A Novel Technique for Strain Measurements in Encapsulated-Layers

With the increasing complexity of both microelectronics and micromechanics has come a greater need to accurately characterise the mechanical properties of deposited layers. In the case of microelectronics mechanical stress can have a detrimental effect on transistor characteristics, whereas in micromechanics the stress may cause buckling or mechanical failure. There are several techniques available for measuring the mechanical stress or strain in deposited layers. If however multiple layers are required there are three considerations; 1) the stress of the total structure, 2) the stress in each layer and 3) the effect of deposited layers on underlying layers. At Delft University a simple method has been developed for measuring strain using a micromachined pointer which can measure both tensile and compressive strain. This method involves the use of a surface micromachined device which must be released in HF. If the layer to be measured is attacked by the sacrificial etchant, an encapsulation in polysilicon is used as a protection layer. This technique can be used to measure intermediate layers. This paper examines the problems of measuring the mechanical strain in multiple layers. Two main questions are addressed. The first is the effect of deposited layers on the mechanical properties of the underlying layers. The second is the use of encapsulation of the layer to be measured. A method has been developed to minimise the effect of the encapsulation on the strain measurements.