Single Crystal Silicon Micromachined Capacitive Microphone

Tu Xiang Zheng

 

In 2006, the present author designed a Micro-Electro-Mechanical System, or MEMS microphone as shown in the above picture. The MEMS microphone is capacitive sensing device. In essence, it operates like a high frequency pressure sensor. It is comprised of two capacitor plates that, under the influence of the sound wave, vibrate with respect to each other. The resulted variation of the capacitance is then amplified by an interface circuit to produce either an analog or digital output signal.

The market for MEMS microphones has been growing steadily. It was reported to reach approximately $1.2 billion in 2015. Of the total market, almost $900 million is coming from smart phone, tablets, and wearable platforms. Additional applications include hearing aids, automotive, virtual reality, headsets, smart home, and internet of thing. As the quality of MEMS microphones continues to improve, new applications such as far field and directional audio are emerging.

Many efforts have been made to fabricate acoustic capacitive microphones. W. Kuhnel et al. have reported a micromachined subminiature capacitive microphone. The described capacitive microphone consists of a membrane chip and a back plate chip. The membrane chip has a silicon nitride thickness of 150 nm and a metallization layer thickness of 100 nm. The back plate chip has an electrode on a silicon bridge. Both the chips are fabricated respectively and then bonded together to form a capacitor.

J. J. Bernstein et al. have reported the fabrication and results of very high sensitivity acoustic transducers fabricated using surface and bulk silicon micro-machining techniques in a manufacturing environment. The silicon microphone described here is a capacitive microphone. The basic movable element is a thin (.about.3 micron thick) diaphragm made from p+ silicon. The p+ silicon is one side of an air gap capacitor. The p+ regions are formed using boron solid source diffusion at high temperatures. The other plate of the capacitor is a 20 micron thick perforated gold back plate formed using electroplating. The air gap is defined using a 2.2 micron thick sacrificial photoresist.

Altti Torkkeli et al. have reported a capacitive silicon microphone. The reported capacitive silicon microphone consists of two freestanding polysilicon membranes, a low-stress bending membrane and a high-stress back plate, which are separated by an air gap. A back chamber is arranged by encapsulation and static pressure changes are prevented with small equalization holes in the bending membrane. The device is fabricated combining bulk and surface micromachining techniques. Silicon substrates are etched in TMAH and sacrificial oxide between the membranes is etched in PSG-etch followed by freeze drying to prevent sticking.

The microphone design has gone through a number of iterations since the fabrication of the first batch of working devices. The most notable efforts have been made to reduce the thickness of the flexible plate and the air gap and lower the bias voltage of the capacitor.

However, it should be pointed out that difficulties have frequently been encountered with such efforts. In a thin plate there are two kinds of forces which resist deflection in response to acoustic signals. The first kind of force includes plate bending forces which are proportional to the thickness of the plate. These forces can be reduced by using a very thin plate. The second kind of force, which resists deflection, includes membrane forces which are proportional to the tension applied to the plate. In the case of a thin plate, tension is generally a result of the fabrication technique and of mismatches in thermal expansion coefficients between the plate and the particular means utilized to hold the plate in place. The thermal mismatched tension lowers the flatness of the plate. Reducing the thickness of the plate and air gap may mean the capacitor plates pulling together under a lower bias voltage.

The present design provided a single crystal silicon micromachined capacitive microphone whose capacitor structure comprises a single crystal silicon substrate, an acoustic cavity recessed from the back side of the substrate, a flexible single crystal silicon plate with the edge clamped to the inside of the substrate and the rear side facing the cavity, a single crystal silicon contained supporting frame having the top surface coated with a thin insulating layer, a stiff and perforated single crystal silicon plate supported at the edge by the supporting frame, an air gap sandwiched by the flexible plate and the stiff plate and surrounded by the supporting frame, and two electrodes disposed around the stiff and perforated plate and interconnecting to the flexible plate and the stiff and perforated plate, respectively.

Compared with the prior art capacitive microphone, it is easy to find that the single crystal silicon micromachined capacitive microphone has the following outstanding features:

Firstly, the single crystal silicon microphone is made from a three layer structure consisting of a single crystal silicon substrate, a thinner epitaxial single crystal silicon layer, and a thicker epitaxial single crystal layer and the prior art microphone is made from a five layer structure consisting of a single crystal silicon substrate, a thin insulating layer, a thin single crystal silicon layer, a thicker oxide layer, and a thicker polysilicon layer. The three layer structure of the single crystal silicon microphone is composed of a same kind of material. In this structure there is no thermal mismatched tension to reside therein. All thermal mismatched tension related problems are able to cancel forever. The five layer structure of the prior art microphone is composed of three different kinds of materials. Due to having different thermal expansion coefficient, thermal mismatched tension always exists between each two different material layers. As is well known, lower tension may result in lowering the sensitivity of the devices and higher tension may result in damage of the devices. Furthermore, a released thin plate with a strong tension often bucks up so that the achievable thickness of the flexible plate and the air gap of the microphone are severely limited.

Secondly, the acoustic cavity of the all single crystal silicon microphone has an opening area smaller than the area of the flexible plate and the acoustic cavity of the prior art microphone has an opening area larger than the area of the flexible plate. A small opening area means less losing mechanical strength and enables to further shrink the microphone size.

Thirdly, the epitaxial single crystal silicon layer for making the stiff and perforated plate has a rest portion with high quality, which can be used to fabricate an electronic circuit, such as a CMOS circuit for conditioning the electronic signals generated by the microphone. For the prior art microphone the top layer is a polysilicon layer that cannot be used to fabricate the CMOS circuit.