MEMS (Micro Electro Mechanical Systems) microphones have a small form factor and are more resistant to heat, vibration and radio frequency interference than the currently widely used electret microphones. Due to its strong heat resistance, MEMS microphones use a fully automated surface mount (SMT) process, while most electret microphones require manual soldering. This not only simplifies the production process, reduces production costs, but also provides greater design freedom and system cost advantages.
Imagine less than half the size of an ordinary microphone with integrated audio signal processing, and the MEMS microphone can be used as an integrated part of a single-chip phone. The full potential of the new MEMS microphones remains to be tapped, but the first products using this technology have demonstrated many advantages in a variety of applications, especially for mid- to high-end mobile phones.
working principleThe Infineon microphone SMM310 contains two chips: MEMS chip and ASIC chip. The two chips are packaged in a surface mount device. The MEMS chip includes a rigid perforated back electrode and a piece of elastic silicon film. MEMS chips are used as capacitors to convert sound pressure into capacitance changes. The ASIC chip is used to detect MEMS capacitance changes and convert them into electrical signals for transmission to related processing devices such as baseband processors or amplifiers. ASIC chips are standard IC technologies. Therefore, this two-chip approach can quickly add additional functionality to the ASIC. This feature can be an additional component such as audio signal processing, RF shielding, or any function that can be integrated into a standard IC.
Performance characteristics
Most of the microphones we use today are electret condenser microphones (ECMs), a technology that has been around for decades. The ECM works by using a vibrating membrane of polymeric material with permanent charge isolation.
Compared to ECM's polymeric material diaphragms, MEMS microphones are very stable at different temperatures and are not affected by temperature, vibration, humidity and time. Due to its high heat resistance, MEMS microphones can withstand high temperature reflow at 260 ° C without any change in performance. This can even save the cost of audio debugging during the manufacturing process due to the small change in sensitivity before and after assembly.
MEMS microphones require an external bias from the ASIC, which is not required for ECM. Effective biasing will maintain stable acoustic and electrical parameters throughout the operating temperature range. The external bias of the MEMS chip also supports the design of microphones with different sensitivities.
Traditional ECMs are typically larger than MEMS microphones and cannot perform SMT operations. SMT reflow simplifies the manufacturing process, and a manufacturing step can be omitted, which is now usually done manually.
The IC is no different from the signal processing electronics in the electret condenser microphone, but it is a technology that has already been put into use. In the electret, an IC must be added, and in a MEMS microphone, simply add additional dedicated functions to the IC. The advantage of this extra feature compared to ECM is that the microphone has a high power supply rejection ratio. In other words, if the power supply voltage fluctuates, it will be effectively suppressed.
The SMM310's intelligent ASIC design results in very low power consumption, only one-third of the standard ECM (the current consumption of the SMM310 is ~70 μA at 1.5-3.3 V supply voltage, as shown in Table 1).
Table 1: Characteristic parameters of the new SMM310 silicon-based MEMS microphone.
Another advantage is the wideband RF suppression performance integrated on the IC, which is especially important for RF applications such as mobile phones, and is important for all devices that work similar to mobile phones, such as hearing aids. The SMM310 has a metal cover that shields the RF.
Another advantage of the small diaphragm of MEMS microphones is that small films with a diameter of less than 1mm are equally lightweight, which means that MEMS microphones can produce PCB noise caused by speakers mounted on the same PCB compared to ECM. Lower vibration coupling.
Wide range of applicationsConsidering the many advantages and system cost of silicon-based microphones, silicon-based microphones will be attractive for mid- to high-end applications where size, heat resistance, vibration and RF are critical, as shown in Figure 1. Application examples.
Figure 1: Example of a mobile phone application with additional 30pF capacitor for common mode rejection
Dimensions refer not only to the footprint of the microphone device, but also to the size of discrete devices that can be eliminated by a higher degree of integration in the ASIC. Applications with the above requirements include: mid- to high-end mobile phones, digital cameras, PDAs or game consoles.
The A/D converter can be easily integrated into the ASIC. By providing a digital interface to the microphone, the audio signal is not distorted by RF noise. This is an advantage for both mobile phones and laptops.
For laptops, silicon-based microphones have another advantage. In the case of VoIP, laptops can be used as phones. Microphone array software allows you to adjust the directional sensitivity of your laptop or the entire space, such as a conference room. However, to calculate the direction of the sound from the delayed signals of different microphones in an array, a microphone with very stable performance, such as a MEMS microphone, is required.
In addition to consumer applications and data processing applications, MEMS microphones are also attractive to the industrial, medical, and automotive industries, from MEMS microphones to machine monitoring, hearing aids, and car hands-free devices. The system cost for mid- to high-end applications is roughly the same. However, MEMS microphones have great potential for development. Now maybe it's a good time to learn about this new technology so that you can benefit from it in the future.
Technical outlookInfineon introduces a new product line, the silicon-based MEMS microphone. The first product in this product line is the SMM310-SMT analog output single-ended microphone. Semiconductor manufacturers have the core capabilities to build this product line. The first is MEMS design and manufacturing capabilities, followed by ASIC (application-specific integrated circuit) design and manufacturing capabilities, and finally high-volume, low-cost packaging capabilities. To date, audio companies have dominated almost the entire MEMS microphone market. Audio companies must rely on semiconductor foundries to provide relevant technology and share profits with them. Now, the entry of semiconductor companies like Infineon means that the market has new options and reduces the risk of component buyers.
Further shrinking of the size will be limited by the standard automated placement tools in the manufacturing process, as the audio port cannot be operated with a vacuum tool. In fact, the limitation is mainly from the MEMS size itself, and the size of the MEMS is less than half that of today's ordinary microphones.
More features will be integrated into the ASIC, and A/D conversion and digital output are the first steps. In addition, standard components such as wind noise signal filtering members can also be utilized. Dedicated interfaces and signal preprocessing will also be a large application area. RF shielding will also be further improved.
There are also many changes in audio. The SMM310 is optimized for vocals, but has a high acoustic sensitivity in the frequency range of 20 Hz to 20 kHz. It's hard to predict when a single-chip videophone with an integrated microphone and good stereo recording will appear, but there is no doubt that we are moving in this direction.
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