Part 5: The InvenSense Story

InvenSense is a company based on the vision of one person – Steven Nasiri, a veteran of the MEMS industry. He came to the United States from Iran in 1974 and completed a BS degree from UC Berkeley. He has been involved with many MEMS sensors companies as cofounders or executives, including SenSym (Honeywell), NovaSensor (GE), Integrated Sensor Solutions (TI), and ISS Nagano. He was hired by Nova Sensors, founded by Kurt Petersen, Janusz Bredzyk and Joseph Lemon (a veteran of Kulite), to manage the manufacturing process.  He also held key management and operations positions at several semiconductor companies, including National Semiconductor, Fairchild Semiconductor and Maxim Integrated Products.

Mr. Nasiri founded InvenSense from his home in early 2003 and received the company’s first funding in April of 2004. It is said that he spent close to one year focused on the unique MEMS fabrication platform and its gyroscope feasibility study. During this period, he developed the novel product concept of the MEMS gyroscope and the low-cost, high-volume fabrication process known as, “Nasiri-Fabrication”. These efforts have led to the filing of the four core patents for the company, including the dual-axis gyro design and the vertical MEMS fabrication process. These patents are now responsible for the development of world’s first integrated dual-axis gyroscopes.


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Fabricating the mechanical structures on the chip is only half of the job done. The chip needs to be encapsulated in vacuum. It is the art of encapsulation that requires a lot of manufacturing resources and defines the yield and cost of the final device. Nasiri’s InvenSense also developed wafer-level packaging and encapsulation schemes. A silicon chip is stacked with a CMOS chip with eutectic bonding to aluminum bonding layer. The bonding process is performed with standard bonding equipment but with special material recipes. The process allows, in one step, wafer scale integration and encapsulation of fully hermetic sealed chambers.

Nasiri’s philosophy is to only start something if it has the potential to be the first of its kind in the world. Nasiri’s deep entrepreneurial spirit drives him to be both strategic and hands-on. He takes the time to understand the details of a problem and remains in the trenches to help work toward the optimal resolution. He won the Ernst & Young Entrepreneur of the Year 2010 Award Recipient in Northern California.  (For further reading, see the following links.  Link 1  pdf backup )

Part 6. Sensor Fusion  

Nintendo Wii is the first to use motion processing seriously in games. Since the beginning, a 1:1 motion control was not possible with accelerometer sensors only. Nintendo thought to a combination between tri-axial accelerometer and multi-axis gyroscope for the wiimote in order to improve playability. Indeed, sensor fusion is the preferred solution for increasing accuracy. Such a use of gyroscope and accelerometer sensors became obvious for Nintendo's engineers.  The Wii MotionPlus accessory is based on IDG 600(Invensense) dual axis gyroscope sensor and X3500W (EPSON) single-axis gyroscope sensor.

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Figure : Sensor fusion can increase motion sensing accuracy. Pitch and roll accuracy with different types of motion sensors. (Source)

The Apple iPhone is said to use nine axis motion sensing. In the future, more axis of sensors may be integrated closely together to satisfy form factor and pricing targets.

Part 7: The STMicroelectronics Story

ST Microelectronics is an Italian-French company based on the 1987 merger between two companies: SGS Microelettronica of Italy and Thomson Semiconducteurs of France. ST Microelectronics is the fifth largest semiconductor company in the world, with MEMS revenue of approximately $450m in 2008 (accelerometers, pressure sensors, ink jet printer heads). In 2008, accelerometers account for $220M, eclipsing its own accelerometer revenue of $27M in 2007. This helps STMicroelectronics gain a market share of 20% in 2008, up from 4% in 2006.

The company has many product lines, including Integrated circuits for specific applications, Memories (flash, EEPROM), Microcontrollers, smartcards, analog circuits power ICs. The company’s revenue in 2009 is $8.51B (billion).  In 2005, it earns revenue of $8.87B with profit of $ 266M.  

The company’s stock is trades at the New York Stock Exchange, Euronext, and the Italian stock exchange Borca Italiana (BIT) under symbol STM. The company owns a network of design centers, front end processing (wafer), and back end processing (assembly). The Agrate facility near Milan, Italy has 8 inch fab facility, whereas the Grenoble, France site has up to 12 inch facilty.

STMicro is a relative late coming in the field of motion sensors.  It entered the MEMS market in 2001. By 2005, the company was earning $3M in MEMS business annually. In November 2006, STMicroelectronics inaugurated a new state-of-the-art 200mm (8-inch) semiconductor wafer fabrication line at Agrate, in the Milan area, dedicated to MEMS devices. This is a bold move, requiring investment on the order of $50M. Benedetto Vigna, General manager of MEMS and Sensors division at STM, made a huge bet. He even invested his own money to convince the STM management to agree with his vision. This bold move allows STM to be prepared for the revolution of CE sensors before other rivals. In doing so, Mr. Vigna made two important decisions: (1) not to “wait and see”; (2) not to outsource fabrication to foundries in the famous Fabless model.

The STMicro company entered the Gyro market in June 2008 to target the growing consumer electronics (CE) market.  The company has been on a rapid pace – it announced single-axis gyro in 2008, two axis gyro in June 2009, and three axis digital gyro (L32G4200D) on February 15, 2010. The L32G4200D is a revolutionary device. It provides angular rate measurement in three axis, using a single sensing element. The entire chip, including both MEMS and ASIC, is housed in a package of 4 mm by 4 mm by 2 mm. ST’s 3-axis digital gyroscope offers a wide set of user-programmable full-scale ranges from ±250 dps up to ±2000 dps, with the low full-scale values for high accuracy of slow-motion sensing and the high range to detect and measure very fast gestures and movements. The device provides a 16-bit data output, together with additional embedded digital features, such as configurable low- and high-pass filters, and boasts excellent output stability over time and temperature.

The L3G4200D 3-axis MEMS gyroscope with digital output has been designed and produced using the same manufacturing-process technology that ST has already successfully applied to more than 600 million motion sensors sold in the market. Volume production began in Q2 2010 and the unit price is $2.9 in quantities greater than 200,000 pieces.

What contributed to STM’s success include at least the following factors:

1.      An earlier bold bet to invest in 8” wafer technology for MEMS, before market starts to establish. The 8” line allows STM to lower cost, good for CE applications.

2.      The STM gyro is based on a novel design utilizing a single element, therefore reducing the dimensions of chips. Small form factor is really important for CE.

The STMicroelectronics Company use one key process flow for wafer level fabrication.  The process, called THELMA (Thick Epi-Poly Layer for Micro Actuators and Accelerometers), uses thick polysilicon as structural material. In the first step, a 2.5 mm thick thermal oxide is grown. Secondly (step 2), a layer of LPCVD polysilicon is grown to form electrical conduction layers. Following this, a 1.6- mm thick layer of sacrificial oxide is grown with low temperature PECVD (plasma enhanced chemical vapor deposition). The wafer is then coated with epitaxial polysilicon layer. The layer is polished layer using chemical mechanical polishing to realize a smooth front surface. This layer can be 15-50 mm thick, depending on the design and application. Metal conductors are deposited and patterned on top of the epi-poly layer. Finally, the epi-poly layer is defined using dry etching (deep reactive ion etching). Finally, the sacrificial layer is removed using vapor or liquid phase hydrofluoric etch.

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The epitaxially deposition polysilicon has good mechanical properties, and can be deposited in an epitaxial reactor. The typical deposition rate is 1.5 mm/min. Earlier work in 1996 showed that the material can be deposited in single-wafer epitaxial reactor using an HCl/dichlorosilane gas system. 

For further reading on epitaxial silicon, start with the following papers:
- Epitaxy, wikipedia link 
- P.T. Gennissen, P.J. French, "application of bipolar compatible epitaxial polysilicon), 1996-1997, many conferences, plus Sensors and Actuators, Vol 62, issues 1-3, 1997. Link 
- M. Furtsch, "mechnical properties of thick polycrystalline silicon films suitable for surface micromachining", thesis, ISBN 3-8265-6594-0, 1999.
- A Patridge, "new thin film epitaxial polysilicon encapsulation", IEEE MEMS 2001, January 2001

The device is encapsulated at a wafer level with frit glass seal. Since the gyros are vibratory devices that must operate near vacuum, remaining gas molecules are absorbed with a SAES Getter material (provided by SAES Getters Group). (For further reading, see links.  Link 1 pdf backup )

Summary

  What do we learn from this?

-         Industry evolve rapidly and it takes tremendous efforts and innovation to stay at the front;

-         Technical superiority is not enough to make an innovation successful - you must solve a problem.  You don't get rewarded for solving half a problem.

-         The success of products are always behind visionary individuals and bold, persistent moves;

-         The MEMS motion sensor business will continue to be exciting and competitive for a long future to come.

Reference:

Patrick L. Walter, “The History of the Accelerometer – 1920-1996”, Sound and Vibration, www.SandV.com, 84-92, 2007. (PLW is a Professor with Texas Christian University, Ft. Worth, Texas). His email is p.walter@tcu.edu.

Steven Nasiri, “Wafer-Scale Packaging and Integration Are Credited for New Generation of Low-Cost MEMS Motion Sensor Products” (InvenSense link pdf archive )

Bernetto Vigna (STMicro MEMS leader), "MEMS Epiphany", IEEE MEMS 2009, Sorrento, Italy, January 26, 2009. (Just do websearch using the title)

 

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