Agilent Measurement Journal - Issue 3 - 2007 - (Page 44)

The unwanted ﬁeld component coupling can be minimized using a balanced, shielded structure as shown in Figure 1. The inner conductors of two coaxial cables are connected at the tip to form a loop. The received power is measured at the two output ports, which consist of two orthogonal modes. The differential mode output is associated with the magnetic ﬁeld and the common mode output is associated with the electric ﬁeld. A shield underneath the inner loop helps to minimize electric ﬁeld coupling. The shield is gapped to prevent interference with the magnetic ﬁeld measurement. temporarily deform after unintentional contact with the device under test (DUT) while still maintaining structural integrity. Under these conditions, silicon would be too brittle and has the added drawback that it is difﬁcult to cut the silicon in such a way to allow the probe to be placed very close (closer than the diameter of the probe) to the DUT. After construction, the thin-ﬁlm probes were bonded to a prefabricated printed circuit board (PCB) that connects the probe to the test equipment. Pictures of the probe connections and of the PCB are shown in Figure 2. After cleaning, the probe shield was attached to the ground plane of the PCB using an electrically conductive epoxy. Following a complete cure of the ground plane-epoxy connection, bonding pads on the probe traces were attached to matching bonding pads on the PCB using gold wire. The gold wire was attached by hand using an electrically conductive epoxy and a microscope. Optical micrographs of the ﬁnal probe assembly are shown in Figure 3. Conductive epoxy Overall length: 1.5 cm (15,000 µm) x–y 100 µm – 60 µm y Probe traces Gold wire Ground plane x x PCB traces Figure 1. Layout of the thin-ﬁlm probe Figure 2. Cross section of the bonded probe 1.5 cm Fabricating the probe The probe was constructed using a series of thin-ﬁlm photolithography processes.6, 7 A thin layer of silver was deposited on a silicon base and then was etched to form the traces and loop that constitute the main part of the probe. The traces were covered with an insulating material, SU-8, which is commonly used as part of many specialty photolithography processes. An additional layer of silver was patterned on top of the SU-8 to form a shield. The wafer was diced to separate individual probes. A single die including a probe was then washed in a chemical bath to separate the silver and SU-8 from the silicon base, thus forming a probe from the shield, SU-8, and traces, with the SU-8 acting as an insulator between the two metal layers. Removing the probe from the silicon creates a probe that is ﬂexible enough to Figure 3. Optical micrographs of the 100-µm probe: full length (top); a magniﬁed view of the wire bonds (lower left); and a closeup of the probe tip (lower right) 44 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

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