Two different approaches to integrated MEMS

by Dick James, Senior Technology Advisor, Chipworks

After the 45nm hype of the last few months, I thought it would be a nice change of pace to go to the other end of the scale, and examine a couple of the more interesting MEMS devices that have shown up recently.

Historically, automotive applications have dominated this market (70%-80%), but now consumer electronics has become a growth area. The advent of MEMS devices in gaming systems, mobile phones, and digital cameras has opened up whole new markets to the business and driven changes in the technology and manufacturing. In particular, the Nintendo Wii and iPhone (and now iPod Touch) have created almost a step-function in unit growth in the last year and a half.

The inertial sensor market can be divided between accelerometers and gyroscopes. In 2006 accelerometers took 11% of the MEMS market, with a compound annual growth rate (CAGR) of 17% [1]. They found use in the automotive sector in applications such as air bag sensors and active suspension systems, and in consumer goods in hard drive protection, mobile phones, pedometers, game consoles, etc.

Gyroscopes are used in rollover detection, camera and mobile phone stabilization, video games, GPS, etc. in the consumer market, taking 10% of the MEMS market with a CAGR of 8% in 2006 1.

Analog Devices’ ADXL330

Analog Devices Inc. is one of the longest-lasting major players in the inertial sensor market, with its well-established iMEMS process (née 1993), which cleverly integrates the mechanical sensor and the electrical signal processing into a single die.

In the iMEMS process, a surface micro-machined 3-4μm thick polysilicon layer is used to form the fixed and moving MEMS structures (proof masses) on the surface of a BiCMOS die. The MEMS formation is done halfway through the BiCMOS, after polysilicon deposition and prior to the aluminum metallization. The MEMS is released during the final steps of the process. The iMEMS can be a two- or three-poly MEMS process, since either poly 1 or diffusions can be used for electrical interconnects for the MEMS elements.

The ADXL330 low-power ±3g three-axis accelerometer achieved a major design win in the Nintendo Wii controller, establishing MEMS devices as the key innovation in a new generation of game systems. It is a clever evolution of the 2-axis ADXL203 part [2], allowing detection of Z-axis motion, while keeping the X- and Y-axis sense essentially unchanged.

Figure 1 shows the decapsulated chip (a), with the silicon lid in the centre covering the MEMS structure, and residual bond wires around the edge of the die. In the close-up (b) we can see the cap partly sawn through, which indicates to us that the that a trench was etched in the cap wafer at same time as the cavity in the cap (c), so that only a partial cut is needed to separate the caps. Removing the lid (d) unfortunately causes some damage to the ASIC part of the die, but leaves the MEMS intact.

Fig. 1 : Decapsulated and de-lidded ADXL330 accelerometer.

The die is 1.9 × 1.9mm (3.61mm2), and the MEMS 0.8mm2 (0.64mm2), ~18% of the die area. The ASIC is made using a 1-poly, 1-metal BiCMOS process, with 2.45μm gates and LOCOS isolation. Poly capacitors and laser-trimmed thin film resistors are also used.

The MEMS structure is formed from three polysilicon layers. Tilt-view and cross-sectional views of the MEMS structure are shown in Figure 2; the fixed and moving beams are formed using the 4μm-thick poly 3. We found a poly 2 shield layer beneath the fixed and movable poly 3 area, which likely serves as the bottom plate of the Z-axis sense capacitor. Poly 1 is used for electrical interconnects, whereas in previous generations the iMEMS process used diffusions.

Fig. 2: Tilt-view and cross-sectional images of ADXL330 MEMS structure.

Figure 3 displays some of the architectural features of the MEMS area. The square features toward the four corners (a) are the springs, also seen in the tilt-view SEM (b). The X and Y inter-digitated sense capacitors measure lateral movement, whilst the Z sense axis is formed between the proof mass and the underlying poly 2 plate.

Fig. 3: Optical and SEM images of ADXL330 MEMS structure.

The ADXL330 sensor is a monolithic, three-axis, force-balanced, capacitive accelerometer. The proof mass, supported by the four springs, is movable along any axis (X-Y-Z) in response to an inertial force. The proof mass is attached to the common electrode of sets of differential capacitors, with the other electrodes of each differential capacitor pair being fixed. Each sense axis requires a separate set of differential capacitors oriented to the appropriate direction. The differential capacitance is measured by applying one of two complementary (180° out-of-phase) square-waves to each of the fixed plates of the differential capacitors.

A timing circuit samples each orientation one at a time. By cycling the voltages on and off for the fixed electrodes of the differential capacitors, the movement of the mass along each of the axes can be determined, and from that the relative accelerations. The electronics required to perform these functions are integrated into the ADXL330 die. The whole thing is packaged in a 4 mm × 4 mm × 1.45mm, 16-lead, plastic lead frame chip-scale package.

Invensense IDG-300

This part is an integrated dual-axis gyroscopic angular rate sensor that uses a very different fabrication technique. Invensense is a fabless MEMS design house, based in California, although they developed their process [3] before porting it to MEMS foundries for manufacture. Although a relatively new entrant to the MEMS industry, they are starting to get design wins in for image stabilization in consumer gadgets such as Pentax cameras and Hitachi camcorders.

Unusually, the device is formed from three wafers — a 490μm-thick lid wafer, the 35μm-thick MEMS wafer, and the ASIC substrate wafer, which is ~470 μm thick. Figure 4 shows different views of the device, with cavities in the lid (c) and ASIC (b), and the thin MEMS die in the middle of the sandwich (d). The lid had a thin layer of aluminum on oxide which has been affected by our decapsulation.

Fig. 4: Plan- and tilt-view and cross-sectional images of IDG-300.

The die is 3.44mm2, with the 1.76 × 3.44mm cap covering the MEMS die. The MEMS sensors themselves are each 1.47mm2 (2.16mm2), totaling ~36% of the die area. The ASIC uses a 0.5μm, three metal, BiCMOS-DMOS process, including on-board EEPROM to program the sensitivity required.

Fig. 5: Details of Y-sensor in IDG-300.

The layout of the Y-sensor is shown in Figure 5; the X-sensor is similar, just oriented at 90° to the Y-sensor (a). The corresponding ASIC area (b) has few features except the bond pads and split electrodes, and the parts of the MEMS die that stayed with the ASIC when we separated the two.

The MEMS is formed of a pair of proof masses over the cavities in the ASIC die, linked via hinges to drive plates over the split electrodes. These plates are similarly linked via torsional hinges to a spring-mounted circular frame around the whole assembly (c). On the outside of the frame are capacitance fingers (d) which move between sensor fingers connected to the ASIC through the bond pads around the edge of the array.

The split electrodes are separated from the MEMS drive plates by about ~0.25μm. When a signal of the right frequency is applied, they electrostatically drive the plates into oscillation such that the proof masses resonate vertically in and out of the plane of the device. When the sensor is rotated about the Y-axis, this induces angular oscillation in the circular frame, proportional to the rate of rotation, which is sensed by the differential capacitors outside the frame. This then feeds into the ASIC circuitry to give a Y angular rate signal to the outside world.

As we commented above, the manufacturing process is a little unusual. The lid cavity is straightforward enough, being KOH-etched, and then oxidized and cleaned. Then we get into a different sequence from other MEMS processes, since the lid is then fusion-bonded to the MEMS wafer, before thinning the MEMS wafer back almost to its final 35μm thickness. ~3 μm-high standoffs are KOH-etched into the MEMS die and coated with germanium, which is then patterned to match the standoffs — these act as the seal around the MEMS cavity, and bond pads for the MEMS electrodes.

The MEMS structure itself is then patterned and etched using a DRIE (deep reactive ion etching) process which does not etch the oxide remaining on the edges and inside of the lid.

Separately, the ASIC die is fabricated up to and including the bond pads and passivation. The final metallization and passivation mask includes openings for bonding the edge seal of the MEMS and vias for the MEMS electrodes. The cavities for the MEMS proof masses are drilled 85μm into the substrate using a Bosch etch (Figure 6), with the characteristic ridged profile (a). (In the Bosch process, alternate etch and sidewall passivation steps are used to control lateral etching, enabling the etching of cavities with high aspect ratios.)

At first glance, the deep etching of the MEMS beams looks remarkably uniform, and I wondered what process had been used — but a close-up (b) shows it is again a Bosch-style process, but with a much shorter cycle time between the etch and passivation steps than those in the substrate etch.

Fig 6: MEMS cavity (a) and MEMS cross-section (b) in IDG-300.

To finish the process, the two parts of the device are bonded together using the opposing germanium and aluminum surfaces, enabling a low-temperature process. After singulation the device is encapsulated in a 6 × 6 × 1.5mm quad flat no-lead (QFN) package.

One of the notable differences in the MEMS structures in the two parts is the nature of the MEMS layers themselves. Analog uses a 4μm thick polysilicon layer, whereas the Invensense uses a 35μm thick single crystal element. This obviously gives very different values to the proof masses, and it is arguable that Invensense could not have made their part using the lighter mass/unit2 of the Analog process — or at least it would have been much larger for the same performance. It is worth noting that the thickest deposited polysilicon MEMS structure seen to date by Chipworks is the ~20μm thick poly 3 MEMS structures found on the Bosch SMB380.

Which brings me to my technological conclusion… today’s MEMS parts with thicker proof masses would not have been manufacturable without the deep etching processes that have evolved over the last few years. Bosch filed their patent [4] back in 1993 — in retrospect, an enabling piece of technology that is helping drive the MEMS revolution that we read about regularly.

On a more commercial level, here are two parts that are making the break into consumer electronics. Neither of them use leading edge CMOS processes for the signal processing, in fact far from it, but that keeps the cost down and lets the MEMS innovation penetrate the new market segments.

[1] Global MEMS Markets & Opportunities Report, Yole Développement, 2007.
[2] S. Lewis, et al., “Integrated Sensor and Electronics Processing for >108 .iMEMS. Inertial Measurement Unit Components,” IEDM Technical Digest 2003, p. 949.
[3] S. Nasiri, A. Flannery, Jr., “Method of Making an X-Y Axis Dual-Mass Tuning fork Gyroscope with Vertically Integrated Electronics and Wafer-Scale hermetic Packaging”, United States Patent 6,939,473 B2.
[4] F. Laermer, A. Schilp, “Method of Anisotropically Etching Silicon”, United States Patent 5,501,893.

DICK JAMES is a 30-year veteran of the semiconductor industry and the senior technology analyst for Chipworks, an Ottawa, Canada-based specialty reverse engineering company that gets inside technology and takes apart ICs and electronics systems in order to provide engineering information for its customers. Contact him at 3685 Richmond Road, Suite 500, Ottawa, ON, K2H 5B7, Canada; ph 613/829-0414, fax 613/829-0515, [email protected],


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