Stuart M. Wentworth

Far-infrared composite microbolometers

Composite microbolometers for use as broadband far-infrared radiation detectors have been constructed. These novel devices utilize nichrome load elements which can be impedance-matched to a planar antenna. The load elements are thermally coupled to tellurium detectors. Room-temperature responsivities of 120 V/W and noise equivalent powers of 6.7*10/sup -9/ W/ square root Hz have been achieved.<<ETX>>

The high-frequency characteristics of tape automated bonding (TAB) interconnects

Tape automated bonding (TAB) tape is modeled as a transmission line, since, above 5 GHz, the tape length is significant compared to the signal wavelength. A test chip was fabricated to allow determination of the microwave characteristics of 3Ms 120-lead, two-layer gold-plated electrodeposited copper tape, which has a polyimide support layer and an inner lead pitch of 8 mils. A characteristic impedance of 100+or-10 Omega was found for the tape based upon measurement up to 18 GHz. This compares well with the theoretical model, which predicts an impedance between 98 and 118 Omega . Theoretical models and practical considerations were used to design a TAB tape with a lower impedance and good signal isolation. This design is presented. >

Complex permittivity and permeability extraction for multilayered samples using S-parameter waveguide measurements

In this paper, a novel technique is presented for accurately extracting the complex constitutive parameters (/spl epsiv//spl circ/, /spl mu//spl circ/) for individual layers of a multilayer sample using S-parameter waveguide measurements. The technique is based on a modified sequential quadratic programming algorithm, which utilizes a large number of initial guess points, thereby alleviating the possibility of local minima trapping. The algorithm was found to be significantly faster and more accurate than traditional global optimization methods such as the genetic algorithm. Computer-generated S-parameter data sets were initially used to establish the achievable accuracy of the algorithm for one-, two-, and three-layer cases. Sensitivity of the S-parameters to changes in the constitutive parameters and layer thicknesses was also investigated. Two-port S-parameter measurements (8.2-10 GHz) were conducted on three material samples in single and multilayer arrangements. The algorithm accurately extracted the complex constitutive parameters for each layer. These values were then compared with values extracted using a modified short-circuit line (SCL) method (single-layer cases only). S-parameters were also generated using the extracted values and compared with the measured data. In all cases, results were found to be in good overall agreement with both the SCL method values and the measured data.

Millimeter wave twin slot antennas on layered substrates

We have constructed a planar, monolithic twin slot antenna integrated with a bismuth microbolometer detector for operation near 94 GHz. In this paper, we briefly discuss the theory of operation for planar antennas on a dielectric stack. We then cover the fabrication details, including a short discussion of a photoresist bridge technique used to fabricate the microbolometer detectors. Measured receiver gain beam patterns are then compared with theoretical patterns for a number of cases.

Calculating Effective Skin Depth for Thin Conductive Sheets

In this presentation the conventional skin depth is compared with a modified skin depth that considers a finite thickness conductor, and with an effective skin depth that also considers wave reflection at the backside of the conductor. The comparison yields recommended thickness ranges over which each version of skin depth is accurate

A twin slot antenna on a layered substrate coupled to a microstrip feed line

A method is described for calculating the input impedance as seen by the detector for a twin slot antenna structure. This technique is then used to analyze several twin slot antenna configurations, leading to improved performance.

5.8 GHz dual-polarized aperture-coupled microstrip antenna

The paper presents the design of a dual-polarized aperture-coupled microstrip patch antenna suitable for passive 5.8 GHz radio frequency identification (RFID) applications. The antenna provides two orthogonal ports with nearly 30 dB isolation and crosspolarization levels of nearly 20 dB for both ports. Using the orthogonal ports, both transmit and receive functions can be implemented on the same antenna. This type of antenna could be very useful for RFID transponders which have to receive energy from an RFID reader and send back identification information.

Improvements in cross ratio invariance techniques for coaxial probe dielectric measurements

In coaxial probe dielectric measurements the use of a cross ratio invariance of three dielectric reference materials and one material under test (MUT) has been a common approach to calibrate and thereafter extract the complex permittivity of a MUT. Although it is generally accepted that the cross ratio is most accurate when one uses at least one reference material that resembles the MUT, there has been relatively little discussion on how different combinations of reference materials affect the accuracy of the extracted data. This work will describe two new approaches for implementing the cross ratio invariance technique by (1) measuring more than three dielectric references and selecting the three that most closely resemble the MUT, or (2) by forming a cross ratio of five reference materials and one MUT. The two approaches are evaluated through simulation and measurement by exploring the error in the extracted properties of 20 independently measured dielectric materials. For both new methods, the percent error in the extracted complex permittivity typically reduces from approximately 0.5% to 0.25% or less compared to the standard approach.

Composite microbolometers with tellurium detector elements

A composite microbolometer has been constructed for possible use as a broadband submillimeter radiation detector. Theory, fabrication, and measurement of these devices are discussed, and a finite element thermal model is introduced. The devices utilize nichrome load elements which can be impedance-matched to a planar antenna. The load elements are thermally coupled to tellurium detectors. Room temperature responsivities of 120 V/W and noise equivalent powers (NEPs) as low as 6.7*10/sup -9/ W/ square root Hz were achieved. Performance appears to be limited by 1/f noise in the Te detector. >

Complex Constitutive Parameter Extraction for Multilayered Samples using S-Parameter Waveguide Measurements

In this paper, a method was presented for accurately determining the complex CP from individual layers within a multilayer sample using S-parameter waveguide measurements. MPSQP, a modification of SQP, provided a simple mechanism to avoid local minima trapping while still obtaining the global minimum in a short amount of time. Comparison of the extracted CP to those of the SCL method showed that the MPSQP algorithm was successful in accurately estimating the CP for three single layer materials (one nonmagnetic and two radar absorbing). Results were also shown for a three layer material. Based on RMS error comparisons, the CP were found to be accurate to within the tolerance of the available measurements