14 nm RMS
The most popular DRIE process is the well-known Bosch process based on the use of alternative steps of SF6 and C4F8. The SF6 is used to etch the Si, and the C4F8 to passivate the surfaces and to achieve the anisotropic etch of the Silicon. This alternation of etching and passivation steps results in a waving side wall profile, also commonly called scalloping. With the control of the gas flows and pressures, this scalloping can be significantly reduced, to as low as 14 nm.

1.2. Etch rate versus exposed area

High Etch Rate > 50 µm/mnS
Fig. 2: High Etch Rate > 50 µm/mnS

I
n 2002, the "I-Speeder" Project developed by Alcatel with Bosch and PerkinElmer resulted in tools that had the fastest etch rate for Silicon etching in the market. The latest "I-Productivity" project has resulted in further improvement of the etch rate and is now exceeding 50 µm/mn, rate with an excellent etch depth uniformity (Fig.2).





1.3. High aspect ratio (SHARP process)
The "SHARP" Process for submicron features. (Courtesy ESIEE)
Fig. 3: The "SHARP" Process for submicron features. (Courtesy ESIEE)

Designers and manufacturers are continuously looking for improved device performances like higher sensitivity of the widespread capacitive detection or actuation of comb structures. Better sensitivity of MEMS devices requires higher aspect ratio features than can be achieved, using the "Bosch" process. The Aspect Ratio (A.R.) of a given feature, like a trench, is defined as the ratio between the depth of the trench and the width. The Alcatel patented "SHARP" (Super High Aspect Ratio Process) allows to increase the aspect ratio in comparison with the "basic" Bosch process. When applying the innovative "SHARP" process to deep etching of very small features, it was demonstrated that an Aspect Ratio as high as 110 was achievable as shown in Fig. 3. For comparison, a "Basic" Bosch process doesn't achieve aspect ratios higher than 30.

1.4. Aspect ratio depending etching

Example of 1% ARDE at 3.0 µm/min Etch Rate
Fig. 4: Example of 1% ARDE at 3.0 µm/min Etch Rate
The ARDE (Aspect Ratio Depending Etching) is sometimes a limitation as the large features are naturally etched faster than the narrow ones. Alcatel has developed the capability to make this phenomena disappear by optimizing the passivation step of the Bosch process. The best results obtained presented less than 1% ARDE at 3 µm/mn etch rate (Fig.4).





1.5 DRIE at Micralyne

Micralyne's DRIE technology has been successfully transferred over to full-scale manufacturing. With etching capabilities varying from under 1µm to over 700µm's in depth, Micralyne's expertise lies in their DRIE recipes. Standard DRIE recipes are incapable of releasing heat sensitive membrane structures. This is because of excessive heat buildup that results in a loss of sidewall passivation and ultimately destruction of the device. Micralyne has developed specific low heat generating recipes that have successfully been implemented into multiple designs and process flows.

Micralyne has also developed multiple ways of mounting DRIE etch samples onto full sized wafer carriers, allowing R&D work to be completed on wafer pieces. This results in reduced material cost and faster experimental results. Micralyne has successfully developed DRIE recipes for over 50 customers/projects. Many customers require very specific etch specifications such as sidewall angle, sidewall roughness, etch rate, uniformity, selectivity, max temperatures, etch stop, low polymer production and others.

Micralyne has extensive experience customizing DRIE recipes to meet these varying customer requirements. Alcatel gives excellent process engineering support that allows Micralyne to remain updated on any new process developments.

Conclusion

The Alcatel DRIE is able to obtain a wide variety of features, in terms of sizes, depths, and shapes. This high versatility is attractive to MEMS manufacturers, such as Micralyne, because the Alcatel DRIE can be applied to a broad range of applications such as physical sensors, optical actuators, biochips and microfluidics. The MEMS market is now mature, with MEMS devices everywhere, from the telecommunication area to the entertainment world for motion monitoring. Size reduction, and new applications for MEMS are creating opportunities for the DRIE to be used in the realisation of micro and nano-features.

- By M. Puech, J.M. Thevenoud, J.M. Gruffat, N. Launay, P. Godinat and O. Le Barillecy, Alcatel Micro Machining Systems -

email: jean-marc.gruffat@alcatelmicromachining.com
web: www.alcatelmicromachining.com


Back to Newslyne


Contact Information: Micralyne Inc.
1911-94 Street, Edmonton, Alberta, Canada, T6N 1E6
Phone: 1.780.431.4400 Fax: 1.780.431.4422

This e-mail has been sent from Micralyne Inc. If you do not want to receive this newsletter,
please click the following link to unsubscribe.