SEMI Technology Symposium (STS) Oxford Instrument CTO Lecture
Semiconductor production is a continuation of the process of taking (photolithographing), shaving (etching), washing (cleaning), and covering the material (deposition). Among them, etching is a step of cutting out a thin film put on a circuit pattern drawn by exposure or vapor deposition. As the circuit pattern becomes finer, more and more precise etching becomes necessary, and 'Atomic layer etching (ALE)' is attracting attention as a promising technology.
Mike Cook, Chief Technology Officer at Oxford Instruments, introduced, at the SEMI Technology Symposium (STS), a SeminCon Korea 2019 event, the ALE test results using silicon, silicon dioxide (SiO2), molybdenum disulfide (MoS2), gallium nitride (GaN), and aluminium gallium (AlGaN), and other materials.
Currently, the widely used dry etching method uses plasma. In the plasma state, several ions hit the etch material, cutting off the circuit pattern or thin film as needed. However, the ion itself has an energy of several electron volts (eV), which can cause undesired damage to the circuit pattern. The chemical structure of the etch surface layer may change or may be converted into fine particles by combining with the specific gas used to clean the chamber.
ALE, which has emerged to solve this problem, is a technique in which reactive gas sticks to the surface requiring etching and removes it with a neutral beam. When the reactive gas is combined with the eutectic layer, the neutral beam separates the two materials into atomic units. It creates a physically smooth surface and isotropic, anisotropic etch as well as conventional dry etch. The etch rate can be adjusted in the desired direction.
CTO Cook noted that in 1996, ALE was possible through Si and GaAs, but stressed that the ability to freely adjust the output while lowering the energy of limited etch materials and neutral beams to less than 50 eV is necessary. At present, Oxford Instruments has created the ALE process using Si, MoS2, AlGaN, and GaN. Chlorine (Cl2) and argon (Ar) were used as reactive gases. The target materials are Si and SiO2.
As a result of the test, it etched 2 to 7 angstroms (0.2 to 0.7 nanometers) per cycle. Cook explained that up to 70 angstroms per minute is possible. In the 3D Si of 25 nanosize, the curve to a depth of 110 nanometers was created. MoS2 removed layers as needed within 40 cycles. What about AlGaN? AlGaN with a thickness of 20~30 nm was etched at 1.4 nm per cycle with uniformity of ± 5% (based on a 200 mm wafer). It was very uniform and accurate to the desired depth.
There are some challenges to overcome. 300mm wafer support and neutral beam output should be fine-tuned. "With the use of a variety of materials away from the laboratory environment, ALE is transitioning to a technology that can be used," said CTO Cook. "With the Atomic Layer Deposition (ALD) on the market, it will be a way to overcome difficulties in the production of semiconductor below 10 nm,” he added.