Performance grows with integration

Performance grows with integration
By combining silicon micromachining designs and processes with advanced mixed-signal CMOS circuitry, many of the early limitations of single-chip sensors can be overcome. As an example of this combination of technologies, a co-integrated pressure sensor consisting of a piezoresistive pressure sensor can be manufactured on the same 6-inch wafer as the CMOS circuitry. The circuitry performs calibration for offset and sensitivity and multiorder temperature compensation and error correction. On-chip E2PROM stores the calibration coefficients for the device.

The manufacturing process flow has been designed to allow all standard CMOS steps to occur at the front end of the process, with the micromachining steps performed after the circuitry is completed.

Here is the thinking behind integration: Piezoresistive pressure sensors need signal-conditioning circuitry to be interchangeable and compatible with most electronic control systems. Each sensor element has individual characteristics and is sensitive to both pressure and temperature.

The signal-conditioning functions include calibration for offset and full-scale variations to make each unit electrically interchangeable; temperature compensation for offset and full scale; and sometimes other functions as well, such as linearity correction, diagnostics and filtering.

Traditional technologies to marry signal-conditioning electronics with sensing elements have resulted in technical and commercial compromises in product cost and performance.

Monolithic approaches have resulted in low cost and small size but in many cases have compromised device performance because of the restrictions placed on circuit and sensor design by the monolithic fabrication process.

Hybrid approaches use either a dedicated ASIC for signal conditioning or a discrete circuit approach. In many cases, those approaches have better performance than monolithic devices and offer flexibility for adapting new designs with simple components changes. However, those hybrid approaches are generally not as low in cost as the monolithic approaches and are also larger and require additional assembly steps. More electrical and mechanical connections mean more concerns about reliability.

A number of technologies can be used to store the calibration and compensation values for each sensor. One involves thick-film resistors that are laser-trimmed after device testing to achieve temperature compensation. Another involves calibration to electronic trimming, where the compensation and calibration values are electronically programmed into the chip-either using a one-time-programmable technology such as fuses, zener diodes or on-chip trimmable resistors; or a reprogrammable technology like E2PROM.

Reprogrammable technologies like E2PROM allow coefficients to be programmed into the device multiple times, allowing for real-time programming during manufacturing or programming after assembly based on the test data for a given device.

The co-integrated pressure sensor at Silicon Microstructures Inc. was a collaboration between MEMS sensor designers and mixed-signal IC designers. This collaboration brings the strengths of each technology to the device design to maximize performance. A standard 0.65-micron mixed-signal CMOS process with E2PROM is used. All of the CMOS processing is performed at the start of the process and the MEMS processing is performed at the end. This process flow results in fewer compromises on the circuitry and better sensor performance.

In addition, this "CMOS first" choice results in a better manufacturing flow because the CMOS process flow remains uninterrupted. With this approach, process controls, based on high-volume production, are maintained.

The MEMS process that follows the CMOS process steps does not adversely affect the CMOS functions. These processes include the silicon etch to form the pressure-sensitive diaphragm; and anodic bonding of a glass substrate for absolute pressure configurations, probing, wafer dicing and inspection.

The circuit on the co-integrated pressure sensor provides for a number of adjustable parameters. Based on production test data, the following parameters can be adjusted in the ASIC by programming the device using the on-board E2PROM.

Preamplifier gain-32 to 152 in 16 steps (16 to 76 at 2.5-volt offset);

• Preamplifier gain sign-1 bit (positive vs. negative output);
• Preamplifier offset-0 to 49.5 millivolts in 16 steps;
• Preamplifier offset sign-1 bit (positive vs. negative input);
• Upward of 20 coefficients in pressure and temperature correction;
• Output clamp-independent adjustment in 40-mV steps;
• Output filter-1 to 256 conversions in 1, 2, 4, 8, 16, 32, 64, 128 and 256 steps, equivalent to approximately 1 to 256 milliseconds.

The circuitry on the co-integrated pressure sensor includes all of the functions for signal conditioning and calibration, including amplification, correction, span calibration, temperature compensation and multiorder nonlinearity correction for pressure and for the temperature coefficients.

The system uses an 11-bit analog-to-digital converter with 8x oversampling to provide for an effective 14-bit data conversion resolution. Data is then processed through an on-board digital signal processor (DSP) where the A/D data is corrected, based on calibration coefficients stored in the on-board E2PROM. The corrected signal is then fed to a 12-bit digital-to-analog converter, which in turn drives the output amplifier. The amplifier has been designed to be able to drive more than 2 nanofarads of capacitance as needed for electromotive force suppression in the automotive environment.

Calibration accuracy

Calibration is done by measuring the sensor at multiple temperatures and pressures. Uncorrected data from the A/D converter is read out through a digital I/O for each data point. The external calibration computer then determines the minimum error and loads the order of the correction in pressure, temperature and correction coefficients into E2PROM. Verification of the calibration can then be performed to verify calibration accuracy as desired.

Before linearization, the uncorrected output exhibits both positive and negative error (with respect to the ideal transfer function). In this case, a third-order pressure transfer function adequately defines the nonlinearity in pressure. Using this curve fit, total error is reduced from about 1.75 percent to about 0.2 percent. The performance of the device can change over temperature, and thus calibrating at other temperatures will increase the pressure accuracy at those temperatures. By minimizing the pressure error at this second temperature, total error is reduced.

The DSP algorithm used can be adapted to easily correct pressure nonlinearities of various orders and further can facilitate correction that might be introduced by temperature-dependent pressure nonlinearities from jelling or other media interfaces.

Sensor correction

After completion of the CMOS processing steps, the wafers are shipped from the CMOS fab to the MEMS fab. Silicon etching is performed along with the final process steps for wafer bonding, dicing and testing. The resulting dice are then either shipped to die customers or assembled into a variety of packages.

A new, single-chip pressure sensor has been built with all signal-conditioning and pressure-sensing functions on the same chip. This co-integrated pressure sensor overcomes many of the performance limitations of previous single-chip designs.

The co-integrated pressure sensor improves performance over previous single-chip pressure sensor solutions while reducing size and cost compared with hybrid configurations. A process flow that is compatible with a standard 0.65-micron CMOS process has been used along with MEMS processing to create the pressure sensor with E2PROM on-chip to store the calibration and compensation values for the sensor. Test results show a dramatic improvement in device performance over the uncompensated result.

This co-integration technology can be extended beyond pressure sensors to other devices as well, including accelerometers, gyros and other sensor types.

Michael L. Dunbar is director, strategic account development, and Henry V. Allen is vice president of engineering at Silicon Microstructures Inc. (Milpitas, Calif.).

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