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Critical thicknass of metallization SLIS


T.I. Danilina - Ph.D. (Eng.), Professor, Tomsk State University of Control Systems and Radioelectronics
I.A. Chistoedova - Ph.D. (Eng.), Assistant Professor, Tomsk State University of Control Systems and Radioelectronics
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The development of high speed very-large-scale integration circuits (VLSI) requires decreasing of all dimensions of elements including thickness of metallization. The conductive film metallization thickness has become less than 50 nm, and that of diffusion barrier layers (DBL) has already reached 10 nm. On a certain growth stage, a film consists of separate isles forming a reticulate structure first and then forming a continuous film. During the transition from an island film into continuous one, its surface resistance becomes several orders higher. There is also an essential change of an optical transmission (reflection) spectrum. The transition to submicronic metallization requires additional research in order to select critical metallization thickness. To solve this problem, electrical and optical measurements of conductive films based on aluminium and titanium with the thickness over the range of 10-100 nm was conducted.
The deposition of aluminium and titanium films on silicon plates was carried out by electron vacuum evaporation under the pressure of 105 Pa using an Orion-B installation with velocity of 0.5 nm/s. The film thickness control was carried out by means of a quartz sensor with accuracy of 0.1 nm. Samples with various thicknesses of conductive films had good adhesion and smooth surface. With growth of film thickness from 10 nm to 100 nm the roughness of aluminium increases from 10 nm to 40 nm, and that of titanium decreases from 50 nm to 10 nm. With the thickness reduction of aluminium and titanium films, the specific surface resistance increases approximately in 20 times, i.e. the known size effect is developed. The conducted research allows us to approximately evaluate the critical thickness of conductive films so that the films with metallization thickness lower than critical one are technologically inexpedient to choose. Thus, the critical thickness is 25-30 nm for aluminium metallization and less than 40-50 nm for titanium metallization.
The reflectance of aluminium films with the thickness of 100 nm is 0.924, and that of titanium films is 0.57. The calculation shows a sharp reduction of the reflectance of aluminium films at the thickness of less than 30 nm. All conducted research allows us to recommend that the critical thickness of conductive metallization films based on aluminium should be more than 30 nm, and of those based on titanium should be more than 50 nm.

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June 24, 2020
May 29, 2020

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