Fabrication of planar silicon transistors without photoresist

Direct-write electron beam lithography in silicon dioxide at low energy

“…Maximal ER values are about 3 for energies of 1.5, 2, and 2.5 keV. This is in agreement with values previously reported for higher energies, 1,3,4,6 indicating that there is a continuity in the physical behavior of SiO 2 irradiated with electrons from 300 keV down to 1.5 keV. As a consequence, 1.5 kV is an acceleration voltage of choice for direct-write EBL since it reaches the maximal ER for the lowest area dose ͑0.07 C cm −2 ͒.…”

“…1,4 The ratio between the etch rate of exposed areas and nonexposed areas is hereafter designed as the ER. In this paper, ER was calculated with the relation ER = TRIT/ TROT, where TRIT is the thickness removed in the trenches and TROT is the thickness removed outside the trenches according to the conventions sketched in Fig.…”

“…Early studies on inorganic resists dealt with the use of SiO 2 because it is the original insulator of Si-based microelectronics. Direct writing in silicon dioxide was first demonstrated by O'Keeffe and Handy in 1968, 1 but it has attracted much attention later in the 1990s after Allee and Broers showed that this technique had nanometer scale capabilities. 2 A significant amount of data has been published at that time for high energy ͑Ͼ50 keV͒ direct patterning of SiO 2 .…”

Glasses for lithography

“…Because of the strong Si-O bond, amorphous silica film is commonly used as a protective layer rather than as a resist. Then it is particularly interesting that the foundation of silica as a resist was laid by O'Keefe and Handy in 1968 when they reported that silica film thermally grown on silicon would dissolve several times faster in the so called 'p-etch' (HF:HNO 3 :H 2 O in 15:10:300) when it was irradiated with 1-15 kV electrons [15]. Unfortunately, the charging of silica film by electron beam easily attracts carbon contamination, which affects the etching process and results have poor reproducibility.…”

“…Therefore, the EB lithographic resolution need not be limited by the damage from secondary electrons. Although the exact mechanism of EB lithographic process is not established, O'Keefe and Handy [15] suggest that the selectivity of EB irradiated silica arises primarily from the beam-induced loss of oxygen on the surface and/or the production of defects in the film [17].…”