Silicon Surface Micromachined Structures for the Stress Measurement of Thin Films
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SILICON SURFACE MEASUREMENT MICROMACHINED OF STRUCTURES THIN FILMS FOR THE STRESS RISTO MUTIKAINEN* and MARKKU ORPANA** Technical Research Centre of Finland, Semiconductor Laboratory, *Otakaari 7 B, FIN-02150 Espoo, **Olarinluoma 9, FIN-02200 Espoo, Finland ABSTRACT The realization of structures with silicon surface micromachining requires very low stresses of the structural elements. When low tensile film strains have been difficult to measure as no double-beam type structures susceptible for buckling at low tensile strains have been in use, a new "diamond beam" structure is proposed for the tensile-to--compressive strain conversion. The device performance is analysed as a strain gauge. The micromachined structures can also be used as substrates for deposited films. The formulation is presented for double beam as substrate for deposited film compressive strain gauging. The structures were applied for strained sputter-deposited Molybdenum, and it was found that large undercut of the double beam edge must be avoided as well as the spacer film thickness must be large enough for beam buckling to be noticeable. INTRODUCTION Silicon surface micromachining is a powerful means to prepare elaborate structures at high precision. When utilizing self-supported structures one has to control the film stress of structural elements to very low values to fabricate the structures and to avoid the undesirable side effects. An easy to use method is to observe the critical length of double beam (supported at both ends) for buckling to occur, which can be used in the compressive regime for film strain determination. Tensile strains are more difficult to investigate, although there have been reports on ring beams as tensile-to--compressive strain converterst. The structure suffers from reduced efficiency at long beam lengths because of the compliant ring-shaped quarters2. An alternative structure is presented in this study based on the newly developed diamond beams as tensile-to--compressive transducers. On the other hand similar structures can also be used as strain gauges for the deposited films. Commonly the bending is measured at single beam ends to derive the strain of the deposited film. An alternative way for this technique is to utilize the buckling structures similar to those for the strain determination of the structural polysilicon film, BUCKLING OF BEAMS FOR FILM STRESS GAUGING Diamond beams A diamond or square-shaped beam is proposed as an alternative to ring beam for tensile-to--compressive strain converter, see figure 1. The straight quarter bars are more rigid than those of ring beam to avoid reduced sensitivity with long beams.
Fig. 1. Diamond beam bars convert the tensile film strain to compressive force on the middle beam, which can lead to buckling.
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When a diamond beam is under tensile strain, the majority of the deflections of a quarter section are caused by the in plane bending moments and by axial force, and to smaller extent by shear. The deformations assumed are in the plane of the beam only, and the sma
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