W. Kent Schubert

Simplified high-efficiency silicon cell processing

Abstract We developed an emitter diffusion process that yields a near-ideal doping profile with a passivating oxide in a single furnace step. Because this process subjects the material to only one high-temperature thermal excursion, bulk lifetime is better preserved. This is especially true for lower-cost silicon materials containing a high concentration of oxygen or carbon. Using this process, we routinely obtain one-sun cell efficiencies over 19% on float-zone material and over 18% on Czochralski material. Using solar-grade Czochralski material, we have demonstrated record efficiencies of 18.3% at one sun and 20.0% under concentration. Simple processes that yield high-performance diffusion profiles are expected to become increasingly important as manufacturers adopt improved techniques for ohmic contacts.

Integrated Chemical Analysis Systems for Gas Phase CW Agent Detection

A miniature, integrated chemical laboratory (μChemLab) is being developed that utilizes microfabrication to provide faster response, smaller size, and an ability to utilize multiple analysis channels for enhanced versatility and chemical discrimination. Improved sensitivity and selectivity are achieved by using a cascaded approach where each channel includes a sample collector/concentrator, a gas chromatographic (GC) separator, and a chemically selective surface acoustic wave (SAW) array detector. Prototypes of all three components have been developed and demonstrated individually and current work is focused on integrating these into a complete analysis system.

Micro acoustic spectrum analyzer

A micro acoustic spectrum analyzer for determining the frequency components of a fluctuating sound signal comprises a microphone to pick up the fluctuating sound signal and produce an alternating current electrical signal; at least one microfabricated resonator, each resonator having a different resonant frequency, that vibrate in response to the alternating current electrical signal; and at least one detector to detect the vibration of the microfabricated resonators. The micro acoustic spectrum analyzer can further comprise a mixer to mix a reference signal with the alternating current electrical signal from the microphone to shift the frequency spectrum to a frequency range that is a better matched to the resonant frequencies of the microfabricated resonators. The micro acoustic spectrum analyzer can be designed specifically for portability, size, cost, accuracy, speed, power requirements, and use in a harsh environment. The micro acoustic spectrum analyzer is particularly suited for applications where size, accessibility, and power requirements are limited, such as the monitoring of industrial equipment and processes, detection of security intrusions, or evaluation of military threats.

Amorphous Diamond MEMS and Sensors

This report describes a new microsystems technology for the creation of microsensors and microelectromechanical systems (MEMS) using stress-free amorphous diamond (aD) films. Stress-free aD is a new material that has mechanical properties close to that of crystalline diamond, and the material is particularly promising for the development of high sensitivity microsensors and rugged and reliable MEMS. Some of the unique properties of aD include the ability to easily tailor film stress from compressive to slightly tensile, hardness and stiffness 80-90% that of crystalline diamond, very high wear resistance, a hydrophobic surface, extreme chemical inertness, chemical compatibility with silicon, controllable electrical conductivity from insulating to conducting, and biocompatibility. A variety of MEMS structures were fabricated from this material and evaluated. These structures included electrostatically-actuated comb drives, micro-tensile test structures, singly- and doubly-clamped beams, and friction and wear test structures. It was found that surface micromachined MEMS could be fabricated in this material easily and that the hydrophobic surface of the film enabled the release of structures without the need for special drying procedures or the use of applied hydrophobic coatings. Measurements using these structures revealed that aD has a Youngs modulus of {approx}650 GPa, a tensile fracture strength of 8 GPa, and a fracture toughness of 8 MPa{center_dot}m {sup 1/2}. These results suggest that this material may be suitable in applications where stiction or wear is an issue. Flexural plate wave (FPW) microsensors were also fabricated from aD. These devices use membranes of aD as thin as {approx}100 nm. The performance of the aD FPW sensors was evaluated for the detection of volatile organic compounds using ethyl cellulose as the sensor coating. For comparable membrane thicknesses, the aD sensors showed better performance than silicon nitride based sensors. Greater than one order of magnitude increase in chemical sensitivity is expected through the use of ultra-thin aD membranes in the FPW sensor. The discoveries and development of the aD microsystems technology that were made in this project have led to new research projects in the areas of aD bioMEMS and aD radio frequency MEMS.

Sensitivity of piezoresistive readout device for microfabricated acoustic spectrum analyzer

A readout mechanism has been developed for measuring the response of mechanical microresonators to be used in an array for a microfabricated acoustic spectrum analyzer. It is based on the piezo-resistive property of polysilicon. The piezo-resistive readout mechanism is constructed in a quarter Wheatstone bridge fashion in which four equal serpentine polysilicon patterns are fabricated on top of a dielectric layer of silicon nitride. Microresonator devices using cantilever and clamped-clamped beam types with piezo-resistive readout mechanisms are fabricated using the surface micromachining technology of SUMMiTTM. The sensitivity of the piezo-resistive mechanism is characterized using 10 volts as supply on the Wheatstone bridge and no amplification of signal. The testing is conducted with electrostatic drive potentials 0-75 volts. Sensitivity of 1-5 millivolts per micron of beam deflection was observed by the characterization.

Post-Processed Integrated Microsystems

This report represents the completion of a three-year Laboratory-Directed Research and Development (LDRD) program to develop a low cost platform for integrated microsystems that is easily configured to meet a wide variety of specific applications-driven needs. Once developed, this platform, which incorporates many of the elements that are common to numerous microsystems, will enable integrated microsystem users access to this technology without paying the high up-front development costs that are now required. The process starts with the fabrication, or acquisition, of wafers which have sparsely placed device or circuit components fabricated in any foundried technology (Si CMOS or bipolar technologies, high-frequency GaAs technologies, MEMS, etc.). We call these “smart substrates.” Using the diverse processing capabilities of SNL, we then intend to “post-process” high-value components in open areas on the front or back of the wafers and microelectronically integrate the added components with the pre-placed circuitry. Examples of post-processed components include sensors, antennas, SAW devices, passive elements, micro-optics, and surface-mounted hybrids. The combination of preplaced electronics and post-processed components will enable the development of many new types of integrated microsystems. Targeted applications include integrated sensor systems, tags, and electromechanical systems.