Market analysis: MEMS packaging moves towards wafer level

Traditional MEMS relied on ceramic packaging for a long time. Although effective, the MEMS industry has been brewing into a wafer-level packaging (WLP) technology, and part of the driving force for this transformation comes from the beginning of more and more foundries. Get involved in the field of MEMS.

"TSMC (Hsinchu, Taiwan) and some other wafer foundries are already talking about manufacturing MEMS parts in the future," introduced Ken Gilleo, MEMS packaging expert and founder of ET-Trends (West Greenwich, Rhode Island), "they also They also have the resources needed to make packages. If they decide to enter this area, they will prefer to directly perform wafer-level packaging. Therefore, large wafer foundries may eventually become the protagonists of innovative MEMS packaging. "

Jerome Baron, an analyst at Yole DÃ©veloppement (Lyon, France), said: "The current MEMS packaging market is mainly driven by TSV and WLP technologies, and the shift from 6-inch to 8-inch wafers has also brought additional impetus. "Manufacturers can package a huge number of sensors on an 8-inch wafer. For example, accelerometers are widely used in iPhone or Wii game consoles, where the inertial sensor area is only 5-7 mm2. Baron said that WLP using TSV can package about 5000 sensors on a wafer, and this Data will increase in the future.

There are many types of MEMS applications, such as MEMS gyroscopes, micromirrors, RF, microprobes, pressure sensors, and some IR sensors. Applications such as MEMS microfluidics and microcooling are also about to be launched. Baron predicted, "In the future MEMS industry, the use of three-dimensional WLP to cap the MEMS device will account for a large part."

According to Yole DÃ©veloppement, the most active players in this field are those that already own 200 mm fabs or those that plan to transform. This includes IDM companies such as Bosch (Gerlingen, Germany), STMicroelectronics (Geneva), Silex (Sweden, Jrflla), Touch Micro-system Technology Corp. (Taiwan, Yangmei Town) and Dalsa (Canada, Ontario) and other MEMS foundries, and Companies such as ASE (Taiwan, Taipei), Xintec (Taiwan, Zhongli) and Nemotek (Morocco, Rabat Technolopolis Park) provide packaging services.

logistic issue

Gilleo said that compared with traditional electronic circuits, MEMS logistics has a big problem. The processed MEMS wafer may be relatively large, fragile, and sensitive to contamination, making it difficult to perform dicing operations. It is necessary to cover or cap the surface before cutting. "Although this protection is not absolutely safe, at least it is much more stable than the uncapped wafers," Gilleo continued. "The wafers capped by the foundry can be shipped to the packaging company for wire bonding And molding, because the MEMS chip is the same as the standard electronic chip at this time, and it can be molded. The cap protects the internal mechanical structure. This method is quite effective, which hinders wafer-level packaging Development. Despite this, we will eventually use wafer-level packaging because it can save costs and enable smaller size packages. "

Many MEMS chips, including accelerometers and micro microphones, have only four to six pins. This puts MEMS products into a unique low-end packaging category, but at the same time, MEMS has special requirements for air tightness. "This is the case with accelerometers," Gilleo added. "I don't think there will be much change in motion detector packaging, because they are now small enough to fit well inside smartphones."

Gilleo suggested that some semiconductor companies that have suffered losses in solar cells may consider introducing MEMS business. "I think we will see some people who jump into the field of solar cells begin to pay attention to MEMS. Since we already have some larger wafer foundries to manufacture MEMS devices, why not test and package at the same time? In the future, there will be drivers of full wafer-level packaging, so it will be an inevitable trend to become a larger fab. "He continued.

Yole also mentioned a trend towards CMOS-MEMS integration, which involves adding MEMS structure layers in pure CMOS processes, which is driven by wafer foundries such as TSMC and UMC (Taiwan, Hsinchu) planning to enter the MEMS market of. CMOS integration is "a real breakthrough because it allows the integration of MEMS and ASIC on a single chip," Baron said. "In most cases today, MEMS and related ASIC chips made using bulk micromachining Are separated from each other (two-chip solution), because the process difference between the two is too large to be compatible. However, these CMOS wafer foundries are changing the integration of MEMS and CMOS, and will soon achieve effective CMOS- MEMS integration. "

Towards the direction of MEMS-CMOS compatibility, some key technologies have been developed, including the use of XeF2 etching or O2 plasma etching to achieve a low-temperature, non-damaging surface micromechanical process for effective structure release in CMOS processes, Baron continued.

Using these processes in CMOS wafers can effectively integrate MEMS devices. At the same time, a single chip can be packaged at the wafer level using TSV, so the final packaging will become simpler, Baron introduced. He went on to say, "If ASIC and MEMS are integrated on a single chip, it will reduce the cost and package size, and also achieve a real breakthrough in integration."

Another area where development trends are appearing is RF MEMS. According to Gillee, it has been 10 years of efforts to bring RF MEMS into the market, but many devices are currently unreliable. "For most RF MEMS devices, there is a point-to-point contact at the switch position," Gilleo explained. "When you switch the current repeatedly, there will usually be metal migration at the contact position, which will cause the MEMS device to Mechanical wear. "

Relays used in space have also faced this problem, but after continuous improvement has become reliable enough. RF devices are turning to multi-frequency applications in smart phones and are developing towards WiMAX and other new G4 systems, so the industry will adopt mature RF MEMS (probably an antenna switch) technology at the right time. RF MEMS requires hermetic packaging, and the entire device needs to be sealed, so device manufacturers use traditional ceramic packaging. Gilleo said, "This field will develop, and at the same time I think the traditional form of packaging will continue to exist. But in the next few years, it will develop to wafer-level packaging."

Testing is another area that deserves attention. "We will eventually see some moves in the field of MEMS testing," Gilleo said. "Although electronic testing is trending towards more transistors and more pins, MEMS is just the opposite. It may only require four connections, so for MEMS, there will be no special need for automatic test equipment (ATE). This will Is a problem. "The foundry may choose to conduct testing while packaging. "If you decide to do the entire job, you must also do the test," he said.

Changes in bulk packaging

MEMS inkjet devices require minimal packaging. According to Gille, the device requires only electrical connections without chip protection. "In these 20 years, the most common customer-tailored automatic soldering (TAB) technology with excellent performance. This technology does not require anything new, so we will not see any changes or real changes in the inkjet head field in the near future. Innovation, "he said.

In the fields of accelerometers and gyroscopes, Gilleo said that he believes that the corresponding packages have been developed relatively well two or three years ago. "If it is only used for airbag control, you can put the corresponding chip in a ceramic package, which costs about a few dollars. This is already very good for the automotive industry. In an airbag with life-saving potential, you will not There is an opportunity to change the package form to save a dime, so they will also use proven ceramic packages, "he said.

By using improved ceramic technology, costs can be further reduced. The industry will adopt and improve the leadless ceramic package carrier (LCCC) technology that has been used in military products for decades. "What is happening is that they will continue to reduce costs and eventually switch to wafer-level capping operations, which solves many problems. Some companies use plastic QFN packaging, but the chips are hermetically capped. , "Gilleo said.

Finally, the field of micro-optical electromechanical systems (MOEMS) will also usher in a new round of accelerated development. "High-end cinemas will develop in the direction of digital 3D, which will bring some driving force to MOEMS. In 2010, there will be some hand-held micro projectors using MOEMS, and it is also possible to embed micro projectors directly in mobile phones. And, because of the photography Mobile phones are becoming more and more complicated. I think we can see some fusion of mobile phone camera packaging and MOEMS packaging. Although the camera does not necessarily need the airtightness like MEMS, the technology used by the two is similar, "Gilleo Introduced in this way.

The good news is that all the technologies used for MEMS packaging already exist, but there are no economic factors driving this change yet. "I think we will see more old packaging formats that have not changed, but when the time is right, there will be a lot of wafer-level packaging IPs that look very good," Gille concluded. Now, we must wait for large fabs to enter this field, and verify whether the MEMS field will actually shift to wafer-level packaging.

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