A.M. Gundlach

Role of topography sensing for infection-structure differentiation in cereal rust fungi

Abstract.Over 90% of the germ tubes of Puccinia graminis tritici (wheat stem rust) and Puccinia hordei (barley brown rust) differentiate appressoria on encountering stomata.There has been controversy as to the role of host topographical signals in the highly precise and efficient induction of these infection structures over stomata by cereal rusts. In the present study, polystyrene replicas of microfabricated silicon wafers, bearing precise microtopographies of defined dimensions, were used to investigate the influence of ridge spacing and height on infection-structure induction by P. graminis tritici and P. hordei. It was found that artificial topographical signals alone can induce a reproducibly high percentage (83–86%) of germ tubes to differentiate infection structures. Multiple, closely spaced (1.5 μm) ridges which were 2.0 μm high provided the most inductive topography. Differentiation on flat surfaces and over single ridges was < 4%. Appressorium induction commonly initiated a cascade of differentiation events involving the formation of infection pegs, vesicles, infection hyphae, and occasionally haustorial mother cells. It is suggested that the close spacing of cell junctions associated with the dumbbell-shaped guard cells of cereal stomatal complexes provide inductive signals for infection-structure formation by cereal rusts in vivo.

Direct measurement of residual stress in sub-micron interconnects

The process-induced stress in interconnect structures in modern integrated circuits has a direct influence on the mean time to failure for the device. As the active devices are aggressively scaled to meet the constant demands of the industry, the interconnect structures are also being driven to smaller dimensions and this is increasing the demands made on material scientists to deliver high quality, stress-free metallization. We have demonstrated the measurement of process-induced stress in a single interconnect structure, fabricated in a CMOS compatible process, using a rotating beam sensor. We have shown its applicability in observing the variation in stress level from differing process conditions. Comparison of the rotation observed in the fabricated sensors with finite element simulation using ANSYS is discussed. The structure is suitable for use in a production environment and is scalable to deep sub-micron features for future technology nodes.

Dependence of process parameters on stress generation in aluminum thin films

The dependence of residual stress on the process parameters for aluminum metallization has been studied using a rotating beam sensor. This shows increasing tensile stress with both the target power and ambient pressure used during the sputter deposition of the aluminum layer. The bulk resistivity of the deposited aluminum has been measured using a Van der Pauw technique on test structures fabricated alongside the sensors and this shows different trends with respect to the target power and ambient pressure. This indicates that the stress in an interconnect feature is dominated by extrinsic components, which result from the mismatch in thermal expansion coefficient between the constituent layers, rather than the defects formed during the sputter deposition of the metallization. This indicates the suitability of the stress sensor technique to the monitoring of interconnect features in a production line environment.

Electrical characterization of platinum deposited by focused ion beam

Focused ion beam (FIB) systems are commonly used to image, repair and modify integrated circuits by cutting holes in passivation to create vias or to selectively break metal tracks. The ion beam can also be used to deposit a metal, such as platinum, to create new connections. These techniques are very useful tools for debugging designs and testing possible changes to the circuit without the expense of new mask sets or silicon. This paper presents test structures which can be used to characterize a FIB induced platinum deposition process. Sheet resistance test structures have been fabricated using a FIB tool and the results of testing these structures are presented. The sheet resistance data has been used to fabricate platinum straps with a known resistance. This extends the capability of the focused ion beam system beyond the deposition of simple conducting straps. The design of the test structures has been improved through the use of current flow simulation to investigate the effects of geometry and misalignment on the measurement accuracy. The results of these simulations are also presented.

Techniques to improve the flatness of reflective micro-optical arrays

Abstract Liquid crystal over silicon is an established technology for reflective spatial light modulators and microdisplays. In this paper, we describe progress in improving two key performance criteria. The first is backplane flatness; we describe recent developments in the use of chemical mechanical polishing to produce optically flat pixel mirrors on top of existing circuit topography; we further describe the use of an assembly technique that reduces chip bow caused by the microfabrication induced stresses in the silicon backplane. The second is liquid crystal layer thickness; we describe the use of a lithographically patterned spacer layer microfabricated on the surface of the silicon backplane to minimize layer thickness variations. Each of the techniques produces improvements in the performance of the final device.

A novel sensor for the direct measurement of process induced residual stress in interconnects

Residual stress in multilevel interconnects is a potential road block for the ITRS. Direct measurement of stress in interconnect tracks has been demonstrated for the first time using a rotating sensor fabricated in metallisation layers. The rotation is observable with a reflected light microscope and is compared with computer simulations using ANSYS. The structure is suitable for use in a production environment and is scalable to deep submicron features for future technology nodes.

Calibration of MEMS-based test structures for predicting thermomechanical stress in integrated circuit interconnect structures

This paper uses a rotating-beam-sensor structure to show that the extrinsic stress from the mismatch in expansion coefficient between the aluminum and the silicon substrate dominates over the compressive stress from the sputter growth. Sintering the layers at temperatures above 150/spl deg/C reduces this compressive stress due to the action of creep. Calibration of the rotation of the device has been undertaken by direct comparison to high resolution X-ray-diffraction measurements and these show that the sensor has a resolution better than 2.8 MPa. Furthermore, we have used the sensor to investigate the variation of in-plane stress with the compliance of the intermetal dielectric, by directly comparing sensors fabricated on SiO/sub 2/ and polyimide layers.

X-ray micro- and nanofabrication using a laser–plasma source at 1 nm wavelength

A picosecond excimer laser–plasma source has been constructed, which generates an x-ray average power of 2.2 and 1.4 W at the wavelengths required for proximity x-ray lithography: 1.4 nm (steel target) and 1 nm (copper target), respectively. The plasma source could be scaled to the 50–75 W x-ray average power required for industrial lithographic production by scaling the total average power of the commercial excimer laser system up to 1 kW. The 1 nm x-ray source is used to micromachine a 2.5 THz microwave waveguide–cavity package with a 48 μm deep, three-dimensional structure, using the LIGA technique. The 1 nm x-ray source is also used to print 180 nm long transistor gates in the fabrication process of field-effect transistors.

Realization of a large-area microbolometer sensor array for submillimeter astronomy applications: SCUBA-2

The realization of a large (40x32) pixel sub-array on a 3-inch silicon wafer brings unique challenges involving the integration of a variety of microfabrication techniques. Design, development and fabrication procedures are described, with conventional MEMS techniques in silicon being used where possible. High resolution imaging in the sub-millimetre range requires a pixel size of the order of one millimetre with a high signal/noise ratio detector, which must be addressed at cryogenic temperatures via a very low noise amplifying system. This has been realized using a combination of Transition Edge Sensors (TES) with amplification and multiplexing (MUX) by Superconducting Quantum Interference Devices (SQUID), which imposes particular requirements in the method of construction. This paper describes the details of the technologies used to overcome the conflicting demands of the different elements. The need to operate at millikelvin temperatures limits the materials that can be selected. Particular attention has been paid to the stresses induced in the structure by overlying films, bump bonding and any thermal processing.

SCUBA-2: Application of LTD Technology

We outline the need for SCUBA 2, its goals and specifications. We give reasons for the choice of the low temperature detector technology of TES arrays and SQUID multiplexers, and describe our pixel and array architecture and progress on the project to date.