Title: 設計與製作低應力薄膜麥克風經由低溫過程
Design and Fabrication of a Low-Stress Membrane for Microphones via a low Temperature Process
Authors: 呂宗憲
Tsung-Shien Lu
Paul C.- P. Chao
Keywords: CMOS麥克風;乾蝕刻;低溫過程;低電壓;CMOS Microphone;Dry Etching;Low Temperature Processes;Low Voltage
Issue Date: 2008
Abstract: 本文發展一個新的製程製作一個低應力震動薄膜的微機電麥克風。此薄膜由鋁製作經由乾蝕刻技術。如此,透過低溫過程構成多層結構可以同時達到薄膜的低殘留應力以及低的pull-in電壓。元件的簡單四層結構是金屬(鋁)、矽、金屬(鋁)以及二氧化矽。在文章所提到的低溫製程的優點,其製程方法為利用濺度鋁製程來形成薄膜及乾蝕刻犧牲層來達到低溫製作。。實驗結果顯示,濺鍍的鋁導致低應力的薄膜;此外,乾式蝕刻技術比濕式蝕刻好,因為乾蝕刻的蝕刻速率可容易的控制以及不會對於元件中的結構和材料有過蝕刻情形,例如等相性蝕刻效果。當蝕刻犧牲層時,鋁金屬層上面會留下dimple。這些dimple可以避免當供給高電壓時上電極和下電極接觸在一起;如此一來電極的壽命經由一些操作會有明顯的增加。量測顯示薄膜的殘留應力可以低到16MPa,共振頻率在20KHz,人們可聽見的範圍上限。被製作的膜可以對於一些應用達到一個滿意的水平,包括微機電麥克風。
This paper aims to develop a novel process to fabricate a low-stressed vibrating membrane for MEMS microphones. This membrane is made of aluminum via the dry etching technique. In this way, the low residual stress in the membrane and low pull-in voltage can both be achieved by the low-temperature process to form a multi-layer structure. The simple five layers for the device are metal (aluminum), silicon, metal (aluminum) and oxide. The merit of low-temperature process offered by this proposed process is made possible by the sputtering of aluminum for membrane and the dry-etching of sacrificial layers. The experimental results show that the aluminum sputtering leads to a low-stressed membrane; furthermore, the dry-etching technique is better than wet-etching, since the etching rate of dry etching can be controlled in an easier way, and with no over-etching to other structures or materials in the device, such as by anisotropic etching effect. Note that dimples remain on the aluminum layer when the sacrificial layer is etched. These dimples can prevent the top and bottom electrodes from contacting each other, when the high voltage is applied; thus, the lifetime of electrodes through a number of operations is significantly increased. Measurements show that the residual stress of the membrane can be as low as 16 MPa, while the resonance (cutoff) frequency is up to 20 kHz, the upper limit of human audible range. It appears that the performance of the fabricated membrane reaches a level of satisfaction for a number of applications, including the MEMS microphone.
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