In one of the first real-world applications of photonic crystals, two companies partnered to produce a photonic crystal enhanced (PCE) micro-hotplate device with applications in areas from military combat identification to commercial and biomedical gas sensing technology. Ion Optics, which manufactures optical-based MEMS gas detection sensors and wavelength-tuned infrared emitters, partnered with IMT, which produced the device using its MEMS prototyping and production capabilities.

Figure 1. (left) An Unpackaged Die in a traditional package with 500,000 etched holes, and (right) aWLP die with high-vacuum, wafer-level packaging.

The device is a silicon micromachined infrared emitter, comprised of a photonic crystal modified micro-hotplate that emits thermally stimulated infrared radiation in a narrow band. The wavelength of light emitted is specified by the structure of the photonic crystal, allowing the device to be tuned to a specific waveband of interest.

The key enabling technology in the device is the development of the photonic crystal, which consists of an array of about 500,000 micron-scale holes etched into a metal-coated dielectric substrate. The submicron photolithography capabilities used to make the device enable tight control of the photonic crystal hole size, resulting in tolerances of approximately 100 nanometers (nm). Due to the tight control of the size and spacing of the photonic crystal holes, the device demonstrates excellent infrared emitter properties. The wafer-level packaging approach was chosen to improve performance. Previously, parts had been packaged serially by vacuum capping individual devices after the wafer was complete. With a proprietary bonding technology and a getter, the device provides

Other advantages of the wafer-level packaging were space savings from attainment of the smallest package possible for the device (chip-sized), low cost of electrical testing, and low cost of burnin, since it is performed more efficiently at the wafer level. The process also eliminated the need for underfilling of solder joints with organic materials, and enhanced the device performance by using minimum-length inter-connections. The MEMS device also operates at elevated temperatures, despite its reliance on thin-film metals, and remains stable at temperatures over 350°C.

Figure 2. The Process Capability Assessment on the 2-micron-diameter etched holes. The process sigma is 0.019 micron for the etched hole diameter, and spec tolerance is ±0.1 micron.

The device can perform the functions of a non-dispersive infrared (NDIR) gas sensor by combining it with a retroreflecting optical cell. Traditional infrared gas sensors exploit the fact that most gases have unique infrared signatures in the 2-14 micron wavelength region. Because each gas has a unique infrared absorption line, infrared gas sensors provide conclusive identification and measurement of the target gas with little interference from other gases. NDIR sensor systems achieve this by assembling many discrete components including light source, optical cell, optical filters, and detectors.

The PCE device generates tuned narrowband infrared radiation at the absorption wavelength of the target gas. Using this light, the device can make accurate optical measurements, eliminating cross-sensitivity and false alarms.

The first application of the technology is in supporting infrared combat identification; specifically avoiding “friendly fire” casualties. The emitter can provide a readily recognizable IFF (Identify Friend or Foe) signal in existing thermal sights. The tenability of the photonic crystal technology allows the emitters to provide efficient illumination in the desired infrared band without detectable cross-talk into adjacent thermal bands. In addition to protecting against friendly fire, the device enables emergency pickup, trail marking, landing beacons, and marking of high-value equipment.

This work was done by Ion Optics and Innovative Micro Technology. For more information, visit Ion Optics at;  visit IMT at .