Basic Research On Preparation Techniques Of Monocrystal-Si-based Conical Nanopores

Solid-state nanopore is one of the most widely used platforms for the third generation gene sequencing technology,especially nanopores in the shape of cone and pyramid(collectively called conical nanopore).With the development of nano-manufacturing technology and deepening research of nanopore,the application of solid-state nanopore has been continuously expanded,for example nanopores can be used as smart gate,ionic field-effect transistor,nano-stencil for lithography and so on.However,there are still many challenges in preparing conical solid-state nanopore,such as the mutual restriction of cost,efficiency and controllability of the nanopores’ size and structure.In order to overcome these bottleneck,we formulate the following research target: explore cheap and efficient preparation method to fabricate conical nanopores with flexible and controllable scale and sizes based on MEMS manufacturing process.Based on the unique anisotropic etching and flexible surface modification characteristics of the monocrystalline silicon,intensive theoretical and experimental research has been carried out to improve the quality of the array.Meanwhile,combined with dielectric breakdown principle,we successfully prepared self-aligned sub-5 nm single nanopore on conical membrane.The anisotropic etching method has obvious advantages in the realization of conical nanopore arrays.However,there are still key issues exist in this method.For example,the existing formation model cannot explain the shape diversity of nanopores fabricated by this method;how to improve the uniformity of the nanopore array;and how to further reduce the average feature size of the nanopore arrays.First,we focused to solve the theoretical issue.In combination with the convex corner overetching behavior and WulffJaccodin-Construction model,the macroscopic formation process of nanopore was modeled and the reason for the diversity of nanopore shapes was successfully explained according to this model.At the same time,theoretical limit size of the nanopore(7.7 ?)was obtained by atomic formation process model based on the structure characteristics of silicon lattices.Then we focused to solve the existing technical problems.Based on theoretical analysis,optimization of the substrate,mask layout and preparation process,the uniformity of the nanopore array has been improved by 10 %,and the average feature size was reduced to 70 nm.To further reduce the average size of the nanopore array,we proposed to shrink the nanopore by plasma-enhanced chemical vapor deposition(PECVD)technology.The model of PECVD shrinkage process was established.Under the guidance of the size control equation,efficient shrinkage of nanopores with large initial feature size(> 100 nm)was achieved and the feature size was reduced to 8 nm.Anisotropic etching method faces great challenge in realizing well controlled single nanopore.In order to realize single conical nanopore with flexible size controllability,a novel method to prepare self-aligned single conical nanopore in pyramidal membrane was proposed based on anisotropic etching and dielectric breakdown principle.As the template for pyramidal membrane,silicon pyramidal cavity was formed by anisotropic etching.Then atomic layer deposition(ALD)and thermal oxidation process were adopted to form pyramidal HfO2 and SiO2 membrane with different thickness distributions,respectively.multi-nanopore were obtained on vertex and side-edge of the former under continuous fabrication voltage.Single nanopores with feature size ranging from sub-5 nm to 50 nm were prepared on tip region of the latter.Multi-level pulse voltage with different fabrication modes were applied to realize flexible size controllability.In the last part of research,electrical characteristic,especially ionic current rectification behavior of the single nanopore in pyramidal membrane was studied by COMSOL simulation and experiments.The effects of nanopore structure,solution concentration and bias voltage on the rectification ratio were analyzed emphatically.Furthermore,the biological molecular detectability of the nanopore on pyramidal membrane was tested using this system.Different translocation modes of λ-DNA through nanopore were observed and recorded.The average blocking current was 800 pA and average dwell time was 1 ms,showing similar detective performance to nanopores prepared by other methods.