Fabrication of biomimetic 3-D structured diaphragms

Abstract

We report on a new approach to the fabrication of 3-D structured diaphragms using integrated surface and deep reactive ion etching (DRIE) bulk silicon micromachining on a silicon-on-insulator (SOI) wafer. Polysilicon diaphragms of $\text{1}\text{mm}\text{×2}\text{mm}\text{×1.2}$ μm, and $\text{1}\text{mm}\text{×2}\text{mm}\text{×2.4}$ μm parylene diaphragms, which are designed for a biomimetic directional microphone and a differential microphone, respectively have been successfully fabricated by our method. The membranes have 20 μm-thick silicon proof masses, solid stiffeners, hollow stiffeners, and 20 μm-deep corrugations to mimic the tympanal membranes of the fly’s ears. Acoustic measurements of the diaphragm using laser vibrometry have demonstrated high directional sensitivity of the device.

Introduction

Non-planar micromachined diaphragms that make use of corrugations and bosses have become key components in various micromechanical devices such as pressure sensors [1], microvalves [2], micropumps [3], and accelerometers [4]. As new application areas are discovered and specialized devices are designed, more complex diaphragms will be required. However, two well-established methods for fabricating 3-D structured diaphragms have certain drawbacks limiting the design flexibility. In the electrochemical etch-stop technique, which combines anodic passivation of silicon with a reverse bias of p–n junction, n-type silicon is formed either by epitaxial growth or ion implantation and diffusion on a p-type wafer. It is impractical to construct a “proof mass” type of 3-D structures on silicon membranes with epitaxially grown n-type layers due to the uniform growth of the epi-layer, whereas, it is attainable with ion implanted and diffused n-type layer by double diffusion [5]. The second method utilizes heavily boron-doped (p⁺⁺) layers as etch-stop layers. In most cases, they are ion implanted and diffused. However, these layers can also be epitaxially grown. The stress induced by the high dopant concentration can considerably alter critical mechanical factors such as the resonant frequency of the diaphragm. In addition, the substantial surface damage from implantation precludes the formation of active electronic devices. In both methods, which employ etch-stop layers created by ion implantation and diffusion, the achievable thickness of the structures on diaphragms is upper-bounded by the maximum dopants diffusion depth (on the order of 15 μm) [6].

Our method integrates surface micromachining with bulk silicon micromachining to fabricate 3-D structured membranes [7]. It utilizes the buried oxide layer in a silicon-on-insulator (SOI) wafer as an etch-stop layer. Deep reactive ion etching (DRIE) allows precision patterning of single-crystal silicon (SCS) structures such as proof masses, and stiffeners with aspect ratios higher than 20. The thickness of the structures and the depths of corrugations are determined by the thickness of the device layer on the SOI wafer. Silicon is etched from the backside of the wafer using DRIE, which can achieve deep etching with very high anisotropy (side-wall angles 90±2°), unlike conventional methods involving wet anisotropic etchants such as EDP, KOH, and TMAH [6]. Various diaphragm materials such as polysilicon, silicon nitride, and polymers can be selected for different applications, because there is no subsequent thermal processing and no wet chemical backside silicon etching in our method.

To demonstrate the potential of our new approach for the fabrication of 3-D structured diaphragms, we present a biomimetic corrugated polysilicon diaphragm with SCS proof masses and solid stiffeners for a directional microphone, and a corrugated parylene diaphragm with SCS proof masses, solid stiffeners, and hollow stiffeners for a differential microphone.