Stress in local isolation structures is studied by micro-Raman spectroscopy. The results are correlated with predictions of an analytical model for the stress distribution and with cross-sectional transmission electron microscopy observations. The measurements are performed on structures on which the S&N4 oxidation mask is still present. The influence of the pitch of the periodic local isolation pattern, consisting of parallel lines, the thickness of the mask, and the length of the bird's beak on the stress distribution are studied. It is found that compressive stress is present in the Si substrate under the center of the oxidation mask lines, with a magnitude dependent on the width of the lines. Large tensile stress is concentrated under the bird's beak and is found to increase with decreasing length of the bird's beak and with increasing, thickness of the S&N4 film.

Micro-Raman spectroscopy is used to study mechanical stress in local isolation structures on silicon substrates (poly-buffered local oxidation of silicon, called PBLOCOS or LOPOS). The influence of processing parameters such as nitride film thickness and width, pad oxide thickness, and field oxide thickness is studied. Also the change of the local stress during the successive processing steps of the isolation is investigated: deposition of the nitride mask, field oxidation, and removal of the nitride mask. The results are explained using a simple analytical model. It is found that stress varies very much during the different processing stages. After deposition of the nitride film, the stress is compressive under the mask lines and tensile outside the lines, close to the line border. The stress magnitude is highly dependent on the thickness of the nitride film. It can be described by edge forces. During field oxidation, this edge-force-induced stress nearly completely relaxes. In LOPOS structures with field oxide, the magnitude and distribution of the mechanical stress is mainly determined by the compressive forces exerted by the field oxide and the bird’s beak on the silicon, and tensile forces which are related to the bending of the nitride film at the edges. The influence of nitride film thickness on the stress at the center of the lines, ωc, is much smaller after field oxidation. For wide lines ωc is compressive and much higher than before field oxidation. For small widths ωc becomes tensile due to the influence of the vertical forces acting at the bird’s beak edge. The tensile stress due to the bending of the nitride film edge is much larger than the field-oxide-induced compressive stress, and dominates the stress picture at the bird’s beak. Only when very thick field oxides and thin nitride films are used, the field-oxide-induced compressive stress becomes visible. This becomes more clear when, in a further process stage, the nitride film is removed. The stress picture is then completely determined by the field oxide and the stress in the active area is always compressive.

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Silicon carbide (SiC) integrated circuits have been suggested for extreme environment operation. The challenge of a new technology is to develop process flow, circuit models and circuit designs for a wide temperature range. A bipolar technology was chosen to avoid the gate dielectric weakness and low mobility drawback of SiC MOSFETs. Higher operation temperatures and better radiation hardness have been demonstrated for bipolar integrated circuits. Both digital and analog circuits have been demonstrated in the range from room temperature to 500 °C. Future steps are to demonstrate some mixed signal circuits of greater complexity. There are remaining challenges in contacting, metallization, packaging and reliability.