Reproducible tapered windows in thermally grown silicon dioxide layer have been produced by depositing a thin layer of silicafilm on the thermal silicon dioxide layer before chemical etching. As densification temperature of silicafilm is varied from 175℃ to 1,150℃, taper angles from 3˚ to 40˚ are obtained.
Analytical model of this graded etching process has been presented and etched profile equations of the silicon dioxide layer have been derived using Fermat's principle of least time. Etched contours for two limiting cases are discussed in correlation with the physical behavior of the etching process. Profiles obtained from scanning electron microscope analysis show good agreement with the theoretical profiles.
As an application of the graded etching process, Schottky diodes with a tapered window have been discussed by numerical analysis and experimental results.
Potential and field distribution within the Schottky diode have been obtained by two-dimensional numerical analysis. The result of calculation shows two peaks of the electric field, one at the contact edge and the other at the edge of the metal electrode. The result also shows that as the taper angle decreases, the principal peak of the electric field at the contact edge diminishes, while the secondary peak at the metal electrode remains nearly unchanged. The magnitude of the two electric field peaks become equal to each other at the taper angle of about 6˚. Hence, for the taper angle of 6˚ or less, field peak at the edge of the metal electrode is responsible for the breakdown. Also, for the taper angle of 6˚ and above, field peak at the contact edge is the cause of the breakdown.
Aluminum-silicon Schottky diodes with a tapered window have been fabricated using the graded etching process with silicafilm. The breakdown voltage of the Schottky diode with the taper angle of 3˚ has been drastically increased to 200V in comparison with 60V for the normal metal-overlap diode. Tapered sidewall Schottky diodes have also been demonstrated to have near-ideal characteristics in terms of the breakdown voltages, forward I-V characteristics and reverse I-V characteristics. The experimental results of the breakdown voltages of the Schottky diodes are compared with the theoretical results for taper angle between 3˚ and 90˚. Excellent agreement between the theoretical and experimental results has been found.
스핀온 소오스의 일종인 실리카필름 (silicafilm)을 고속식각층(fastetching layer) 으로 사용하여 thermal $SiO_2$ 의 경사창을 제작할 수 있는 새로운 방법을 소개하였다. 또한 이의 응용으로서 경사창 구조 쇼트기다이오드 (tapered sidewall Schottky diode) 를 제안하였으며 수치해석법과 실험에 의하여 기존의 metal-overlap Schottky diode와 그 성능을 비교 하였다.
본 연구 결과를 요약하면 다음과 같다.
1) 실리카필름의 열처리 온도를 175℃-1,150℃ 로 변화시킴으로써, 산화막의 경사각을 3˚-40˚로 조절할 수 있었다.
2) 경사창 구조 쇼트키다이오드가 기존의 metal-overlap diode에 비해 역방향 특성(파괴전압, 누설전류)이 현저히 개선되었다.
3) 이론적인 계산결과에 의해서도, 경사창구조 다이오드의 파괴 전압이 기존의 metal-overlap 다이오드에 비해 상당히 크며, 실험결과와도 잘 부합되고 있다.
산화막의 경사창 구조는 비단 쇼트키다이오드 뿐만 아니라, 일반적으로 각종 반도체 소자의 파괴전압을 증가시키는 데 일익을 담당할 수 있으리라 기대된다.
