Investigation of thin-film transistors using a combination of focused ion beam etching and cross-sectional transmission electron microscopy observation

The nanometer-scale multilayered structure of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) used as elements for active matrix liquid crystal displays is investigated using a combination of focused ion beam (FIB) etching and cross-sectional transmission electron microscopy (X-TEM). This paper presents how faulty TFTs fabricated on a glass substrate can be characterized by X-TEM to achieve pinpoint accuracy. X-TEM specimens are prepared by FIB etching from TFTs including relatively large microvoids in their multilayers, without dropping of delicate edges. Cross-sectional bright-field TEM micrographs clearly show the details around the microvoids. This pinpoint X-TEM identifies faults and gives an analysis of their causes. In this paper, conventional low-angle ion milling and FIB etching are compared. Ion irradiation damage during FIB etching is also discussed.