Clostridium perfringens isolated from sheep and goat with enterotoxaemia at necropsy and from healthy animals at slaughter were typed using specific PCR assays for the detection of the alpha-, beta- and epsilon-toxin genes. Clostridium perfringens isolated from all 52 animals with pathological signs of enterotoxaemia showed the presence of the alpha- and epsilon-toxin genes but were devoid of the beta-toxin gene. These strains could therefore be identified as type D, characteristic for clostridial enterotoxaemia of sheep, lambs and goats. In contrast, Cl. perfringens isolated from 11 of 13 healthy animals only contained the alpha-toxin gene which is typical for type A. Two of the healthy animals contained Cl. perfringens with the alpha- and epsilon-toxin genes. However, when several individual Cl. perfringens colonies were analysed from each of these two animals, only a small percentage was found to contain the epsilon-toxin gene, whereas the majority of the colonies were of type A with the alpha-toxin gene only. This is in contrast to the findings from the diseased animals which contained practically only type D Cl. perfringens. The beta-toxin gene was not found in any Cl. perfringens isolate from goat and sheep. Comparison of the PCR data with results obtained by the classical biological toxin assay using the mouse model showed a good correlation.
We have used chemical vapor deposition to grow ternary tungsten-based diffusion barriers to determine if they exhibit properties similar to those of sputter-deposited ternaries. A range of different W-B-N compositions in a band of compositions roughly between 20 and 40% W were produced. The deposition temperature was low, 350"C, and the precursors used are well accepted by the industry. Deposition rates are high for a diffusion barrier application. Resistivities range from 200 to 2oooO pQ-cm, the films with the best barrier properties having -1000pQ-cm resistivities. Adhesion to oxides is sufficient to allow these films to be used as the adhesion layer in a tungsten chemical mechanical polishing plug application. The films are x-ray amorphous as-deposited and have crystallization temperatures of up to 900°C. Barrier performance against Cu has been tested using diode test structures. A composition of W.23B.49N.28 was able to prevent diode failure up to a 700°C, 30 minute anneal. These materials, deposited by CVD, display properties similar to those deposited by physical deposition techniques.
The specific contact resistivity of CU with (cx+f9-Ta, TiN, a-W, and amorphous-%&~$T~~ barrier almn is meamred uaing a novel four-point;probe approach. Geometrkak the test stx+uctUres ctrnsist of colinear seta of W-plugs t o act, as current and voltage probes that contact the bottom of a planar CulbarriwlCu stack. Underlying Al ~X&XCOMM~B liok the plugs to the current gource a d voltmeter. The center-to-center distance of the probes ranges f h r n 3 to 200 pm. Using a relation developed by Vu et al., a contact resistivity of roughly 7 x IOa9 C2 cm2 i t 3 obtained for all tested b,arsier/Cu combinations. 'By reflective-mode small-angle X-ray scattering, the similarity in contact resistivity among the barrier filmpi may be related t o interfiicial impurities absorbed from the deposition process. In measuring the Bpe& C O I I~G~ reaiatance between two metals, device mmufactwers often implement a %eIvh'' structure [I]. In its simplest € o m , the atructure consists of a via electrically connecting two planes of metal. Aprobe to 30urce or sink the current contacts each plane. Two additionat prqbes are used t o measure the voltage, acro~e the planea. By knowing the rmietivity d ~patial dimensions of t h e via material, the contribution of the inhdacid xwi&anc:e of the via with the planar metals may be &acted horn the hkd measwed resistance. h CZr8eB where. the contact resistance is very low, numerow vim may be chained together to increase the summed p o t e n h h to measurable levels. AlEhough Kclvin structures are conceptually simple and do mimic the environment witbin 811 integrated circuit, they are often d i E c d t In implement and can suf3Fer from curren%crowding effects &31. Fabrication u~d y requires exposing the sample to air after deposition of each metal. Consequently, h-situ surface cleaning ip, often required to remove native oxides from the metals if very low resistances are desired. In many deposition systems, in-situ cleans are not possible.
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