Silicon Microphones and Speakers: Technology, Market Analysis, and End Applications

Abstract

INTRODUCTION

Portable and consumer electronics device (products that utilize a microphone or microspeaker in its construction) manufacturers are continually attempting to find ways to improve profitability and gain (or retain) market share. Two common approaches are (a)reduce total cost of the cell phone unit or (b) increase price through technology features that consumers are willing to pay a premium.

These companies have done an excellent job of reducing the overall costs associated with building their products, focusing on various areas, including the following:

• Improving the supply chain through stronger supplier relations and more supplier involvement.
• Reduction in the total number of components and negotiating lowest component costs by leveraging volume.
• Improving manufacturing efficiencies by forcing standardization and increasing automation (reducing manual assembly).
• Utilizing low cost manufacturing regions.

Despite these requirements, microphones and microspeakers have shown only limited success in satisfying the needs of device producers.

The traditional electret condenser microphone, commonly referred to as ECM (Electret Condenser Microphone) has been the standard microphone option available to device designers. Manufacturers of portable devices and consumer electronics have had no other option than the traditional ECM. ECMs have provided customers with relatively low piece part cost, but fail to satisfy users in the areas of ease-of assembly, lowest installed cost, and advancements in value-added features. The industry needed a better solution.

Manufacturers have been forced to aggressively pursue cost reductions to simply maintain profitability. Component suppliers in turn have been pushed to the limits with regards to piece part cost reductions, but without sacrificing performance and features. However, after year over year price concessions, and flattening global unit demand for some devices such as mobile phones, double-digit piece part price reductions can not be expected with the existing components. Therefore, manufactures are forced to pursue other cost saving measures.

Many portable and consumer electronics devices contains hundreds of sub components, most of which are surface mounted directly to the primary printed circuit board within the device. By utilizing high speed, automated pick-and-place equipment, components can be placed and terminated to a circuit board at rates greater than 39,000 parts per hour. For an electronic device manufacturer, the actual cost of placing a single component utilizing this equipment becomes negligible manufacturers want to maximize the number of components that can be placed utilizing automated pick-and-place.

For those components that cannot be fed through the automated equipment, manufacturers must utilize off-line, secondary assembly operations to place the remaining components. Although there are only a handful of components that fall into this category, there is still a strong need to minimize any off-line component assembly. For this reason, companies have attempted to drive suppliers to develop surface mountable components that can be implemented directly into their high-speed assembly, without sacrificing performance.

Currently, there are only a handful of components that cannot be assembled in this manner. Two of these components are the microphone and microspeaker. With at least one microphone or microspeaker used in every electronic device analyzed in this study, the need for a replacement to the traditional ECM is increasing in importance.

The construction and design of the ECM prohibits it from being assembled with standard high-speed surface mount equipment. The diaphragm and backplate materials (typically Teflon or Mylar) cannot withstand the high temperatures associated with a typical surface mount reflow process. Reflow temperatures can run as high as 260 degrees Celsius for as long as 30 seconds, well in excess of what these materials can handle without degradation of performance or complete failure.

In addition, ECMs have a charged backplate (typically 200V-300V) that is implanted at the manufacturer. If for any reason the charge is reduced or removed, the dynamic response of the microphone quickly degrades. More often than not, this is caused by excessive heat. This is why ECMs are not specified over 85°ree;C and cannot be soldered to a printed circuit board through automated surface mount processes.

For a high volume consumer microphone it is essential that a close tolerance of the microphone sensitivity is achieved by the MEMS design. In order to avoid the influence of film stress Knowles Acoustics, for example, decided to pursue an approach with a so-called free-floating diaphragm. The microphone diaphragm is supported, but not physically attached, by posts that follow a circular pattern around the edge of the diaphragm. Thus, the diaphragm is ‘free-floating' within the MEMS structure. The thickness of the diaphragm is only 1&mum, the diameter of the active 11V, these support posts establish a gap between the diaphragm and the back-plate of 4 microns resulting in an active capacity of 0.5pF.

One of the challenging issues of the silicon microphone is the packaging. Besides size and cost constraints it is mandatory totake into account a high reproducibility as well as batch fabrication processes. Pack continuously and can handle the heat produced during the reflow soldering process if ttoucakkt 260°ree; for 30secs.